Coesite in Australasian microtektites.

Tektites and microtektites (i.e. very small tektites) are pieces of glass formed as ejecta from impact events. They are found in strewn fields which may extend thousands of kilometres from the original impact site. Current models of their formation suggest that they form from the material being impacted, as a spray of droplets of material melted by the rapid heating of the impact of an asteroid or comet fragment at speeds of over 3 km per second. However, while we have a general picture of how these events unfold, much of the detail is unclear. For example, many tektites from the Australasian Strewn Field contain inclusions of material, which have for a long time been accepted as fragments of the original rock trapped within a matrix of glass melt. This view has recently been challenged by an alternative hypothesis, that the matrix material may be a condensate from rock which was vapourised during the impact, and that the most distant microtektites, from Antarctica, may have formed from material which was vapourised by heat from the atmospheric shock wave before the impacting object touched down. This would help to explain why the Antarctic tektites lack inclusions, whereas in those from close to the presumed impact site, in Southeast Asia, they may make up as much as 5% of the mass of the tektite.

In a paper published in the journal Geology on 3 June 2025, Luigi Folco, Enrico Mugnaioli, and Matteo Masotta of the Dipartimento di Scienze della Terra and the Centro per la Integrazione della Strumentazione  at the Università di Pisa, and Billy Glass of the Department of Geosciences at the University of Delaware, present the results of a study of four microtektites from the Australasian Strewn Field.

The Australasian Strewn Field covers about 15% of the Earth's surface, and formed about 800 000 years ago through the hypervelocity impact of a chondritic body. It is the youngest and largest of five known Cainozoic strewn fields, with the others  being the North American Strewn Field, which is about 34.86 million years old (Eocene), the Central European Strewn Field, which is about 14.7 million years old (Miocene), the Côte d'Ivoire Strewn Field, which is 1.07 million years old (Pleistocene), and the Central American Strewn Field, which is about 820 000 years old (Pleistocene). No crater has been found which can be linked to the Australasian Strewn Field, although high pressure phases have been found in tektites and other ejecta which strongly indicate that the formation of the field was linked to a crater-forming event, probably in Southeast Asia or the surrounding seas, or possibly in northwest China.

Several previous studies have established that Australasian microtektites show Australasian microtektites with distance from the presumed impact site. For this reason Folco et al. selected three microtektites from deep-sea locations close to the putative impact site, SO95-17957-2,04 and ODP 1144A,01 from the South China Sea, and ODP 769A,15_26 from the Sulu Sea, and one, FRO 2.9-1, from a site in the Transantarctic Mountains. These samples were analysed using a combination of optical microscopy, microanalytical scanning electron microscopy, dual beam microscopy, and microanalytical transmission electron microscopy coupled with three-dimensional electron diffraction.

The Australasian Strewn Field microtektites from deep-sea settings are spheroid in shape and dark brown in colour, with a transluscent laustre and numerous inclusions. They range from 350 to 700 µm in maximum elongation, and dominated by silica phases, compositional bands (schlieren), and vesicles. SO95-17957-2,04 and ODP 1144A,01 have normal composition, whereas ODP 769A,15_26 has a high nickel content (232 µg/g). The Antarctic specimen is a pale-yellow transparent sphere 485 µm in diameter with normal composition,  devoid of vesicles, with only one microscopic silica-rich inclusion,  a few tens of micrometres across with diffuse boundaries. This is considered to be fairly typical of Australasian microtektites from Antarctica, although it is the only Antarctic Tektite known with a silica-rich inclusion.

Micrographs of sectioned Australasian microtektite SO95-17957-2,04. (A) Microtektite is pale brown with teardrop shape. It shows folded schlieren (Sch), microscopic vesicles (V), and mineral inclusions (arrowed). Thick white arrow points to inclusion studied in this work. Optical microscope image, plane polarized light. (B) Same petrographic features as in A in backscattered electron (BSE) image. White rectangle outlines field of view of image in panel C. (C) Close-up BSE image of a quartz (Qtz) + lechatelierite (L) + coesite (Coe) inclusion. Silica phases in the inclusion can be distinguished by their different electron density contrast, which increases from lechatelierite to quartz to coesite. A diffusive boundary layer (Dbl) discontinuously surrounds the inclusion. Dashed line traces location of the dual beam microscopy section. Folco et al. (2025).

Examined under the scanning electron microscope, the microtectites from deep-sea environments were found to contain a inclusions which comprise a matrix of vesiculated lechatelierite (shock-fused quartz glass), with variable proportions of microscopic quartz grains and submicroscopic coesite grains (coesite is a form of silica dioxide which only forms at very high pressures). These inclusions are surrounded by diffusive boundary layers a few microns thick. The quartz grains tend to be arranged around the edge of the inclusions, and themselves be surrounded by grains of coesite, while the interior of the inclusions tends to be dominated by lechatelierite. The quartz grains tend to be anhedral and heavily fractured, while the coesite grains comprise  polycrystalline aggregates of nanoscopic crystals set in silica glass. Towards the interior of the inclusions, the coesite grains become smaller, and comprise a higher proportion of silica-glass. The interior part of the inclusions, while dominated by vesiculated lechatelierite, contain many of these low-coesite 'coesite grains'. All three of these microtektites have essentially the same structure, although ODP 769A,15_26 and ODP 1144A,01 are more vesiculated than SO95-17957-2,04.

The Austrolasian microtektite ODP 769A,15_26. (A) Optical microscope, plain polarised image of the sectioned spherule, showing dark brown colour, prolate shape, schlieren, microscopic vesicles and transparent to partly opaque mineral inclusions. (B) The same textural and compositional features seen under scanning electron microscope, back scattered electron imaging mode. The white rectangle outlines the area of the next panel. (C) Back scattered electron view of a quartz + lechatelierite + coesite inclusion. Silica phases in the inclusion can be distinguished by their different contrast, which increases from lechaterlierite to quartz to coesite. The dashed line marks the position of the dual beam film featured in the next panel. (D) Transmission electron microscope image of a dual beam section showing tens of submicroscopic anhedral coesite grains dispensed in vesiculated lechatelierite. Inset: a close-up view of one coesite grain showing characteristic (010) polysnthetic twinning. The dashed line in panel (C) traces the location of the DB section seen in panel (D). Abbreviations: sch, schlieren; V, vesicle: Qtz, quartz; Coe, coesite; L,  lachatelierite. Thin white arrows indicate inclusions; the thick white arrow indicates the coesite bearing inclusion studied in detail. Folca et al. (2025). 

At the edge of the inclusion in microtektite SO95-17957-2,04 studied with dual beam microscopy, coesite could be seen forming euhedral crystals which overgrow the quartz grains. Towards the lechatelierite core of the inclusion, the coesite can be seen to be segmented in submicroscopict abular grains by a fine network of silica glass veinlets producing the polycrystalline aggregates. These coesite grains show a tartan-like texture similar to that seen in microcrystalline coesite aggregates in silica glass found in shocked porous sandstones. The proportion of silica glass between the segments of coesite within the grains increases towards the core of the inclusion, where anhedral nanoscopic grains of coesite can be found floating free within the lechatelierite core. 

Transmission electron microscopy images of electron transparent dual beam microscopy section of quartz + lechatelierite + coesite inclusions from Australasian deep-sea sediment microtektites. (A) Whole section of coesite-bearing inclusion in microtektite SO95-17957-2,04. White rectangle outlines area featured in panel (B). (B) Textural relationships between quartz (Qtz), coesite (Coe), and lechatelierite (L). Few microscopic quartz relicts at periphery of inclusion are overgrown by euhedral coesite grains with polysynthetic (010) twinning. Toward the core of the inclusion dominated by lechatelierite, coesite is segmented by a network of silica glass veinlets producing polycrystalline aggregates, which then disaggregate with increasing amount of silica glass. White rectangle traces area featured panel (C). (C) Close-up view of euhedral coesite (top) adjacent to polycrystalline aggregate with subhedral outline (bottom). (D) Reconstruction of reciprocal space sampled by three-dimensional electron diffraction from a twinned coesite grain. This picture displays a view of the diffraction volume along hh0 vector. Projections of 00l* and hh0* vectors are indicated. (E) Nanoscopic anhedral coesite grain with embayed crystal boundaries embedded in lechatelierite in microtektite ODP 769A,15_26. (F) Several nanoscopic anhedral coesite grains dispersed in an area of about 2 µm² of lechatelierite in microtektite ODP 1144A,01. Broken-apart grains are arrowed. Vesicle (V). Folco et al. (2025).

Inclusions in the Transantarctic Mountains tektite, FRO 2.9-1, could be seen to be featureless glass undr the scanning electron microscope, with diffuse contact with the glass matrix of the tektite. This host matrix was composed largely of silica, with smaller amounts of aluminium oxide, iron oxide, magnesium oxide, titanium oxide, calcium oxide, potassium oxide, and sodium oxide. The transmission electron microscope confirmed that this composition did not vary through the tektite.

The Australasian microtektite ODP1144A,01. (A) Optical microscope, plane polarized image of the sectioned particle. It is a brown glass broken tear drop. Few microscopic vesicles, and faint schlieren and few mineral inclusions are visible. (B) The same textural and compositional features seen under the scanning electron microscope, back scattered electron imaging mode. The white rectangle outline the area of the next panel. (C) Back scattered electron close-up view of a highly vesiculated quartz + lechatelierite + coesite inclusion. Silica phases in the inclusion can be distinguished by their different contrast, which increases from lechatelierite, to quartz to coesite. The dashed line marks the location of the dual beam film featured in the next panel. (D) Transmission electron microscope image of a dual beam section showing a polycrystalline aggregate of submicroscopіс anhedral coesite grains set in lechatelierite. The dashed line in panel (C) traces the location of the dual beam featured in panel (D). Abbreviations: Sch, schlieren; V, vesicle; Qtz, quartz; Coe, coesite; L, lechatelierite. Thin white arrows indicate inclusions; The thick white arrow indicates the coesite bearing inclusion studied in detail in this work. Folco et al. (2025).

Coesite is a fairly common mineral in settings where quartz-bearing rocks have been subjected to shock metamorphism. Whether it occurs as a metastable phase in shocked rocks that have experienced peak pressures and temperatures much beyond its stability field (i.e. pressures in excess of 10 gigapascals and temperatures in excess of 2700°C) has been debated since the 1960s. There are three current models of coesite formation. The first suggests that coesite may form in a silica melt as pressure decreases rapidly following an impact event. The second model suggests that coesite forms within silica glass at very high pressures, without any melting actually occurring. The third model also sees coesite forming within solid quartz, although this time in porous sandstones as the peak of the pressure wave passes through. 

In the Australasian microtektites, coesite appears to be overgrowing quartz crystals, which Folco et al. interpret as a sign that they formed while the matrix was under high pressure, but still in a solid state. However, the adjacent polycrystalline aggregates consisting of submicroscopic elongated grains of coesite pervaded by silica glass veinlets do indicate that some melting has occurred, possibly of the coesite itself, and the nanoscopic anhedral coesite grains dispersed in the surrounding lechatelierite as evidence to significant melting and dispersal of coesite aggregates. In this scenario the coesite nanocrystals re relicts formed by the melting and dispersal of larger, pre-existing coesite crystals during the shock-metamorphism process.

The presence of coesite in the Australasian microtektites provides information about the location of the putative Southeast Asian impact site. Coesite forms at very high temperatures, but is unstable unless cooled rapidly; it has been estimated that after 10 seconds at very high temperatures then all coesite will have transformed into cristobalite. Since there is no cristobalite in the Australasian microtektites, it can be inferred that quenching was much more rapid in this instance, possibly aided by reactions such as the transformation of quartz and coesite melt into lechatelierite, which is endothermic (absorbs heat).

The abundant quartz and lechatelierite inclusions found in Australasian tektites are generally accepted to be indicative of a quartz-bearing target rock which underwent melting and fusion during the impact event. The boundary between these inclusions and the glass matrix of the microtektites shows varying levels of diffuseness, suggesting that these particles underwent varying levels of digestion into the matrix. The presence of coesite in tektites from deep-sea environments off Southeast Asia supports the idea that these tektites are from close to the impact site, and represent melt spherules formed by compression and depression of the impacted rock during crater formation. The absence of coesite from the Transantarctic Mountains tektite, FRO 2.9-1, could imply that this remained at higher temperatures for longer, allowing all coesite to be reabsorbed, possibly implying this tektite was exposed not just the heat from the origianal impact, but also from deceleration in ambient air or atmospheric re-entry. This would explain the near-total absence of inclusions in these tektites, and also the homogenous distribution of elements and isotopes observed. There is no structure in the microtektite indicative exposure of high pressure, which supports the idea that these more distant microtektites formed by rapid heating of the target rock prior to the actual impact.

The presence of pressure-related minerals and structures in microtektites from the South China and Sulu seas strengthens the argument for a Southeast Asian impact event.

See also...

Looking for the origins of garnets from Lower Nubia.

From about the third century BC, garnets became highly valued gemstones to the peoples of the Mediterranean Basin, the Middle East, India, Sri Lanka, Southeast Asia, and China, along with other hard gemstones, such as emerald, aquamarine, and sapphire. Transparent red garnet was used to make engraved rings or seal stones; inlays in diadems; earrings or necklaces or even small sculptures; thin, doubly polished plates in cloisonné jewellery, as well as simple beads, merely drilled to be assembled into necklaces, bracelets, or applied to garments. Studies of garnets dating from the Hellenistic Period to Early Medieval times have identified seven distinctive garnet types (A-G), with unique compositions, some of which have been linked to sources in Europe, India, or Sri Lanka, while the source of others remains unknown.

Garnets were also used as gemstones prior to the Hellenistic Period in several places, although the origin and typology of these is less well understood. Green grossular, a calcic garnet, often intergrown with green vesuvianite was used to make seals, beads, and amulets by the Indus Valley Civilization, and to much lesser extent in ancient Mesopotamia. This material was worked in Harappa, Mohenjo-Daro, and Loal-Mari on the Indus River, and was probably sourced in Balochistan and northern Pakistan. Both grossular and vesuvanite have hardnesses of between 6.5 and 7.0 on the Moh scale (i.e. are slightly softer than quartz) making it possible to work these materials with the tools available to the Indus Valley Civilization.

Red aluminous garnet, however, is harder than quartz, making it much more difficult to work without specialist tools, and is rare in Asia before the advent of the Iron Age. Surprisingly, this material was worked early in Northeast Africa, with red garnet beads known from Predynastic Egypt and the contemporaneous A-group Cultures of Lower Nubia. The use of red garnet continued in Egypt till around the end of the New Kingdom, after which the mineral is seldom found. Thus, the red garnets of Egypt and Nubia are the oldest known examples of the working of this mineral.

In a paper published in the journal Archaeometry on 7 March 2024, Albert Gilg of Engineering Geology at the Technical University of Munich, Joanna Then-Obłuska of the Antiquity of Southeastern Europe Research Centre at the University of Warsaw, and Laure Dussubieux of the Elemental Analysis Facility at the Field Museum, present the results of an analysis of 34 garnet beads from burials in Lower Nubia, dated from the late A-Group to the Post-Meroitic, an age range of about 3200 BC to about 600 AD, as well as two garnets from separate alluvial deposits near the Fourth Cataract of the Nile in the Bayuda Desert of Upper Nubia.

Ancient Nubia is divided into Lower Nubia, which lay between the First and the Second Cataracts of the Nile, and Upper Nubia, to the south of the Second Cataract. Gilg et al. selected beads excavated  from graves in Qustul, Adindan, and Serra East, in the collection of the Museum of the Institute for the Study of Ancient Cultures at the University of Chicago, associated with the Early Nubian A-group Culture, the Middle Nubian C-group and Pan Grave cultures, and the New Kingdom, Napatan, Meroitic, and Post-Meroitic/Nobadian periods.

The A-group Culture (roughly 3700 to 2800 BC) and C-group Culture (roughly 2300 to 1550 BC) are known to have been wealthy societies, due to their location at a junction of trade routes between Egypt and the Mediterranean to the north and the African interior to the south. The Pan Grave people (2200 to1550 BC) lived in small, dispersed groups in the Eastern Desert. All of these peoples traded to differing extents with the Pre-Kerma and Kerma cultures of Upper Nubia. Between about 1570 and about 1069 BC Nubia was controlled by the Egyptian New Kingdon, then between 747 and 656 BC, Egypt was ruled by the Kushite 25th Dynasty, which ruled an area from the confluence of the Blue and White Niles to the Mediterranean. This interval forms part of the Napatan Period in Nubia, which lasted from about 750 BC to about 350 BC, and was another period of wealth in Lower Nubia. This was followed by the Meroitic Period, from about 350 BC to about 350 AD, when Lower Nubia became an intermediary in trade between the Kingdom of Meroë in Upper Nubia and the Hellenic and Roman rulers of Egypt. Between about 350 and 600 AD Lower Nubia was Kingdom of Nobadia, which often had less peaceful relations with both Egypt and the Blemmye peoples who controlled the Eastern Desert and the Red Sea Coast.

Map of Nubia. Gilg et al. (2024).

Gilg et al. analysed garnet beads from tombs 11, 17, 22, 24 of Royal Cemetery L at Qustul, which have been dated to Late A-Group/Naqada III (Naqada III is the final phase of Predynastic Egypt), as well as the late A-Group tombs W19 and V59; garnets from these tombs were commonly accompanied by similarly shaped carnelian beads. Another bead was from the  C-Group Phase III tomb T12B at Adindan, which is of equivalent age to the Egyptian 18th Dynasty (the first dynasty of the New Kingdom). Five more beads come from tombs K74 and K93 at Adindan, which were associated with the Pan Grave Culture; notably, garnet beads are more common in Pan Grave Culture burials than C-Group Culture burials, despite the two being roughly contemporary. Six more beads came from New Kingdom tombs VC45 and R19 at Qustul, three from the Amenhotep III to Amarna Period and three from the post-Amarna Period. A single short barrel bead came from the 25th Dynasty/Napatan tomb W43 at  Qustul. Also from Qustul came an oblate bead from the Meroitic tomb Q465, and a truncated hexagonal bicone bead from the Post-Meroitic tomb Q143.

Beads associated with the A-Group culture are all less than 5 mm in diameter and 2.5 mm wide. They were shaped into short cylinders, barrels, or oblates with a relatively poor polish, and perforated from each end by irregular pecking. Similar beads are known from Predynastic Upper Egypt, which were presumably made in the same way, possibly from the same people. A single bead of similar appearance has also been found at Mehrgarh in Pakistan, which is exotic to that site, but of unknown origin. C-Group and Pan Grave Culture beads are also typically poorly polished and of imperfect shape, though the shape varied slightly, with both rounded and short-barrel beads found. These beads were perforated from each end by drilling, forming either cylindrical or conical holes; the smooth nature of the hole suggests the drill made from a hard stone, such as flint, or possibly copper. Similar perforations have been observed in Middle Kingdom garnet beads from Egypt. Workshops producing carnelian beads are known from A-Group and C-Group sites in Lower Nubia, and while no trace of garnet-working has been found at these sites, the similarity between the carnelian and garnet beads suggests that the garnet beads are also likely to have been manufactured locally.

Microphotographs of garnet beads from lower Nubia. (a) A-group bead with irregular pecked hole (ISAC 13); (b) Pan Grave bead with a smooth drilled hole (ISAC 21); (c) C-group bead with silver beads (ISAC 20);  (d) New Kingdom long barrel-shaped beads with poor polish (ISAC 29-31); (e) Meroitic irregular oblate bead (ISAC33); (f) Post-Meroitic facetted bead (ISAC 34); (g) drill hole (about 1 mm in diameter) with concentric deep grooves from a diamond tipped drill (ISAC 34); (h) tiny short- and long-prismatic colourless inclusions (ISAC 34). Scale bar is 500μm. Gilg et al. (2024). 

The New Kingdom beads showed much improved shaping. The majority of these beads were globular in shape, but also present were unusually long barrel to tubular shapes with a length of up to 7.9 mm and a diameter of 4 mm. These beads all have a low polish, and again are drilled from both ends. The Napatan and Meroitic beads were similar in form to the Pan Grave and C-Group beads.

None of these beads had a high polish, something seen in Egyptian beads from the Great Aten temple at Amarna (18th Dynasty), which were made by polishing with corundum powder as an abrasive, a technique apparently unknown in Upper Nubia. How the Upper Nubian beads were polished is unclear at this time.

The youngest, Post-Merotic bead differs from all others in the study, having a faceted shape (a hexagonal truncated bicone), a well-polished surface, and deep, regularly spaced, concentric grooves in the drill hole, probably indicative of the use of a diamond drill bit. Similar garnet beads are known from Arikamedu in southern India, and sites in southern Sri Lanka. Microscopic examination of this bead revealed the presence of many tiny, short- and long-prismatic, colourless inclusions, something seen in Sri Lankan garnets but not garnets from southern India. Though this is not sufficient evidence to confirm the origin of this bead, Gilg et al. consider it highly likely that this bead comes from South Asia, and probably Sri Lanka.

Chemically, all of the garnets, including the alluvial samples from Upper Nubia were found to be of similar composition, with the exception of the single Post-Merotic bead. These beads have an almandine-rich composition, with a low calcium content (the Post-Merotic bead has a pyrope-rich composition with a low calcium content). Compositionally, these beads do not fit into any of the types used to classify Hellenistic to Early Medieval garnets, with magnesium oxide-calcium oxide ratios intermediate between type A and type B contents, combined with a high manganese and yttrium, low chromium composition not seen in either of these types. This suggests that the beads were made from alluvial garnets sourced from deposits in Upper Nubia, and that garnets from this source were not used in the Hellenistic to Early Medieval periods.

Almandine-rich garnet was the first mineral harder than quartz to be worked in northeast Africa, apparently being sourced at a site in the Bayunda Desert of Upper Nubia at least 670 km south of the most southerly known occurrence of worked garnet beads in Lower Nubia. These Upper Nubian deposits appear to have been the only source of garnets used in manufacturing for at least 3500 years. 

See also...

Seventh largest diamond ever found discovered in Botswana.

The Lucara Diamond Company has announced discovering what is believed to be the seventh largest diamond ever found at its Karowe Mine in eastern Botswana, in a press release issued on 8 August 2023. The diamond is described as weighing 1080.1 carats (216.02 g), and to measure 82.2 x 42.8 x 34.2 mm. Importantly, the gemstone is reported to be a Type IIa top white diamond, which is to say a diamond with almost no impurities, a type of diamond which make up only about 1-2% of all diamonds discovered, and which are correspondingly more valuable than other diamonds of similar size. 

The new Lucara diamond. Lucara Diamonds.

The largest diamond ever found is the Sergio Diamond, found at Lençóis in Bahia State Brazil, in 1895, by Sérgio Borges de Carvalho, after whom it is named, which weighed 3167 carat (633.4 g). Surprisingly, the Sergio Diamond was not found within a diamond mine, but on the surface. The Sergio Diamond was a carbonado, a type of diamond with a black colour, a micro-porous structure, and a high graphite and amorphous carbon content, as well as frequently containing inclusions of other minerals or metals. Notably, some of the inclusions found in carbonado diamonds are extremely rare on Earth, and they have very low proportions of the isotope carbon¹³ compared to other diamonds, as well as radioactive inclusions, again not found in other diamonds. All caronado diamonds subjected to uranium-lead isotope dating have been found to be about 3 billion years old, and almost all carbonado diamonds come from two locations, Brazil and the Central African Republic. This has led to speculation that these diamonds are derived from an extra-terrestrial body which impacted the Earth in the distant past, although no hypothesis as to how such a body could have formed has ever gained widespread acceptance. Because of their hardness, carbonado diamonds were widely sought for use in drill bits in the nineteenth century, although they have been replaced by more modern materials today. Despite its exceptional size (most carbonado diamonds are smaller than a pea), the Sergio Diamond was sold for £6400 in London in September 1895, then broken up to make diamond drill bits.

An engraving of the Sergio Diamond published in Popular Science Monthly in 1906. Wikimedia Commons.

The second largest diamond ever discovered, and the largest gemstone-quality diamond, was rhe Cullinan Diamond found at Cullinan in what is now Gauteng Province, South Africa, in April 1905, which weighed 3106 carat (621.2 g) when it was found. The Cullinan Diamond was purchased by Louis Botha, the Prime Minister of the Transvaal Colony, and given to the British King Edward VII, who had it cut into nine large gemstones and a number of smaller fragments known as 'The Brilliants'. The largest of these cut stones, known as Cullinan I or the Star of Africa, has a mass of 530.4 carat, and is mounted on the Sceptre with Cross, part of the British Crown Jewels, which is carried by the monarch at their coronation.

(Left) The uncut Cullinan Diamond in 1908. (Right) The Star of Africa Diamond in the Sceptre wirh Cross in 1919. Wikimedia Commons.

The third largest diamond ever found is the Sewelô Diamond, recovered at Lucara's Karowe Diamond Mine in Botswana in April 2019, which weighs 1758 carats (352 g). This was the largest diamond ever found in Botswana, and its name was chosen by a competition organised by Lucara, meaning 'rare find' in Setswana. The Sewelô Diamond was purchased by the French fashion house Louis Vuitton, with the intention of having it cut into smaller gems.

The Sewelô Diamond. The diamond has a black crust formed of pitted carbon, but is gemstone quality beneath. Lucara Diamonds.

The fourth largest diamond ever discovered is an unnamed diamond found at Lucara's Karowe Mine in June 2021. This diamond had a mass of 1174.76 carats, and measuring 77 x 55 x 33 mm. This gem is considered to be of variable quality, although with a significant proportion of high quality diamond.

An unnamed diamond found at Karowe Mine in June 2021. Lucara Diamonds.

The fifth largest diamond ever discovered was the Lesedi De Rona Diamond, found at the Karowe Mine in November 2015. Like the new diamond, this was a Type IIa top white diamond, and has a mass of 1111 carat (222.2 g) when it was found. At that time, it was the largest diamond ever found in Botswana, and the third largest diamond ever found, prompting Lucara Mining to organise a national competition in Botswana to chose a name. The winning name, Lesedi De Rona, translates as 'Our Light' in Setswana, and was chosen by Thembani Moitlhobogi of Mmadikola. The diamond was purchased by the London-based jeweller Graff, and cut to form one large diamond, the 302.37 carat Graff Lesedi De Rona Diamond, and 66 smaller gemstones.

The uncut Lesedi De Rona Diamond in 2015. Lucara Diamonds.

The sixth largest diamond ever discovered was found at the Debswana-owned Jwaneng Mine in southern Botswana in June 2021, and had a mass of 1098 carat (219.6 g), measuring 73 x 52 x 27 mm. 

The unnamed diamond found at Debswana's Jwaneng Mine in southern Botswana in June 2021. Reuters.

Thus, the new diamond is the seventh largest diamond ever discovered, the sixth largest gemstone quality diamond ever discovered, the sixth largest diamond ever found in Africa, the fifth largest diamond ever found in Botswana, the fourth largest diamond extracted from the Karowe Mine, and one of only seven diamonds ever found with a mass of greater than 1000 carat. 

That five of these seven diamonds have been found in Botswana, and four of them from a single mine, since 2015 is not a coincidence. but marks the introduction of new technology pioneered at the Karowe Mine. Modern mines typically use crushing machinery to extract diamonds from their parent rock, but this is generally thought to break up a significant proportion of larger diamonds. The Karowe Mine uses X-ray fluorescence technology to scan ore before it passes into the crushing equipment, thus allowing for the machinery to be stopped and particularly large diamonds to be recovered. 

Flow chart showing the processing and sorting of diamonds at the Karowe Mine. Lucara Diamonds.

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Investigating the origin of a gold-coloured talc bead from the Kongo Kingdom of West-Central Africa.

Personal adornments are considered one of the key cultural traits of Modern Humans, and appear to have been more-or-less ubiquitous in all Human societies for at least the past 40 000 years. Adornments are used to convey information about an individuals social status, beliefs, gender identity, kinship, and affiliations, as well as personal taste, with these symbols typically being recognizable to other members of the society in which they live. Early adornments, in the form of perforated shells presumed to have been strung together as beads, appear in Africa north and south of the Sahara, as well as parts of the Middle East from about 82 000 years ago, with possible earlier use by the Aterian Culture of North Africa as long as 130 000 years ago (claims have been made for beads older than this, although these are not generally accepted anymore).

However, while there is strong evidence for these ancient items having been beads, with examples such as shells from Blombos Cave in South Africa, which show signs both of having sustained wear while hanging on a string, and of having been painted with red ochre, there is a gap in the archaeological record between about 70 000 and 450 0000 years ago, during which no beads are known, followed by a sudden re-appearance of beads, and their spreading to every inhabited landmass by about 40 000 years ago. These later beads are no longer made exclusively of shell, with bone and stone also being used as bead-making materials, and the appearance of pendants as well as simple strings of beads, and evidence that beads were being traded over quite long distances, at least in Southern Africa.

By the middle of first millennium BC, semi-precious stones were being widely used to make beads, and traded over wide areas, with examples having been used to demonstrate trade links between West and East Africa, the Indian Ocean rim, and Europe. However, while these high-status items appear to have been traded over wide areas, many people were still using beads made from local materials, with examples of locally-produced-and-used beads from Africa including the jasper beads known from Niger, a variety of stone and mineral beads from around Lake Turkana, in Kenya, and the stone beads of Njoro River Cave, also in Kenya.

Talc, talc-schist, soapstone, and steatite, are soft rocks easily carved into beads or other items, found widely in sub-Saharan Africa. Despite this, beads made from these materials are somewhat unusual on the continent. Talc is the softest of these minerals, and with a rating of 1 on the Mohs scale, is generally rated the softest mineral of any kind. It is variable in colour, ranging from white to green, depending on the nature and position of the metal ions within its crystal structure. Furthermore, while it's softness makes it easy to carve, it can be hardened by heating, reaching 3-5 on on the Mohs scale when heated to above 800°C and as much as 7 (equivalent to quartz) when heated to above 1000°C.

Talk which had apparently been collected with the intention of manufacturing something has been recovered from the Iron Age Thaba ya Batswana archaeological site, a stone walled enclosure to the south of Johannesburg, thought to date to about 1700 AD. Talk and soapstone were also utilized by early herders in East Africa to make beads and pendants. Away from Africa, soapstone items were widely used to make personal adornments in the Near East during the Neolithic, with items known from sites such as Çatalhöyük in Turkey, and Abu Hureyra, Tell Aswad, Dja’de el-Mughara, and Tell Halula, all in Syria, as well as sites in Israel, as well as and several Upper Palaeolithic Aurignacian sites from France, several sites associated with the Indus Valley Civilization in northern India, dated to between the 5th and 2nd millennia BC, the 10 000-year-old Arch Lake human burial site in New Mexico, and 15-16th century AD sites in eastern North America.

In a paper published in the journal Archaeological and Anthropological Sciences on 7 February 2023, Mafalda Costa of the HERCULES Laboratory at the University of Évora, and the Archaeometry Research Group at Ghent University, Pedro Barrulas, also of the HERCULES Laboratory at the University of Évora, Maria da Conceição Lopes of the Research Center in Archaeology and the Department of History, Archaeology and Arts at the University of Coimbra, João Barreira, also of the Research Center in Archaeology at the University of Coimbra, Maria da Piedade de Jesus of the Angolan Instituto Nacional Do Património Cultural, Sónia da Silva Domingos of the Angolan Centro Nacional de Investigação Científc, Peter Vandenabeele, also of the Archaeometry Research Group, and of the Raman Spectroscopy Research Group, at Ghent University, José Mirão, also of the HERCULES Laboratory, and of the Department of Geosciences at the University of Évora, present the results of a chemical and mineralogical characterization of a broken mustard-gold-colored stone bead excavated from the Lumbu archaeological site within Mbanza Kongo, the pre-colonial capitol of the  Kongo Kingdom, which covered much of what if now northern Angola and the west of the Democratic Republic of Kongo between the thirteenth and nineteenth centuries AD.

The Kongo Kingdom is considered to have been highly socially stratified and centralized society prior to its first contact with Portuguese explorers in 1483. The Portuguese introduced Christianity to the nation, as well as a range of new technologies, and integrated it into a transatlantic trade network. The capital of this kingdom, Mbanza Kongo, was founded in the mid fourteenth century, and is considered to be the oldest continuously occupies settlement in West Central Africa, today being the capital of Zaire Province in northwest Angola. The Lumba archaeological site, uncovered during a three-month archaeological site in 2014, is thought to have either been a royal residence or a ceremonial court. The site comprises a dry-stone building made from irregularly shaped blocks, arranged in an uneven fashion, which led to the walls having considerable variation in thickness. Two radiocarbon dates have been obtained from pieces of charcoal from different parts of the site, these being 1637 and 1950.

The 2014 excavations produced a range of ceramic vessels, including Portuguese-made pottery, fragments of clay pottery, and a total of 53 beads, 52 of which were glass 'trade-beads' of European manufacture, but one, identified as 'Type 44' is a broken mustard-gold-colored bead, made from a softer material, initially thought to be clay. The surviving bead-fragment is about 8.7 mm in width, 9.0 mm long, and has a maximum thickness of 8.9 mm, with a central perforation with a maximum diameter of 5.6 mm. The unbroken end is rounded, possibly by contact with other beads on a string. In form, the bead resembles a monochrome elongate glass bead. It was found in Stratigraphic Unit 6 of area H17 at Lumba, along with several hexagonal blue glass beads made in Bohemia in the nineteenth century, and some blue and green glass beads thought to be of Venetian manufacture.

Stereomicroscope images of Type 44 bead found in Lumbu (Mbanza Kongo, Angola). Costa et al. (2023).

Examination of Type 44 under a stereomicroscope revealed it was made from a fine-grained material, with white, black, and dark brown mineral inclusions. The fine grains were found to be made from platy grains of magnesium silicate with a maximum size of 10 µm, making the substance talc. The mineral inclusions range from 5 to 30 µm in size, and appear to be a mixture of zircons, iron oxides, and iron-titanium oxides. During the examination, the bead was accidentally scratched with a fingernail, suggesting it has a hardness of about 2.5 on the Mohs scale. This could be consistent with a talc bead which has been heat-treated to harden it although mineral inclusions can also harden talc this much.

Trace element, and rare earth element, analysis has been used to distinguish talc-minerals from different environments. Talc which formed in marine environments tend to be more enriched in heavier rare earth elements, and depleted in lighter members of the group, as well as having very low levels of cerium and high levels of lanthanum, while talcs from terrestrial hydrothermal systems tend to be enriched in both heavy and light rare earth elements, but deficient in those in the middle of the range. Talcs from ultramafic environments tend to be enriched in chromium and nickel, which often reach levels around 2000 parts per million, while talcs from carbonate environments tend to be very low in these elements.

Type 44 is enriched in heavy rare earth elements, depleted in cerium, and enriched in lanthanum, suggesting that it is made from talc which was precipitated from seawater. The specimen also has a boron concentration typical of seawater, and very low levels of both chromium and nickel, with suggests it formed in a carbonate-rich marine environment, probably with a small amount of detrital material.

Mbanza Kongo sits on rocks of the 575-550-million-year-old Schisto-Calcaire Subgroup, a sequence of sedimentary rocks including of limestones, dolostones, siltstones, and shales with rare intercalations of sandstones, cherts, and evaporites. Two types of talc have been recovered from this sequence, the Pseudo-Oolithe de Kisantu, in which talk oolites are found within quartz-rich layers within a dolomite, and an authigenic hydrothermal talc, which is found associated with copper and lead-zinc mineralizations.

The talc oolites appear to have been formed diagenetically from the remineralization of magnesium-rich clay, and are quite unlike the material from which Type 44 was made. The low levels of chromium and nickel in the material appear to rule out the hydrothermal talc. The talc-schists of Nigeria can also be ruled out as an origin, as these formed within an ultramafic environment. The pattern of rare earth elements and boron concentration seen in the material from which Type 44 is made could support it having been manufactured from an unknown talc-source within the Schisto-Calcaire Subgroup, although such a talc would be likely to have aluminum and iron oxide levels similar to those of the Pseudo-Oolithe de Kisantu, while these are at much higher levels in the Type 44 than in the oolites. However, the Schisto-Calcaire Subgroup outcrops over a wide area in northern Angola, as well as in the vicinity of the Congo River near Luozi, and near Inkisi and Kisantu within the Democratic Republic of Congo, with much of this area, including the area directly around Mbanza Kongo, being very poorly explored by geologists, making it quite possible that unknown talc-deposits within this sequence. Furthermore, the range of materials used in pottery-making in the Kongo Kingdom suggests that the people of the kingdom had a good understanding of the local geology prior to contact with the Portuguese. and even produced talc-rich pottery in the provinces of Nsundi, Mbata, and Mpemba (which is where Mbanza Kongo is located). Since carved talc objects are known from other African kingdoms which were contemporary with Kongo, and likely to have been in contact with it, such as Esie in southwestern Nigeria and Great Zimbabwe in the southeast of modern Zimbabwe, it is quite likely that the people of Kongo had the geological knowledge to seek, locate, and exploit talc resources.

Although Type 44 is only a single (broken) bead, it represents the first known example of a locally made item apparently used for personal adornment. The bead appears to have been made within the Kongo Kingdom, using a source of material familiar to the people of the kingdom, but lost to modern geologists, although it is (obviously) not possible to say quite where this source was. The bead bears a notable resemblance to the elongated monochrome glass beads made in Europe from the sixteenth century onwards, and known to have been exported to the Kongo Kingdom, where glass-making was not known, and appears to represent local craftsmen responding to an introduced European fashion trend.

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Evidence for present-day volcanism on Venus.

The crust of Venus consists mostly of basaltic rock, which is in contact with its hot caustic atmosphere. Chemical reactions between Venus’ basaltic crust and its atmosphere, i.e. weathering, modify the surface’s mineralogy and composition and affect its visible to near-infrared spectral characteristics and radar backscatter. Without liquid water, weathering, based on experimental and modeling work, is suggested to be geologically slow and includes mainly oxidation reactions that produce coatings of hematite and/or magnetite on surfaces of iron-bearing mineral grains. However, the rates of oxidation on Venus, and how these weathering results affect visible to near-infrared reflectance spectra, are not well understood and are needed to constrain the ages of lava flows measured by the Venus Express mission.

 

An artist's impression of the Venus Express space probe. European Space Agency.

The thick atmosphere of Venus prevents the acquisition of high spectral resolution data in the visible to near-infrared, which contain crucial sources of mineralogical inferences about planetary surfaces. Venus’ carbon dioxide-rich atmosphere is relatively transparent in only a few spectral windows in the near-infrared (at 1.01, 1.10, and 1.18 μm), which limits the characterisation of its surface mineralogy. Radar backscatter and emissivity of the surface can also be used to constrain mineralogy and rock physical properties by their surface dielectric and magnetic permeability properties. However, both radar and visible to near-infrared spectroscopic results cannot define the mineralogy of Venus’ crust alone, and one must also invoke constraints on specific mineral stability from experimental and geochemical modeling.

The rocks in Venus’ lowlands are in contact with an atmosphere dominated by carbon dioxide and trace sulphur species at about 460°C and roughly 92 bars (metamorphic conditions on Earth) and, therefore, should be altered from their original basalt mineralogy. However, in the absence of liquid water, the alteration (or weathering) is predicted to be limited to oxidation and/or sulphurisation along surfaces and cracks. Basalts on the Venus surface are predicted by thermodynamic modeling and experimental results to oxidize, producing mainly iron oxides (magnetite and/or haematite), pyroxene, silica, and anhydrite, with possibly minor iron disulphide (pyrite), aluminosilicates (e.g. andalusite), cordierite, alkali feldspar, enstatite, and forsterite, depending on the chemistry of the original rock and of the assumptions of atmospheric composition.

These surface coatings of weathering minerals will affect the reflectance (and emissivity) of Venus’ surfaces in visible and near-infrared wavelengths of light, including the near-infrared windows through Venus’ atmosphere. Earlier studies have used the near-infrared windows to constrain physical properties of the surface, including its mineralogy, chemistry, and the ages of lava flows. Near-infrared emissivity variations at 1.02 μm (emissivity is effectively the difference between unity and reflectance) have been used to distinguish surfaces rich in ferous iron-bearing silicates, with high emissivity (low reflectance), from those rich in hematite, with lower emissivity (high reflectance); the near-infrared emissivity variations, in turn, can be used to constrain weathering (unweathered ferous iron-bearing silicates versus oxidized/weathered products containing hematite). Thus, near-infrared emissivity can be used as an indicator of relative age of erupted material since young flows will be less weathered and should not have a signature of haematite. It has been suggested that some lava flows at large volcanoes were younger than 2.5 million years and possibly even younger than 250 000 years based on these flows having high emissivity; however, without experimental constraints on the rates of weathering/oxidation of iron and knowledge of how these affect near-infrared emissivity spectra, there is large uncertainty in the age of these flows.

In a paper published in the journal Science Advances on 3 January 2020, Justin Filiberto of the Lunar and Planetary Institute, David Trang of the Hawai’i Institute of Geophysics and Planetology at the University of Hawai‘i at Mānoa, Allan Treiman, also of the Lunar and Planetary Institute, and Martha Gilmore of the Department of Earth and Environmental Sciences at Wesleyan University, present visible to near-infrared reflectance spectra for olivine crystals, which are likely common in Venus surface basalts, that have been oxidised in Earth’s atmosphere at 600° and 900°C for a range of durations, in order to determine the rate at which olivine grains become coated with secondary minerals during weathering, characterize how the surface coatings affect visible to near-infrared spectra, and place bounds on the ages of lava flows on the basis of their measured near-infrared spectra emissivity values.

Oxidisation of olivine produces haematite coatings consistent with reaction products thought to be on Venus; therefore, while Filiberto et al.'s experimental results are under terrestrial atmospheric conditions, the results are applicable to the oxidation mineralogy of the surface of Venus.

Filiberto et al. obtained visible to near-infrared reflectance spectra of samples of olivine that had been oxidised under Earth air in an earlier study; gem-quality crystals (approximately 1 cm in size) from San Carlos, Arizona and China. The crystals were purchased from mineral dealers and verified by appearance and composition to be consistent with the advertised sources. The crystals were oxidised in a box furnace, under air, at 600° and 900°C and were removed after durations of 0.2, 1, 5, 25, 125, and 625 hours. Samples to be oxidised at 900°C were placed directly on a plate in the furnace; samples to be oxidised at 600°C were placed in open-ended alumina crucibles. These temperatures were chosen for consistency with the established methods; for olivine oxidation; for comparison, the surface of Venus is about 460°C.

The oxidation state of the Venus surface atmosphere is predicted to be at or above the magnetite-haematite buffer. The difference between Earth’s atmosphere and the Venus carbon dioxide-rich atmosphere is a limitation on the applicability of Filiberto et al.'s experiments. However, other recent experimental results show similar time scales of alteration as in Filiberto et al.'s experiments, with oxide minerals forming within days without providing spectral analyses. Oxidation rates depend on temperature (in addition to the oxidation state); the rates should obviously be greater at 900°C than at 600°C, and Filiberto et al expect the latter to be of the same order as that for the Venus surface. The 900°C experiments were included because including these results provides a more advanced weathering reaction that can be observed with the visible to near-infrared measurements. Last, the effect of surface coatings on the visible to near-infrared spectra is directly applicable as the oxidation mineralogy is expected to be similar to that on the surface of Venus. Therefore, Filiberto et al.'s results provide a direct constraint on the time scales of Venus weathering.

Visible to near-infrared reflectance spectra of one unaltered olivine (China-10) and all oxidised olivine crystals were measured from 350 to 2500 nm with a Spectral Evolution oreXpress spectrometer with its benchtop reflectance probe. Raw measurements were normalized against the reflectance of a standard white panel. The magnetic properties of the samples were measured with a vibrating sample magnetometer, and their mineralogies were determined by Raman spectroscopy. The Raman spectrometer used a dual-laser (758 and 852 nm) excitation and fluorescence mitigation strategy involving successive heating of the laser.

In the 900°C experiments, reddish-brown surface coatings with specular luster began to appear after only 12 minutes; the olivine was completely coated after 5 hours. With increasing oxidation time, the coating became darker red-brown in colour, and the specular luster disappeared. In the 600°C experiments, the surface coating developed same as for the 900°C experiments, but progressed more slowly and never fully coated the olivine grains. Even after 1 month of simulated Venus weathering, green unreacted olivine was still visible through the coating. A thick section of an olivine crystal oxidized at 900°C for 625 hours was analysed in a previous study, and showed three distinct morphologies of iron-oxide oxidative alteration formation: (i) surface coating; (ii) crack filling; and (iii) within the olivine crystal lattice. Recent experimental results of Venus rock–atmosphere interaction under more realistic atmospheric conditions (carbon dioxide-dominated, nickel-nickel oxide buffer) confirm that the rates and alteration minerals in Filiberto et al.'s study are representative of those at Venus surface conditions. Specifically, those experiments produced iron oxide coatings on olivine within 1 week, consistent with both the mineralogy and alteration time scales of Filiberto et al.'s results.

 

Images of olivine crystals before and after oxidation. (A) For the 900°C experiments, and (B) for the 600°C experiments. Images are arranged in the order of increasing time of alteration from left to right. Results show a decrease in the green coloration of olivine and the formation of coating an increase in time of oxidation. The coating is initially metallic before becoming dull red with oxidation time. Delia Enriquez-Draper in Filiberto et al. (2020).

Raman spectra of the altered samples were dominated by the scattering peaks of olivine even after 625 hours of oxidation. Raman signatures of the alteration products, magnetite and haematite, became stronger with alteration duration. Raman spectra also showed characteristic scattering peaks consistent with small proportions of enstatite and quartz in the most oxidided samples (those altered for 625 hours and 900°C); these phases are expected products of oxidation of iron in olivine. Some samples showed small Raman peaks consistent with clinohumite, which was previously interpreted to be contamination from humidity but is more likely to be present as intergrowths in the original olivine.

The minerals Filiberto et al. interpret to be present in their visible to near-infrared reflectance spectra are the same as those identified in the Raman spectra. The reflectance spectra of the unaltered olivine are consistent with pure iron-bearing olivine, with its characteristic broad absorption band centered at 1000 nm and no absorptions from other phases. However, oxidation at 900°C for only 12 minutes changed the visible to near-infrared spectra significantly, unlike Raman spectra, which were dominated by olivine features. With increasing oxidation duration, the olivine absorption around 1000 nm became weaker. Although the Raman spectra show that the crystal is still mostly olivine after oxidation, changes in the reflectance spectra suggest that the alteration process only occurred at the surface, as the penetration depth of Raman spectroscopy is deeper than that of reflectance spectroscopy. With yet longer alteration durations, reflectance spectra became almost flat, consistent with the development of a coating of magnetite on the olivine crystals. After 1 month of oxidation at 900°C, spectral features characteristic of hematite appeared: a shoulder near 700 nm and an absorption near 860 nm. This observation suggests that magnetite forms first during oxidation, followed by a conversion of magnetite to hematite with increasing alteration time scales. After 1 month of oxidation at 900°C, the visible to near-infrared spectra show no features characteristic of olivine (i.e., the broad 1000-nm absorption), even though the bulk sample remained predominantly olivine. 

 

Visible to near-infrared reflectance for unoxidised and oxidized crystals of olivine. (A) Oxidisation at 900°C and (B) oxidisation at 600°C, offset for clarity based on increasing time of oxidation. China-10 was also measured as the unaltered olivine crystal reference for both temperatures. Spectra show a decrease in olivine features with increasing time of oxidation by first becoming relatively featureless and then showing a haematite signature. Filiberto et al. (2020).

The spectra for the 600°C experiments show a similar but less severe flattening. The olivine absorption at 1000 nm weakened but never fully disappeared. This is consistent with the visually observed cloudiness of the olivine and the formation of magnetite/haematite coatings that did not fully enclose the olivine.

The Visible Infrared Thermal Imaging Spectrometer on Venus Express detected the Venus’ surface through three spectral windows at 1.01, 1.10, and 1.18 μm. The results here highlight an important issue for the detection of olivine (and other iron-bearing silicates) in this spectral region. For the 600°C experiments, the 1000-nm olivine band weakened after only 1 month of oxidation, which suggests time scales of several years for it to be completely obscured at Venus surface conditions. In the 900°C experiments, the 1000-nm reflectance band of olivine is entirely absent after 1 month; instead, these experiments show spectral features consistent with magnetite or haematite. The colour of Venus’ surface rock and regolith at the Venera 9 and 10 landing sites is consistent with that of red (pigmentary or nanophase) haematite. Therefore, near-infrared detection of igneous iron-bearing minerals at the Venus’ surface may be dominated by thin coatings of iron-oxide minerals complicating the measurement of primary igneous materials from orbit and challenging efforts to remotely resolve the bulk mineralogy of the Venus surface.

 

An artist's impression of the Russian Venera 9 lander on the surface in 1975. European Space Agency.

To place estimates on the ages of lava flows, previous studies have used the near-infrared windows through the Venus’ atmosphere to investigate variations in emissivity variations. High emissivity (or low reflectance) values are from ferrous iron-bearing igneous minerals (dominantly olivine with pyroxene), whereas ferric iron-bearing alteration minerals (specifically haematite) have lower emissivity (or higher reflectance). On the basis of this emissivity contrast, as well as from radar investigations, recent work has suggested that some lava flows at large volcanoes on Venus are younger than 2.5 million years and possibly even younger than 250 000 years. The large uncertainty in the age estimate from is due to a lack of constraints on alteration rates on Venus and how quickly that alteration affects the near-infrared reflectance and emission. If the inferences of are correct, that unweathered ferrous iron-bearing silicates are responsible for the high-near-infrared emissivities of some lava flows, our results suggest that these high-emissivity lava flows are not millions or even thousands of years old but were emplaced at most a few years before detection. If so, then Venus is volcanically active today because our experimental results show that the emissivity/reflectance signature of olivine should be obscured by oxide coatings within months to years. This active volcanism is consistent with episodic spikes of sulphur dioxide in the atmosphere measured by both the Pioneer Venus Orbiter and the Venus Express, which could have been produced by eruptions that formed young lava flows.

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