W is for Wiluite

with grossular

The two large crystals are each about 6 mm long.

Life in the USA is not normal. It feels pointless and trivial to be talking about small looks at the fascinating natural world when the country is being dismantled. But these posts will continue, as a statement of resistance. I hope you continue to enjoy and learn from them. Stand Up For Science!

Originally posted on Facebook in April. The first Sunday in April is Geologists’ Day, first established in the Soviet Union in 1966.

Here’s a Russian specimen with two minerals from their type locality at the Akhtaragda River mouth, Vilyui River Basin, Mirninsky District, Sakha, Russia. My geologist friends will know of grossular garnet but may not be aware that it was first discovered in this remote part of Siberia, about 80 km northwest of Mirny, the nearest large town (home to a big diamond mine, whose pit is seen in the Google image below).

Abraham Gottlob Werner first named grossular kannelstein (German for ‘cinnamon stone’) for orange-brown varieties in 1803, but in 1808 he changed the name to grossular, for the color (the word refers to gooseberry, scientific name Ribes grossularium) of the original light green specimens from Russia.

There’s green grossular on my specimen, but the prominent 6-mm brown crystals are wiluite, a complex silicate isostructural with vesuvianite. Wiluite’s formula, Ca₁₉MgAl₄(Al,Mg)₈(B,◻)₄◻[Si₂O₇]₄[(SiO₄)₁₀]O(O,OH)₉ is barely different from vesuvianite, Ca₁₉Fe³⁺Al₄(Al₆Mg₂)(◻₄)◻[Si₂O₇]₄[(SiO₄)₁₀]O(OH)₉ with the main differences more magnesium vs iron, and essential boron in wiluite, using the formulas given on MinDat. The squares in those formulas represent vacancies in particular crystallographic positions.

Wiluite was named in 1998 for its occurrence along the Vilyui River (also spelled Wilui). When I got the specimen in 1987 (labeled vesuvianite), ‘wiluite’ was well known as a variety of vesuvianite, but it was defined as a new mineral in 1998 (Groat and others, 1998, Wiluite, a new mineral species isostructural with vesuvianite, from the Sakha Republic, Russian Federation: Canadian Mineralogist 36, 1301-1304). I’m sure these crystals are wiluite, even though there was no reason to argue with the label vesuvianite in 1987.

The locality for this specimen is unusual: it’s from a large xenolith (‘strange rock,’ a piece of country rock caught up in an igneous intrusion of molten material), more than 200 meters long, trapped within a gabbro-dolerite sill (a relatively thin igneous body more or less parallel to the beds of rock it intrudes) that was intruded as part of the huge Permian-Triassic volcanic event that produced the Siberian lava flows that may have been a factor in the end-Permian mass extinction about 250 million years ago.

The rocks in the xenolith are Ordovician carbonate rocks that were deposited about 460 million years ago. The molten gabbro that surrounded the 200-meter sliver of limestone and dolomite carbonate rocks dismembered and metamorphosed them, and there was also exchange of chemicals between the carbonates and the gabbro (metasomatism), so these rocks are technically skarns, rocks which have both changed form (metamorphism) and have grown minerals with new chemistry (metasomatism).

But it was even more complicated than that. Apparently the rock here is considered to be a rodingite-like aposkarn. Yeah, I had to look both of those up. Aposkarn means it formed not as the temperature was going up, but after a peak in the sanidinite facies at more than 800°C had been reached (Galuskina and others, 2010, Eringaite, Ca₃Sc₂(SiO₄)₃, a new mineral of the garnet group: Mineralogical Magazine 74: 365-373) and temperatures were decreasing, or an alteration of older skarn by later, cooler hydrothermal waters; it approximately means ‘retrograde skarn.’ And rodingite is essentially a skarn in serpentinized ultramafic rocks (low silica, high magnesium and iron) which form parts of the igneous intrusive body surrounding the carbonate xenoliths. So that part of the skarn is often in the igneous rock rather than the carbonate; I’d have used the word endoskarn for that. The name comes from occurrences along the Roding River, New Zealand.

Whew. I was going to add some things about the tectonics of the Vilyui River basin, because I actually worked on it for oil exploration, but all that jargon and special conditions above seem like enough for one Geologists’ Day. So I’ll just make a cartoon of the igneous sill with its big xenoliths and call it a day.