Somewhere in these 500+ postings I noted my purchase of afghanite, a sodalite-related mineral that is rather uncommon in the mineral record. The afghanite was a bright blue specimen, sort of a massive blob from Afghanistan. At the recent Coulee Rock Club show I could not turn down an afghanite specimen displaying two blue crystals. In addition, my quest for blue minerals was helped along by purchasing a nifty specimen of lazurite from the same locality in Afghanistan.
Afghanite is a rare, blue to colorless, but often beautiful sapphire blue, feldspathoid mineral (group of alkali aluminosilicate minerals similar to feldspars but with lower silica content) with a complex chemical composition [(Na,K)22Ca10(Si24Al24O96)(SO4)6Cl6]. It has a vitreous luster, is translucent to transparent and has a “medium” hardness of ~6 (Mohs). Interestingly it has a white streak when rubbed on rough porcelain (streak plate) and is colorless when observed in a thin section under a polarizing microscope. Although crystals may be faceted into beautiful gems, specimens of gem quality are quite rare as most crystals are brittle with noticeable fractures and are small. Many specimens of afghanite have a massive habit and collectors look for nice hexagonal crystals with steep terminations on terminal prisms.
Afghanite was discovered (at least as a mineral different from the famous Lapis) in the 1960s in the Sar-e-Sang district of Badakhshan Province in Afghanistan, therefore receiving its name. Afghanite is primarily found in Afghanistan; however it is also known from the U,S, (St. Lawrence County, NY), Siberia, Russia, Germany, Tajikistan, Italy, and Canada but most specimens on the market come from the Type Locality where calcite-rich sedimentary rocks interact with intrusive igneous magma in a high temperature contact metamorphic environment producing marble hosting unique feldspathoid minerals (afghanite, lazurite, sodalite, hauyne, etc.); a skarn environment producing perhaps the best lapis lazuli in the world.
Another common blue mineral at Sar-e-Sang is lazurite, an aquamarine to midnight blue member of the Sodalite Group and Felspathoid Family. It has a resinous luster, is opaque, has a bright blue streak (hauyne has white), and is quite brittle. It is closely related to afghanite described above and its chemical formula has been debated and changed several times, such as: its blue color was created by lapis lazuli (actually a rock); is a sulfide dominant analogue of hauyne; a sulfide-bearing variety of hauyne; but in 2021 Sapozhnikov and others redefined lazurite as Na7Ca(Al6Si6O24)(SO4)(S3) · H2O with at least a quarter of the sodalite cages (the molecular framework) filled with S3-1. Hauyne (Na3Ca(Si3Al3)O12(SO4)) has about three quarters of the sodalite cages filled.
This paragraph says to me: 1) those rockhounds who understand are ways above my pay grade; 2) it would seem almost impossible for an ole plugger like me to identify specimens without calling for some work by an electric gizmo like an XRD; 3) right now I will rely on identifications by the dealers and/or the minerals listed at the collecting localities while remembering the warning on MinDat, As such, most of the pictures showing "lazurite" do not show true lazurite. It is a complicated mineral to understand, and I suggest further searching on MinDat. And BTW, compare lazulite with lazurite :)
REFERENCES CITED
Sapozhnikov, A. N., Tauson, V. L, Lipko, S.V., Shendrik, R.Y., Levitskii, V. I., Suvorova, L. F., Chukanov, N.V., Vigasina, M. F., 2021, On the crystal chemistry of sulfur-rich lazurite, ideally Na7Ca(Al6Si6O24)(SO4)(S3)–·nH2O: American Mineralogist, Vol. 106, No. 2.
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