Hexagonal tan platy crystal of borocookeite with almost no matrix. Borocookeite is an extremely rare Li-Al borosilicate mineral, known from only two localities worldwide. From: Ze Pinto, Conselheiro Pena, Doce Valley, Minas Gerais, Brazil. — Photo: David Hospital · CC BY-SA 4.0
Borocookeite
A boron-bearing lithium chlorite — a member of the greenish phyllosilicate group common in low-grade metamorphic rocks.
Intro adapted from Wikipedia (CC BY-SA).
Borocookeite is an IMA-recognized mineral species in the silicate class, with the chemical formula LiAl₄(Si₃B)O₁₀(OH)₈.
It crystallizes in the monoclinic crystal system (space group Cc). It is a member of the Chlorite, Clay structural group.
Key properties: Mohs hardness 3; specific gravity 2.62; Light pinkish gray streak; Greasy luster.
IMA symbol Bckt, IMA status: approved, named in 2000.
Found in miarolitic cavities in gem-bearing, zoned, complex, Li-bearing granitic pegmatite.
It is found associated with Elbaite, lepidolite, danburite, boron-rich muscovite, laumontite, quartz, albite.
The prefix indicates the boron-dominant analogue of cookeite.