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Formula: LiAl4(Si3B)O10(OH)8
Phyllosilicate (sheet silicate), chlorite group,
lithium- and boron- bearing mineral
Crystal System: Monoclinic
Specific gravity: 2.62 measured, 2.69 calculated
Hardness: 3
Streak: Light pinkish grey
Colour: Light grey with a pinkish or yellow hue
Environments
Localities
At the type locality, the Sosedka pegmatite vein, Malkhan pegmatite field, Krasnyi Chikoy, Krasnochikoysky District,
Zabaykalsky Krai, Russia, borocookeite occurs as a late-stage pocket mineral. Borocookeite, as well as
boron-rich cookeite, is light grey
with a pinkish or yellow hue and occurs as a dense, massive aggregate or thin crusts and snow-like coatings on crystals
of quartz, tourmaline and
feldspars from miarolitic cavities. Fragments of
elbaite, danburite and
albite are included in the borocookeite mass. In some pockets, the
coating is composed of borocookeite and boron-rich
muscovite which are not distinguishable visually.
Borocookeite, as well as other boron-rich phyllosilicate minerals,
crystallised from evolved residual solutions in miarolitic cavities at temperatures not less 240oC. The
ratio of activities of K, Li, B, F, and H2O in the mineral-forming fluids of isolated evolving pockets
determined whether borocookeite or boromuscovite formed
separately or together.
(AM 88.5.830-836).
Borocookeite from the Malkhan Pegmatite Field -
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