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Formula: KAl2(Si3B)O10(OH)2
Phyllosilicate (sheet silicate), dioctahedral mica group,
mica group, boron-bearing mineral
Crystal System: Monoclinic
Specific gravity: 2.81 measured, 2.89 calculated
Hardness: 2½ to 3
Streak: White
Colour: White
Common impurities: Ti,Fe,Mn,Mg,Ca,Li,Na,Cs,P
Environments
Localities
At Řečice, Žďár nad Sázavou District, Vysočina Region, Czech Republic, the deposit is hosted in an
elbaite-subtype
pegmatite carrying abundant
boron-rich minerals: dominant
tourmaline (schorl to
elbaite and rare liddicoatite),
and subordinate tusionite,
boron-bearing polylithionite and
boromuscovite. The pegmatitic unit contains numerous,
randomly distributed pockets lined with crystals of orthoclase,
quartz, two generations of red to pink
elbaite, subordinate albite, minor
polylithionite and two generations of boromuscotive.
Dominant boromuscovite I consistes of a mixture of 83 vol% 2M and 17 vol% 1M polytypes. Boromuscovite II
forms rare overgrowths on larger flakes of boromuscovite I and is slightly Fe-, Mg-enriched. The hydrothermal
fluids that precipitated boromuscovite were relatively depleted in Li, Rb, Cs and F, but were B-rich. high
activity of boron is maintained from the magmatic to the hydrothermal stage, as
indicated from abundant early schorl to late Li-, F- poor
boromuscovite
(EJM 11.4.669–678).
The Sosedka pegmatite vein, Malkhan pegmatite field, Krasnyi Chikoy, Krasnochikoysky District, Zabaykalsky Krai, Russia,
is of concentric-zonal structure; three types of pocket were recognised by
mineral composition:
A: quartz – lepidolite – Mn – Li –
Al – tourmaline (± pollucite,
hambergite, borocookeite,
boromuscovite, danburite, light pink
beryl);
B: quartz – adularia –
axinite (± laumontite);
and
C: quartz and laumontite
(± B-containing cookeite).
Each type of pocket contains feldspars of specific composition and structure.
This indicates that pockets formed in strongly different conditions, though some pockets of different types are
situated as close as 0.5 to 2 m from each other within a zone. The reported data disagree with the common model
implying the formation of zonal pegmatitic bodies as a
result of crystallisation differentiation within the vein
(Russian Geology and Geophysics 53.6.522–534).
Borocookeite, in which Al is replaced by B relative to
cookeite, occurs as a late-stage pocket mineral in the Sosedka and Mokhovaya
pegmatite veins.
Borocookeite, and boron-rich
cookeite, is light grey with a pinkish or yellow hue and occurs as a dense,
massive crypto-flaky 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 (or boromuscovite) 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 than 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 88.830–836).
At the type locality, the Little Three Mine, Ramona, Ramona Mining District, San Diego County, California, USA,
boromuscovite, in which Al is replaced by B relative to muscovite,
occurs as a late-stage, postpocket rupture mineral. It is white to cream coloured and occurs as a porcelaneous veneer
and coating on primary minerals. The average grain size is less
than 4 microns, but the coatings may be as much as 1 cm or more thick. Fragments of
topaz, albite,
elbaite, and other pocket minerals are included in the coating. The
boromuscovite precipitated from a late-stage hydrothermal fluid, and occurs only as a snowlike coating.
Boromuscovite crystallised from the evolved fluid phase present after the growth of the
primary minerals in the pocket. The coexistence of the 2M and 1M
polytypes is consistent with growth in the interval 350 to 400oC
(AM 76.1998-2002).
Associated minerals include lepidolite,
quartz, microcline and
topaz
(HOM).
Boromuscovite from the Little Three Mine -
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