Boromuscovite

boromuscovite

lepidolite

microcline

topaz

Images

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

Pegmatites

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: quartzlepidolite – Mn – Li – Al – tourmalinepollucite, hambergite, borocookeite, boromuscovite, danburite, light pink beryl);
B: quartzadulariaaxinitelaumontite);
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 - Image

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