Skip to Main Content
GEOREF RECORD

Boromuscovite, a new member of the mica group, from the Little Three Mine pegmatite, Ramona District, San Diego County, California

Eugene E. Foord, Robert F. Martin, Joan J. Fitzpatrick, Joseph E. Taggart and James G. Crock
Boromuscovite, a new member of the mica group, from the Little Three Mine pegmatite, Ramona District, San Diego County, California
American Mineralogist (December 1991) 76 (11-12): 1998-2002

Abstract

Boromuscovite, ideally KAl (sub 2) (Si (sub 3) B)O (sub 10) (OH,F) (sub 2) , in which (super [4]) Al is replaced by B relative to muscovite, occurs as a late-stage, post-pocket rupture mineral within the New Spaulding Pocket, main Little Three pegmatite dyke, Ramona district, San Diego County, California. The mineral is white to cream-coloured and occurs as a porcellanous veneer and coating on primary minerals. The average grain size is < 3-4 mu m, 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; it occurs only as a snow-like coating. The mineral is a mixture of about equal amounts of 2M (sub 1) and 1M polytypes. Refined unit-cell parameters for both polytypes are significantly smaller than those for muscovite: 2M (sub 1) a 5.075(1), b 8.794(4), c 19.815(25) Angstrom, beta 95.59(3) degrees ; 1M a 5.077(1), b 8.775(3), c 10.061(2) Angstrom, beta 101.31(2) degrees . D (sub calc) for the 2 M (sub 1) and 1M polytypes are 2.89 and 2.90 g/cm (super 3) , respectively; D (sub obs) of the mixture is 2.81 g/cm (super 3) . In aggregate, the mineral has H. 2.5-3, white streak, a dull to porcellanous lustre, perfect 001 cleavage, a poor parting on 010, poor to subconchoidal fracture, and no fluorescence or luminescence. Optical properties are (for Na light): biaxial (-), alpha 1.557, beta 1.587, gamma 1.593 (all +- 0.002), 2V (sub alpha) 44(2) degrees, dispersion r > v weak. For the 2M (sub 1) polytype, O.A.P. perpendicular (010), alpha :c = -1 degrees, beta :a = +2 degrees, gamma = b; alpha is in the acute angle of beta and beta is in the obtuse angle. The mineral shows no absorption or pleochroism. Boromuscovite crystallized from the evolved fluid phase present after the growth of the primary minerals in the New Spaulding Pocket. The coexistence of the 2M (sub 1) and 1M polytypes is consistent with growth in the interval 350-400 degrees C, a range consistent with fluid-inclusion geothermometry on pocket minerals in related pegmatitic occurrences.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 76
Serial Issue: 11-12
Title: Boromuscovite, a new member of the mica group, from the Little Three Mine pegmatite, Ramona District, San Diego County, California
Affiliation: U. S. Geol. Surv., Denver, CO, United States
Pages: 1998-2002
Published: 199112
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 20
Accession Number: 1992-013955
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
N32°30'00" - N33°30'00", W117°40'00" - W116°10'00"
Secondary Affiliation: McGill Univ., CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom
Update Code: 1992
Program Name: USGSOPNon-USGS publications with USGS authors
Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal