Boromuscovite, ideally KAl2(Si3B)O10(OH,F)2, in which [4]Al is replaced by B relative to muscovite, occurs as a late-stage, postpocket mpture mineral within the New Spaulding Pocket, main Little Three pegmatite dike, Ramona district, San Diego County, California. The mineral is white to cream colored and occurs as a porcelaneous veneer and coating on primary minerals. The average grain size is less than 3–4 μm, 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 snowlike coating.

Chemical analysis yielded (wt%): SiO2 48.1, Al2O3 28.1, B2O3 7.0, CaO 0.1, MgO 0.15, Fe2O3 0.1, MnO 0.08, P2O5 <0.05, TiO2 <0.01, K2O 11.0, Na2O <0.05, Li2O 0.05, Rb2O 0.52, Cs2O 0.05, F 0.76, H2O+ 4.55, H2O- 0.22, O = F 0.32, total 100.46. An empirical formula calculated on the basis of 12 O + OH + F is: (–0.89Rb0.02Ca0.01)∑0.92(Al1.93Li0.01-Mg0.01)∑1.95(Si3.06B0.77Al0.17)∑4.00O9.82[(OH)2.02F0.16]∑2.18. The mineral is a mixture of about equal amounts of 2M1, and 1M polytypes. Refined unit-cell parameters for both polytypes are significantly smaller than those for muscovite: 2M1 a = 5.075(1), b = 8.794(4), c = 19.815(25) Å, β = 95.59(3)°; 1M a = 5.077(1), b = 8.775(3), c = 10.061(2) Å, β = 101.31(2)°. Dcalc for the 2M1 and 1M polytypes are 2.89 g/cm3 and 2.90 g,/cm3, respectively. Dobs of the mixture is 2.81 g/cm3.

In aggregate, the mineral has a Mohs hardness of 2.5–3, white streak, a dull to porcelaneous luster, perfect {001} cleavage, a poor parting on {010}, poor to subconchoidal fracture, and no fluorescence or luminescence. Optical properties are (for Na light): biaxial (−), α = 1.557, β = 1.587, γ = 1.593 (all ±0.002), 2Vx meas = 44(2)°, 2Vx calc = 47.5°, dispersion is r > v weak. For the 2M1 polytype, the OAP is normal to (010), X:c = −1°, Y:a = +2°, Z = b; X is in the acute angle of β and Y is in the obtuse angle of β. 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 2M1 and 1M polytypes is consistent with growth in the interval 350–400 °C, a range consistent with fluid-inclusion geothermometry on pocket minerals in related pegmatitic occurrences.

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