Brammallite

Brammallite is a sodium rich analogue of illite. First described in 1943 for an occurrence in Llandybie, Carmarthenshire, Wales, it was named for British geologist and mineralogist Alfred Brammall (1879–1954).

Brammallite
General
CategoryPhyllosilicate
Formula
(repeating unit)
(Na,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2·(H2O)]
IMA symbolBmr[1]
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Identification
ColorWhite
Crystal habitEarthy clay like
CleavagePerfect on {001}
Mohs scale hardness2+12 - 3
LusterDull, earthy
StreakWhite
DiaphaneityTranslucent
Specific gravity2.83 - 2.88
Optical propertiesBiaxial (-) 2V: Measured: 5° to 25°
Refractive indexnα = 1.535 - 1.570 nβ = 1.555 - 1.600 nγ = 1.565 - 1.605
Birefringenceδ = 0.030 - 0.035
References[2][3]

Believed to be a degradation product of paragonite, like illite it is a non-expanding, clay-sized, micaceous mineral. Brammallite is a phyllosilicate or layered silicate. Structurally, brammallite is quite similar to muscovite or sericite with slightly more silicon, magnesium, iron, and water and slightly less tetrahedral aluminium and interlayer potassium.

It occurs as aggregates of small monoclinic white crystals. Due to the small size, positive identification usually requires x-ray diffraction analysis.

References


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