Brammallite

Brammallite
General
Category	Phyllosilicate
Formula (repeating unit)	(Na,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2·(H2O)]
Identification
Color	White
Crystal habit	Earthy clay like
Crystal system	Monoclinic prismatic 2/m
Cleavage	Perfect on {001}
Mohs scale hardness	2½ - 3
Luster	Dull, earthy
Streak	White
Diaphaneity	Translucent
Specific gravity	2.83 - 2.88
Optical properties	Biaxial (-) 2V: Measured: 5° to 25°
Refractive index	nα = 1.535 - 1.570 nβ = 1.555 - 1.600 nγ = 1.565 - 1.605
Birefringence	δ = 0.030 - 0.035
References	[1][2]

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).

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

1. http://www.mindat.org/min-816.html Brammallite at Mindata.org
2. http://webmineral.com/data/Brammallite.shtml WebMineral