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Title: Brammallite  
Author: World Heritage Encyclopedia
Language: English
Subject: List of minerals, Llandybie, Illite, Cefn Coed Colliery Museum
Publisher: World Heritage Encyclopedia


Category Phyllosilicate
(repeating unit)
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.


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