Brammallite

Phyllosilicate mineral in the dioctahedral mica group

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Brammallite
General
Category	Phyllosilicate minerals
Group	Mica group, dioctahedral mica group
Formula	(Na,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂·(H₂O)]
IMA symbol	Bmr^[1]
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Identification
Color	White
Crystal habit	Earthy clay like
Cleavage	Perfect on {001}
Mohs scale hardness	2+1⁄2 – 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	^[2]^[3]

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

Believed to be a degradation product of paragonite through the process of sodium leaching, though can also occur when muscovite, or other aluminous materials are altered^[4]. Just as illite (whose mineral series it is in) it is a non-expanding, non-plastic, 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.

Similar to illite, and of rare use, it can be used as a flux in high fire ceramic bodies due to its high insoluble sodium content, but unlike common sodium-containing fluxes (i.e. feldspar) deposits of it have the advantage of its extremely small clay-like particles that remove the need for grinding, but unlike other clear minerals it does not expand or develop plasticity/shrinkage with water. Brammallite shares all of these ceramic properties with illite, but these two minerals differ in their sodium and potassium contents.