Brammallite

Brammallite is a sodium rich analogue illite. First found in 1943 in Llandybie, Carmarthenshire, Wales, it is named after British geologist and mineralogist Alfred Brammall (1879-?).

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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.

The chemical formula is given as (Na,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)].^[1]

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

See also

• Illite
• Paragonite

References

1. ^ http://www.mindat.org/min-816.html