Brammallite is a sodium rich analogue of illite. First described in 1943 for an occurrence in Llandybie , Carmarthenshire, it was named for British 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 with slightly more silicon , 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.