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Brammallite is a sodium-rich analogue of illite. First described in 1943 for an occurrence in Llandybie, Wales, it was named for United Kingdom 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. Just as illite 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, it can be used as a ceramic flux in high fire ceramic bodies due to its high insoluble sodium content, but unlike common sodium-containing fluxes (i.e. feldspar) it has the advantages of its extremely small clay-sized particles that necessitate the need for grinding yet has the unique feature of being a clay but not expanding or becoming plastic with water. Brammallite shares all of these physical properties with illite, but these two substances differ in their sodium and potassium contents.
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