| Brammallite | |
|---|---|
| General | |
| Category | Silicate mineral |
| Chemical formula | (Na,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2·(H2O)] |
| Identification | |
| 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-?).
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.