| Brammallite | |
|---|---|
| General | |
| Category | Phyllosilicate |
| Formula (repeating unit) | (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 Na 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 flux in high fire ceramic bodies due to its high insoluble sodium content, but unlike common Na 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. All of which it shares with illite, with the difference being their contents of corresponding sodium or potassium amounts.