brammallite

{{Infobox mineral | name = Brammallite | category = Phyllosilicate minerals | group = Mica group, dioctahedral mica group | image = | caption = | formula = (Na,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2·(H2O)] | IMAsymbol=Bmr | molweight = | color = White | habit = Earthy clay like | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | twinning = | cleavage = Perfect on {001} | fracture = | tenacity = | mohs = – 3 | luster = Dull, earthy | polish = | refractive = nα = 1.535 – 1.570 nβ = 1.555 – 1.600 nγ = 1.565 – 1.605 | opticalprop = Biaxial (−) 2V: Measured: 5° to 25° | birefringence = δ = 0.030 – 0.035 | dispersion = | pleochroism = | fluorescence= | absorption = | streak = White | gravity = 2.83 – 2.88 | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Translucent | other = | references = }}

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).