Formula: Pb2Fe3+(Te6+O6)Cl
Crystal System: Trigonal
Specific gravity: 5.812 calculated for the empirical formula
Hardness: 2
Streak: Pale yellow
Colour: Yellow, tending to reddish orange

Hydrothermal environments

Müllerite is a relatively new mineral, approved in 2019.


There are three co-type localities, the Aga Mine, the Bird Nest drift and Bird Nest E3, all on Otto Mountain, Baker, Soda Mountains, Silver Lake Mining District, San Bernardino county, California, USA. The Otto Mountain massif contains the world’s most extensive assemblage of tellurium oxysalt (a salt containing oxygen as well as a given anion) minerals, as well as being the type locality for 16 minerals (as of Februry 2024): agaite, andychristyite, bairdite, chromschieffelinite, eckhardite, fuettererite, hagstromite, housleyite, markcooperite, mojaveite, müllerite, ottoite, paratimroseite, telluroperite, thorneite and timroseite.
Although it has been found in most of the collecting areas on Otto Mountain, müllerite is relatively rare. It occurs on recrystallised quartz fracture surfaces and in small vugs in association with acanthite, cerussite, hematite, khinite, phosphohedyphane, thorneite and timroseite. Müllerite is a secondary oxidation zone mineral and is presumed to have formed by oxidation of earlier formed tellurides, most likely hessite, with chalcopyrite and galena.
Müllerite occurs as hexagonal tablets and thin plates up to 0.2 mm across, but usually much smaller. Crystals may occur as isolated tablets, intergrown ball-like clusters or scattered flakes. They are yellow, tending to reddish orange, and have a pale yellow streak and subadamantine to greasy lustre (CM 58.4.413-419).

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