Specific gravity: 9.16
Hardness: 1 to 2
Streak: Yellowish to brownish white
Colour: Yellow-green, olive-green, reddish brown
Solubility: Soluble in hydrochloric acid
At the type locality, Durango, Mexico, sillénite is a secondary
mineral formed by the oxidation of bismuth-bearing minerals,
bismutite and bismuthinite
(Mindat, Webmin, Dana, HOM).
At Fuka, Okayama prefecture, Japan, sillénite occurs on calcite in a hydrothermal vein in gehlenite-spurrite gneiss (Dana).
At Munihuaza, Mexico, sillénite is associated with shattuckite, ajoite and mottramite variety duhamelite (HOM).
At Monapo, Mozambique, sillénite occurs in a pegmatite (Dana).
Common impurities: Al,Fe
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