Phyllosilicate (sheet silicate), gillespite group
Specific gravity: 3.33
Solubility: Readily decomposed by hydrochloric acid
Common impurities: Al,Mn
At the Gun claim, Wilson Lake, Itsi Mountain, Watson Lake mining district, Yukon, Canada, gillespite is associated with pellyite, sanbornite, taramellite, fresnoite, muirite and baryte (HOM).
At the Upper Ross River, Yukon, Canada, gillespite occurs as a granular aggregate of anhedral crystals to 2 mm. Small rough grains containing taramellite are embedded in the gillespite (AM 69.358).
At the Madrelena mine, Tres Pozos, Baja California, Mexico, La Madralena mine gillespite occurs as elongated aggregates surrounded by sanbornite (AM 69.358).
At the type locality, Dry Delta, Alaska Range, Yukon-Koyukuk Census Area, Alaska, USA, gillespite occurs in a glacial moraine associated with celsian, hedenbergite and quartz (HOM).
At Big Creek, Rush Creek deposit, Big Creek-Rush Creek Mining District, Fresno county, California, USA, gillespite is most prominent in some quartz-rich rocks with or without sanbornite. Some of this gillespite, the coarsest known, appears as ragged crystals with conspicuous cleavage up to several inches in size (AM 69.358).
At the Trumbull Peak barium-silicate deposit, Trumbull Peak, Clearing House, Clearing House Mining District, East Belt, Mariposa county, California, USA, gillespite probably forms in a contact metamorphic zone. Associated minerals include sanbornite, celsian, diopside, tourmaline, quartz and pyrrhotite (HOM, AM 28.372-390).
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