Montroydite is a rare mercury oxide. It forms orthorhombic, orange-red to pale yellow, transparent to translucent, long prismatic  or equant crystals with pyramidal terminations often striated. It also occurs as wormlike, tubular, or spherical aggregates consisting of minute prismatic crystals. Montroydite can also be massive or banded, with crystals often bent or twisted. It was named in 1903 After Montroyd Sharp, an owner of the mine at type locality: Terlingua district, Brewster Co., Texas, USA. Other notable localities include: Red Elephant mine, Lake Co. (associated with calomel, mercury, cinnabar, and eglestonite); Clear Creek, San Benito Co.; and near Redwood City, San Mateo Co., California, USA.
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