Formula: CrO(OH)
Oxide hydroxide mineral, paramorph of bracewellite and grimaldiite, chromium-bearing mineral
Specific gravity: 4.67 calculated
Streak: Yellow-brown, greenish brown
Colour: Reddish brown, golden brown, greenish brown, green
Common impurities: Fe,Al,V

Metamorphic environments


At the Outokumpu mining district, Outokumpu, North Karelia, Finland, Guyanaite occurs in chromium-rich tremolite skarn, metaquartzite, and chlorite veins (HOM).

At the type locality, the Merume river, Kamakusa, Potaro-Siparuni Region, Guyana, guyanaite occurs as microcrystalline aggregates and alluvial grains, and very rarely as prismatic crystals, up to 0.1 mm in size, in an alluvial placer deposit. The guyanaite forms fine-grained intergrowth with other chromium oxide-hydroxides Associated minerals include quartz, pyrophyllite, mcconnellite, grimaldiite, gold, gahnite, eskolaite and bracewellite (Mindat, HOM).

At Moses Rock, San Juan County, Utah, USA, guyanaite has been identified in a xenolith of chromium-rich omphacite from the Moses Rock diatreme in the Navajo Volcanic Field. It occurs as the dominant phase in small clusters of accessory minerals, intergrown with kosmochlor-rich omphacite, zincochromite, eskolaite and carmichaelite. The assemblage is interpreted as the result of metasomatism of chromite-bearing serpentinite by slab-derived fluids during subduction. At the time of entrainment of the xenolith the rock was undergoing prograde metamorphism, with guyanaite dehydrating to eskolaite plus water (AM 99.1277-1283).

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