Protocaseyite

protocaseyite

ammoniozippeite

gypsum

postite

Images

Formula: [Al₄(OH)₆(H₂O)₁₂][V₁₀O₂₈].8H₂O
Hydrated decavanadate
Crystal system: Triclinic
Specific gravity: 2.45 measured, 2.448 calculated
Hardness: 2
Streak: Orange-yellow
Colour: Saffron-yellow
Luminescence: No fluorescence under UV
Solubility: Insoluble in water, soluble in dilute hydrochloric acid
Environments

Sedimentary environments
Hydrothermal environments

Protocaseyite is a new mineral, approved in 2021 and to date (June 2025) reported only from the type locality.

Localities

At the type locality, the Burro Mine, San Miguel County, Colorado, USA, protocaseyite occurs in low-temperature, post-mining, secondary mineral assemblages, formed on montroseite- and corvusite- bearing sandstone by oxidation in a moist environment. Associated minerals include ammoniozippeite, gypsum and postite.
Crystals of protocaseyite are saffron-yellow, thick blades, with a pale orange-yellow streak, vitreous lustre, brittle tenacity, curved fracture and two very good cleavages. The crystal structure contains both the [V₁₀O₂₈]⁶⁻ decavanadate polyoxoanion and a novel [Al₄(OH)₆(H₂O)₁₂]⁶⁺ polyoxocation (AM 107.6.1181-1189, HOM).

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