Gatewayite

gatewayite

vanarsite

packratite

morrisonite

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Formula: Ca6(As3+V4+3V5+9As5+6O51).31H2O
Polyoxometalate mineral, arsenic- and vanadium- bearing mineral.
A polyoxometalate is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks (Wiki).
Crystal System: Monoclinic
Specific gravity: 2.34 measured, 2.337 calculated
Hardness: 2
Streak: Greyish blue
Colour: Very dark greenish blue
Luminescence: Nonfluorescent under UV
Solubility: Insoluble in water at room temperature, easily soluble in dilute hydrochloric acid at room temperature
Environments

Sedimentary environments
Hydrothermal environments

Gatewayite is a new mineral, approved in 2015 and to date (November 2022) reported only from the type locality.

Localities

At the type locality, the Packrat mine, Gateway, Mesa county, Colorado, USA, vanarsite, packratite, morrisonite and gatewayite were found in the main tunnel level. The Packrat mine is in the Uravan Mineral Belt, in which uranium and vanadium minerals occur together on montroseite- and corvusite-bearing sandstone. They are also closely associated with pharmacolite and a potentially new vanadate mineral. Other secondary minerals found in the mine include andersonite, ansermetite, calcite, dickthomssenite, gypsum, hewettite, hummerite, lasalite, magnesiopascoite, martyite, munirite, navajoite, pascoite, picropharmacolite, postite, rossite, native selenium, sherwoodite and uranopilite.
Gatewayite forms from the oxidation of montroseite-corvusite assemblages in a moist environment. Mining operations have exposed unoxidised and oxidised phases. Under ambient temperatures and generally oxidising near-surface conditions, water reacts with pyrite and an unknown arsenic-bearing phase (perhaps arsenopyrite) to form aqueous solutions with relatively low pH (acid). The various secondary vanadate phases that form depend upon prevailing conditions and the presence of other cations such as Na+, Ca2+ and Mg2+.
Gatewayite blades are up to 0.5 mm long and form divergent intergrowths; the mineral also occurs as crude prisms, up to 1 mm long, with rounded faces, and as composite crystals consisting of subparallel intergrowths of narrow prisms up to 2 mm long (CM 54.1.145-162).

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