Autunite

minerals

torbernite

montmorillonite

uraninite

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Formula: Ca(UO2)2(PO4)2.10-12H2
Hydrated normal phosphate
Epitaxial mineral: torbernite
Specific gravity: 3.2
Hardness: 2 to 2½
Streak: White to yellowish
Colour: Yellow, due to the uranyl ion UO2+2 (Mindat)
Solubility: Autunite is moderately soluble in hydrochloric, sulphuric and nitric acid
RADIOACTIVE
Environments:

Pegmatites
Hydrothermal environments

Autunite is a secondary mineral found chiefly in the zone of oxidation and weathering derived from the alteration of uraninite or other uranium minerals in hypothermal (high temperature) veins. Associated minerals include meta-autunite, torbernite, phosphuranylite, saléeite, uranophane, uranophane-β and sabugalite (HOM).

Localities

At the type locality, Saint-Symphorien-de-Marmagne, Montcenis, Saône-et-Loire, Bourgogne-Franche-Comté, France, autunite occurs in a granite pegmatite (Mindat).

At Hagendorf and Pleystein, Bavaria, Germany autunite occurs in pegmatites (Dana).

At the Apex Mine, Lander county, Nevada, USA, crystals of autunite to 3 mm have been found covering the surfaces of quartzite/phyllite breccias, and small tablets to 2 mm occur locally within open stress fractures in monzonite. In the finest specimens freestanding crystals to 3 mm occur in brittle and heavily fractured quartzite clasts caught up locally in fault breccias. Rarely, two generations of autunite crystallization have been noted in these clasts. Noteworthy epitactic overgrowths of autunite on torbernite/ metatorbernite have also been noted in a number of specimens (R&M 87.3.270-276)

At the quarry of the New Hampshire Mica and Mining Company in Alstead, New Hampshire, USA, autunite has been found in small micaceous flakes lying between plates of albite variety cleavelandite (AM 13.578-579).

Near Mount Spokane, Spokane county, Washington, USA, autunite is developed in vugs, fractures, and shear zones in granitic rock. Analyses of the autunite suggest discrete water losses at about 90oC, 145oC, and 220oC. The first water loss probably represents dehydration to meta-autunite I. The dehydrations at about 145oC and 220oC may involve interlayer water as in montmorillonite. Unless a saturated atmosphere is maintained, fully hydrated autunite quickly reverts to meta-autunite I, which is the most stable form under ordinary conditions of temperature and humidity. Irreversible dehydration from meta-autunite I to meta-autunite II takes place at about 80oC (AM 45.99-128).

Common impurities: Ba,Mg

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