Alstonite

minerals

ankerite

witherite

sphalerite

Formula: BaCa(CO3)2
Anhydrous normal carbonate, paramorph of barytocalcite and paralstonite
Specific gravity: 3.67 to 3.711
Hardness: 4 to 4½
Streak: White
Colour: Colourless to snow white, yellow-gray; pale gray, pale cream, pink to pale rose-red
Solubility: Soluble in dilute hydrochloric acid
Environments

Carbonatites rarely
Hydrothermal environments

Alstonite typically occurs in low-temperature hydrothermal lead-zinc deposits; it is rare in carbonatites. Associated minerals include calcite, baryte, ankerite, siderite, benstonite, galena, sphalerite, pyrite and quartz (HOM, Mindat).

Localities

At the New Brancepeth Colliery, Deerness Valley, County Durham, England, UK, a few speciments of alstonite have been collected from the baryte - witherite vein (SY p124).

There are two co-type localities, the Brownley Hill Mine, Nenthead, Alston Moor, Eden, Cumbria, England, UK, and the Fallowfield Mine, Acomb, Northumberland, England, UK.
At Brownley hill alstonite occurs in low temperature hydrothermal deposits associated with calcite, baryte and witherite (Dana, SY p125).
At Fallowfield alstonite is usually associated with witherite pyramids in the Great Limestone. At lower levels of the vein structure, cavities in sandstone host an assemblage of late-stage primary minerals, including alstonite to 2.5 cm. Even lower down the vein crosses shale, then enters sandstone again where alstonite crystals to 1.5 cm occur with calcite and ankerite (Minrec 31.3.240-242).

At the Nentsberry Haggs Mine, Alston Moor, Eden, Cumbria, England, UK, crystals of alstonite occur partly or completely altered to a calcite-baryte crust. These occur on sphalerite, quartz, baryte and broken surfaces of wall rock (JRS 17.42).
Baryte and calcite pseudomorphs of alstonite have been found here (SY p124).

At the Ozark-Mahoning Mine, Ozark-Mahoning group, Cave-in-Rock, Hardin county, Illinois, USA, tiny alstonite crystals are found with benstonite and more commonly with witherite and azurite. Microscopic grains of alstonite with malachite have been found only on the dumps (R&M 85.3.211).

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