Formula: BaCa(CO3)2
Anhydrous normal carbonate, paramorph of
barytocalcite and
paralstonite, barium-bearing
mineral
Crystal System: Triclinic
Specific gravity: 3.67 to 3.711 measured, 3.67 calculated
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 Smallcleugh Mine, Nenthead, Alston Moor, Eden, Cumbria, England, UK, a specimen has been found with
baryte pseudomorphing a 4mm white
alstonite crystal perched on a larger shiny black
sphalerite crystal on a quartz matrix
(AESS).
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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