Ankerite

ankerite

witherite

dolomite

kutnohorite

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Formula: Ca(Fe2+,Mg)(CO3)2
Anhydrous normal carbonate, dolomite group, forms a series with dolomite and with kutnohorite
Specific gravity: 3.121
Hardness: 3½ to 4
Streak: white
Colour: brown, white to grey, yellowish-brown, greenish
Solubility: Moderately soluble in hydrochloric and sulphuric acid
Environments:

Carbonatites
Hydrothermal environments

Ankerite is a common but not essential constituent of dolostone.

Ankerite is formed in low-grade metamorphosed ironstones and sedimentary banded iron formations and in carbonatites. It can be authigenic, diagenetic, or the product of hydrothermal alteration of carbonate sediments and in hydrothermal sulphide veins. Associated minerals include siderite and dolomite (HOM, Mindat, Webmin).

Localities

The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. ankerite is a secondary carbonate that has been detected associated with ranciéite (AJM 22.1.42).

At the Mount Lyell mines, Queenstown district, West Coast municipality, Tasmania, Australia, dolomite-ankerite crystals to 3 cm occur in veins in the sulphide ore, with chalcopyrite, siderite and quartz crystals (AJM 21.2.24).

At the Telfer gold mine, Western Australia, ankerite crystals occur on quartz in veins in sandstone associated with hematite-goethite pseudomorphs after pyrite, malachite and rare clinoatacamite (AJM 12.1.29).

At the Francon quarry, Montreal, Quebec, Canada, ankerite is an early to late stage mineral in the cavities, often closely associated with siderite. It is commonly observed in cavities containing dawsonite - celestine - goethite - hematite assemblages. Other associated minerals include calcite, dolomite, analcime, fluorite and weloganite, and very rarely also baddeleyite, dachiardite-Na, mordenite and bastnäsite (Minrec 37.1.20).

At the Příbram District, Central Bohemian Region, Czech Republic, ankerite is associated with siderite (Dana).

At Neudorf, Harzgerode mining district, Harz, Saxony-Anhalt, Germany ankerite has been described as crystals to 2 mm dusting quartz and siderite, with calcite(?) and marcasite(?) perched, in turn, on the ankerite crystals (Minrec 43-1.28).

At the Taxco mining district, Taxco de Alarcón, Guerrero, Mexico, ankerite has been reported only from the Hueyapa vein of the San Antonio mine, where it has been found as crystals to 2 mm coating calcite crystals (Minrec 42.5.421).

In a small flat deposit, in the old workings to the west of the Killhope Lead Mining Museum in Upper Weardale, County Durham, England, UK, early minor dolomitisation of the limestone was followed by extensive alteration to ankerite, often containing galena and pyrite, and cavity formation. The cavities were then infilled with some ankerite and other minerals such as galena, sphalerite, pyrite and minor chalcopyrite, with the gangue minerals fluorite, quartz, calcite, baryte, witherite and barytocalcite. Ankerite is extensively altered to limonite (JRS 13.61-63).

At Westernhope Old Mine, Weardale, County Durham, England, UK, ankerite is uncommon. Crystals have not been found, but a few blocks of massive ankerite to 10 cm across have been found associated with, and in some cases encrusted by quartz (JRS 13.67).

At Short Grain, Deer Hills, Caldbeck, Allerdale, Cumbria, England, UK, although no specimens of dolomite or ankerite were found, a few quartz specimens preserve saddle-shaped rhombic casts that are characteristic of these minerals. Limonite pseudomorphs after either dolomite or ankerite are also present (JRS 12.50).

At the Hay Gill Copper Mine, Roughton Gill, Caldbeck, Allerdale, Cumbria, England, UK, several lumps of a carbonate, highly oxidised to goethite, were found on the spoil heaps, usually associated with quartz. Rarely small crystals to 2 mm and pseudomorphed by goethite had formed in small cavities. All this material is considered to have been ankerite. On some specimens quartz had apparently been brecciated and then cemented with the ankerite, indicating that the ankerite had formed later in the paragenesis than the quartz (JRS 14.64-65).

At the Brownley Hill mine, Alston Moor, Eden, Cumbria, England, UK, ankerite occurs with galena crystals on a matrix of galena and limestone, with minor fluorite (AESS).

At the Nentsberry Haggs mine, Alston Moor, Eden, Cumbria, England, UK, ankerite occurs with sphalerite and baryte (AESS).

At the Rampgill mine, Alston Moor, Eden, Cumbria, England, UK, ankerite occurs with sphalerite and gypsum variety selenite (AESS).

At the Smallcleugh mine, Alston Moor, Eden, Cumbria, England, UK, ankerite is very common. Crystals are generally 2 to 3 mm across, and they can often be seen to have grown around sphalerite and pyrite, but both of these minerals also occur on ankerite (JRS 18.20). Ankerite also occurs with galena on quartz here, and with pyrite (AESS).

At the Colorado Mine, Whitlock Mining District, Mariposa county, California, USA, cleavable masses of ankerite occur, rarely associated with crystalline gold (R&M 84.5.407).

At the Eldorado mine, Ouray county, Colorado, ankerite occurred sparingly as microcrystals lining vugs in the crystalline quartz-baryte vein (R&M 84.5.422)

At Antwerp, Jefferson county, New York, USA, ankerite is associated with hematite (Dana).

Alteration

ankerite-dolomite and quartz to augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2
(DHZ 2A p384)

ankerite-dolomite and quartz to diopside-hedenbergite and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2
(DHZ 2A p274)

calcite, Fe2+ and Mg2+ to ankerite and Ca2+
4CaCO3 + Fe2+ + Mg2+ = 2Ca(Mg0.5Fe0.5)(CO3)2+ 2Ca2+
Ankerite is believed to be formed from calcite hydrothermally according to the above reaction (DHZ 5B p228)

Common impurities: Mn

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