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Formula: Ca(Fe2+,Mg)(CO3)2
Anhydrous normal carbonate, dolomite group, forms a series with
dolomite and with
kutnohorite
Crystal System: Trigonal
Specific gravity: 2.93 to 3.10 measured, 3.11 calculated
Hardness: 3½ to 4
Streak: white
Colour: brown, white to grey, yellowish-brown, greenish
Solubility: Moderately soluble in hydrochloric and sulphuric acid
Common impurities: Mn
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(Mg
Ankerite is believed to be formed from calcite hydrothermally according to the
above reaction
(DHZ 5B p228)
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