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Ankérite Quartz Pérou.jpg
Ankerite on quartz from Peru.
Category Carbonate mineral
(repeating unit)
Strunz classification 5.AB.10
Crystal system Trigonal
Rhombohedral class
Space group Trigonal
H-M symbol: (3)
Space group: R3
Unit cell a = 4.8312(2)
c = 16.1663(3) [Å]; Z = 3
Color Brown, yellow, white
Crystal habit Chrystals rhombohedral with curved faces; columnar, stalactitic, granular, massive
Twinning Simple twins on {0001}, {1010}. {1120}
Cleavage Perfect on {1011}
Fracture Subconchoidal
Tenacity Brittle
Mohs scale hardness 3.5–4
Luster Vitreous to pearly
Streak White
Diaphaneity Translucent to transparent
Specific gravity 2.93–3.10
Optical properties Uniaxial (-)
Refractive index nω = 1.690 - 1.750 nε = 1.510 - 1.548
Birefringence δ = 0.180 - 0.202
Dispersion Strong
References [1][2][3]

Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates with formula: Ca(Fe,Mg,Mn)(CO3)2. In composition it is closely related to dolomite, but differs from this in having magnesium replaced by varying amounts of iron(II) and manganese. Forms series with dolomite and kutnohorite.[2]

Ankerite on pyrite from King County, Washington

The crystallographic and physical characters resemble those of dolomite and siderite. The angle between the perfect rhombohedral cleavages is 73° 48', the hardness is 3.5 to 4, and the specific gravity is 2.9 to 3.1. The color is white, grey or reddish to yellowish brown.

Ankerite occurs with siderite in metamorphosed ironstones and sedimentary banded iron formations. It also occurs in carbonatites. In sediments it occurs as authigenic, diagenetic minerals and as a product of hydrothermal deposition.[1] It is one of the minerals of the dolomite-siderite series, to which the terms brown-spar, pearl-spar and bitter-spar have been historically loosely applied.

It was first recognized as a distinct species by W. von Haidinger in 1825, and named for Matthias Joseph Anker (1771–1843) of Styria, an Austrian mineralogist.[2]

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 This article incorporates text from a publication now in the public domainChisholm, Hugh, ed. (1911). "Ankerite". Encyclopædia Britannica (11th ed.). Cambridge University Press. 

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