Ankerite

Ankerite
General
Category Carbonate mineral
Formula
(repeating unit)		 Ca(Fe,Mg,Mn)(CO3)2
Strunz classification 5.AB.10
Crystal system Trigonal
Crystal class Rhombohedral (3)
H-M symbol: (3)
Space group R3
Unit cell a = 4.8312(2)
c = 16.1663(3) [Å]; Z = 3
Identification
Color Brown, yellow, white
Crystal habit Chrystals rhombohedral with curved faces; columnar, stalactitic, granular, massive
Twinning

Simple t

{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. It forms a series with dolomite and kutnohorite.[2]

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.[4]

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.[4]

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]

It has been found in Western Tasmania, in mines in Dundas, Tasmania.

See also

References

  1. 1 2 Handbook of Mineralogy
  2. 1 2 3 Ankerite on Mindat.org
  3. ↑ Ankerite on Webmineral
  4. 1 2  One or more of the preceding sentences incorporates text from a publication now in the public domain: Spencer, Leonard James (1911). "Ankerite". In Chisholm, Hugh. Encyclopædia Britannica. 2 (11th ed.). Cambridge University Press. p. 58.
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