Ankerite

Ankerite
240px Ankerite on quartz from Peru.
General
Category	Carbonate mineral
Formula (repeating unit)	Ca(Fe,Mg,Mn)(CO3)2
Strunz classification	05.AB.10
Crystal symmetry	Trigonal rhombohedral H-M symbol: (3) Space group: R3
Unit cell	a = 4.8312(2) Å, c = 16.1663(3) Å; Z=3
Identification
Color	Brown, yellow, white
Crystal habit	Chrystals rhombohedral with curved faces; columnar, stalactitic, granular, massive
Crystal system	Trigonal
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

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.

File:Ankerite, Pyrite - Goldmyer Hot Springs, King Co, Washington, USA.jpg

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. 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.

See also

• List of minerals
• List of minerals named after people

References

• Mineral Galleries

 This article incorporates text from a publication now in the public domain: