Ankerite

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]

Ankerite
General
CategoryCarbonate mineral
Formula
(repeating unit)		Ca(Fe,Mg,Mn)(CO3)2
Strunz classification5.AB.10
Crystal systemTrigonal
Crystal classRhombohedral (3)
H-M symbol: (3)
Space groupR3
Unit cella = 4.8312(2)
c = 16.1663(3) [Å]; Z = 3
Identification
ColorBrown, yellow, white
Crystal habitChrystals rhombohedral with curved faces; columnar, stalactitic, granular, massive
TwinningSimple t {0001}, {1010}. {1120}
CleavagePerfect on {1011}
FractureSubconchoidal
TenacityBrittle
Mohs scale hardness3.5–4
LusterVitreous to pearly
StreakWhite
DiaphaneityTranslucent to transparent
Specific gravity2.93–3.10
Optical propertiesUniaxial (-)
Refractive indexnω = 1.690 - 1.750 nε = 1.510 - 1.548
Birefringenceδ = 0.180 - 0.202
DispersionStrong
References[1][2][3]

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.

  • Ankerite on pyrite from King County, Washington
  • Ankerite on quartz from Peru

See also

References
  1. Handbook of Mineralogy
  2. Ankerite on Mindat.org
  3. Ankerite on Webmineral
  4.  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 (ed.). Encyclopædia Britannica. 2 (11th ed.). Cambridge University Press. p. 58.
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