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
gollark: No, I don't have 6727.
gollark: 8148, now.
gollark: No, I removed those last week, they were cluttering my computer.
gollark: Well, it would DISprove it if there was a small counterexample, but there may not be.
gollark: IT'S GOLLARIOUS, YOU :BEES: :BISMUTH1: :SMALL_STELLATED_DODECAHEDRON:.

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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