ANKERITE
Ankerite – Ca(Fe2,Mg,Mn2+,)(CO3)2 – is a relatively rare carbonate mineral. It does not constitute an ore.
It is classified within the dolomite group, and its composition is very similar to that of dolomite, though with the substitution of Mg by varying amounts of Fe and Mn. It forms a solid-solution series with dolomite and another with kutnohorite. Three varieties exist (Mn-bearing, Ni-bearing, and a grayish-white type). It exhibits slight effervescence with cold dilute HCl and strong effervescence with hot dilute HCl.
Caution is advised regarding specimens labeled “ankerite,” as many are actually Fe-rich dolomites (where Mg > Fe). The classification of these carbonates has changed; it is necessary to consult current definitions.
1. Characteristics
Crystal system: Trigonal rhombohedral.
Color: Brown, yellow, yellowish-brown, bronze, greenish, white to gray.
Habit: Crystals typically feature curved faces, resembling a saddle shape – a characteristic also seen in dolomite and calcite. They may also occur in prismatic, tabular, pseudo-octahedral, columnar, or stalactitic forms, or as granular or massive aggregates.
Cleavage: {10-11} perfect.
Tenacity: Brittle.
Twinning: Simple on {0001}, {10-10}, and {11-20}.
Fracture: Subconchoidal, irregular.
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: White.
Lustre: Vitreous, pearly.
Diaphaneity: Transparente.
Density (g/cm³): 2.93 to 3.10
2. Geology and Deposits
Ankerite occurs with siderite in low-grade metamorphic ironstones and also in banded iron formations (BIFs).
It occasionally occurs as a primary igneous mineral in carbonatites (ferrocarbonatites) and other carbonate-rich alkaline igneous rocks. It may occur in sulfide-bearing hydrothermal veins.
Ankerite is one of the products of hydrothermal alteration of carbonate sediments, occurring in a range of dolomite-bearing rocks and in some rocks that have undergone metasomatism.
In dolomitic sediments, it occurs in gravels, sands, and muds as an authigenic (diagenetic) mineral, forming grains, concretionary masses, and veins.
Finally, hydrothermal alteration of mafic and ultramafic igneous rocks can generate dolomite and ankerite associated with serpentine, talc, magnesite, and other Mg-bearing minerals.
3. Mineral Associations
It occurs associated with carbonates such as calcite, dolomite, rhodochrosite, magnesite, and siderite.
It occurs with sulfides such as pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena.
Also with fluorite, barite, and rutile, and silicates (quartz, albite, chlorite, serpentine, talc, and clay minerals).
4. Transmitted Light Microscopy
Refraction indices: nω: 1.690 – 1.750 nε: 1.510 – 1.548
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate.
Cleavage: perfect rhombohedral, like calcite, dolomite, rhodocrosite, siderite and other carbonates.
Habits: Massive, granular, concretionary, prismatic, tabular, pseudo-octahedral.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Birefringence of 0.180 to 0.202: pearly colors of very high orders, up to the 10th order, difficult to recogize and extremely similar to the interference colors of other carbonates.
Extinction: Probably paralell on longitudinal sections and symmetrical in basal sections.
Elongation sign: No information available.
Twins: It may exhibit twins arranged parallel to the longer or shorter diagonal of the rhombus formed by the cleavage.
Zoning: No information available.
CONVERGENT LIGHT
Character: U(-)
2V angle: No.
Alterations: No information available.
May be confused with: Under transmitted light, it is very difficult – or even impossible – to distinguish between calcite, dolomite, siderite, magnesite, ankerite, and other carbonates; the use of other analytical techniques is necessary.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the recommended analytical method for identifying ankerite. However, preparing a polished section is important for identifying the opaque minerals associated with ankerite, such as pyrite, chalcopyrite, pyrrhotite, sphalerite, and galena.
Sample preparation: The characteristics of ankerite under reflected-light microscopy are intermediate between dolomite and siderite.
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: Dolomite and siderite.
Many specimens cataloged as “ankerite” are actually iron-rich dolomites; it is not possible to identify ankerite under reflected light.
General Characteristics: xx