35 Facts About Ankerite

What is Ankerite?

Ankerite is a unique mineral that belongs to the rhombohedral carbonate group. Its chemical composition and physical properties make it a subject of interest for geologists and mineralogists alike. Let's dive into the fascinating world of ankerite with these 35 facts.

1. Chemical Formula
   Ankerite's chemical formula is Ca(Fe,Mg,Mn)(CO3)2, indicating it contains calcium, iron, magnesium, and manganese.

2. Crystal System
   It crystallizes in the trigonal crystal system, also known as the rhombohedral system.

3. Crystal Class
   The crystal class of ankerite is rhombohedral (3), with a space group of R3.

4. Unit Cell Parameters
   The unit cell parameters for ankerite are a = 4.8312(2) Å and c = 16.1663(3) Å, with a unit cell volume of approximately 741.6 ų.

Where is Ankerite Found?

Ankerite can be found in various geological settings, often associated with other minerals. Its occurrence is widespread but specific to certain environments.

5. Occurrence
   Commonly found in medium and low-temperature hydrothermal veins, ankerite is often associated with dolomite, calcite, siderite, and various sulphides.

6. Composition
   Ankerite is closely related to dolomite but differs by having magnesium replaced by varying amounts of iron(II) and manganese. It forms a series with dolomite and kutnohorite.

7. Occurrence in Sediments
   It occurs in sediments as authigenic, diagenetic minerals and as a product of hydrothermal deposition. It is also found in low-grade metamorphosed ironstones and sedimentary banded iron formations (BIF).

8. Occurrence in Carbonatites
   Ankerite is one of the minerals found in carbonatites, which are igneous rocks rich in carbonate minerals.

Physical Properties of Ankerite

Ankerite exhibits a range of physical properties that make it identifiable and distinguishable from other minerals.

9. Hardness
   It has a hardness of 3.5 to 4 on the Mohs scale.

10. Specific Gravity
    The specific gravity ranges from 2.93 to 3.10 g/cm³.

11. Cleavage
    Ankerite has perfect cleavage on {1011}.

12. Color
    The color can vary from white, grey, brown, yellow, or reddish to yellowish brown. It often changes slowly when exposed to surface conditions, turning from pearly white to brown or rusty.

13. Luster
    Ankerite has a vitreous to pearly luster.

14. Streak
    The streak of ankerite is white.

15. Transparency
    Typically translucent to transparent in thin pieces, although it can be opaque in thicker specimens.

16. Fracture
    The fracture is subconchoidal to hackly, and the mineral is brittle.

Crystal Habits and Associations

Ankerite can form various crystal habits and is often found alongside other minerals.

17. Twinning
    It often exhibits twinning, particularly on {0001}, {1010}, and {1120} planes.

18. Crystal Habit
    Ankerite can form rhombohedral crystals with curved faces, columnar, stalactitic, granular, and massive forms.

19. Associated Minerals
    Often associated with dolomite, siderite, baryte, quartz, fluorite, and ore minerals like tetrahedrite, chalcopyrite, and pyrite.

Industrial and Historical Significance

Though not widely used industrially, ankerite has historical significance and specific applications.

20. Industrial Uses
    Limited industrial use, primarily in special construction or refractory materials. It was also used as a poor quality iron ore in countries lacking this metal or during difficult periods in history.

21. Historical Significance
    First recognized as a distinct species by Wilhelm von Haidinger in 1825 and named after Matthias Joseph Anker, an Austrian mineralogist.

Geological and Structural Insights

Understanding the geological and structural aspects of ankerite provides deeper insights into its formation and stability.

22. Geological Settings
    Forms in various geological settings, including hydrothermal veins, metasomatic rocks, and low-temperature alteration of carbonates.

23. Crystal Structure
    Similar to dolomite and calcite, consisting of alternating layers of cations and [CO3] carbonate groups. The accommodation of metal atoms of different sizes leads to a small rotation of the CO3 groups around the threefold axis compared to calcite.

24. Compressibility
    Studies show that iron-rich ankerite exhibits a high degree of compressibility, essential for understanding its phase stability under different conditions.

25. Phase Stability
    DFT calculations indicate that pure CaFe(CO3)2 ankerite has never been found naturally or synthesized in a laboratory.

26. Rietveld Refinement
    Rietveld refinement of the ambient-conditions powder XRD pattern of iron-rich ankerite suggests specific atomic coordinates that align with calculated values and other literature results.

Optical and Chemical Properties

Ankerite's optical and chemical properties further distinguish it from other minerals.

27. Optical Properties
    Exhibits uniaxial optical properties, with refractive indices ranging from nω = 1.690 to 1.750 and nε = 1.510 to 1.548. The birefringence is moderate, with a maximum value of δ = 0.180 to 0.202.

28. Dispersion
    Shows strong dispersion, a characteristic feature of many carbonate minerals.

29. Fluorescence
    Exhibits weak fluorescence, typically orange in color.

30. Solubility
    Soluble in acids, common among carbonate minerals.

31. Magnetism and Radioactivity
    Not magnetic and does not exhibit radioactivity.

Formation and Occurrence in Specific Environments

Ankerite forms in specific environments, often through unique geological processes.

32. Formation in Hydrothermal Veins
    Forms in hydrothermal veins through the low-temperature alteration of carbonates. Often associated with dolomite, calcite, and siderite.

33. Formation in Metasomatic Rocks
    Can also form in metasomatic rocks, which have undergone chemical alteration due to interaction with fluids.

34. Occurrence in Precambrian Iron Formations
    A dominant carbonate in Precambrian iron formations, particularly in the "algoma" type ores. These formations are rich in iron and characterized by banded ironstones.

35. Occurrence in Saliferous Marls
    Found in saliferous marls, sedimentary rocks rich in salt and other minerals. Often associated with dolomite in these deposits.

Ankerite's Unique Role in Geology

Ankerite stands out as a unique mineral with its blend of calcium, iron, magnesium, and manganese carbonates. Found in hydrothermal veins, metasomatic rocks, and Precambrian iron formations, it showcases a variety of colors from white to reddish brown. Its trigonal crystal system and rhombohedral class give it distinct physical properties like a hardness of 3.5 to 4 on the Mohs scale and a specific gravity of 2.93 to 3.10 g/cm³. Though not widely used industrially, ankerite's historical significance as a poor-quality iron ore can't be overlooked. Its compressibility and phase stability are crucial for understanding geological processes. Whether found in carbonatites or saliferous marls, ankerite's presence helps geologists piece together Earth's history. This mineral, often mistaken for ferroan dolomite, remains a key player in the study of mineralogy and geology.