The mineral known as ankerite is a member of the group of minerals that includes dolomite (calcium-magnesium carbonate), kutnahorite (calcium-manganese carbonate) and ankerite (calcium-iron carbonate). All these minerals share a similar structure and their chemical composition can vary among the three members so that we find ferroan (iron rich) dolomites and magnesian (magnesium rich) ankerites. Ankerite is a rare mineral and is not mined for a specific purpose except as a component of rocks being excavated for crushed stone. Historically, iron ores that formed from the weathering of ankerite rich rocks were once mined and smelted here in Indiana

The mineral ankerite derives its name from Mathias Joseph Anker (1771-1843). He was an Austrian mineralogist

The mineral ankerite is a combination of the elements calcium, iron, carbon and oxygen. Chemically it is a calcium, iron carbonate. The elements magnesium and manganese, among others, may freely substitute for the iron in ankerite and the pure calcium iron carbonate is almost never found in nature. The ideal composition of ankerite is:

CaO (Calcium oxide) 28.01 %

MgO (Magnesium oxide) 10.07 %

FeO (Iron oxide) 17.95 %

CO2 (Carbon dioxide) 43.97 %

The specific gravity of ankerite is 2.97. This means that the pure form of ankerite weighs 2.97 times as much as an equal volume of pure water. The iron content of ankerite gives it a slightly higher specific gravity than dolomite and can be used to distinguish ankerite from dolomite. Ankerite, like dolomite, is between 3.5 and 4 on the Mohs scale of hardness. This means that it is harder than a piece of copper (a penny, for example) and softer than typical mild steel (like a knife blade).

Ankerite is slightly soluble in water and will dissolve in carbonated ground water like limestone (calcite) and dolomite do. Unlike either calcite or dolomite, ankerite will not completely dissolve. The ferrous iron in ankerite oxidizes to ferric iron and precipitates as brown to reddish-brown masses of Alimonite@, or impure iron oxides. Some of these masses of iron oxides were used locally to feed small charcoal iron furnaces many years ago.

Ankerite may be a component of limestones and dolomites in Indiana, but wouldprobably go unrecognized as a part of the gray rocks we see in mines, quarries, road cuts and outcrops. Only where ankerite had weathered to form iron oxides and we see the brown to reddish brown masses and stains in the rocks would there be any clue that ankerite was formerly present. Crystals of ankerite are fairly common in geodes and are usually white to pale brown or tan curved crystals like dolomite. These curved crystals will often be coated by a thin layer of iron oxides so that they take on a brown or reddish-brown color and this helps to distinguish them from dolomite. Figures 1 through 4 show ankerite crystals progressively coated with and replaced by iron oxides.

Ankerite is not used directly as an industrial mineral; however, veins of ankerite are frequently associated with other metals and have been mined extensively as a source of iron in some locations. Veins of ankerite in Austria and Czechoslovakia were exploited for iron ore and also mined along with silver and lead ores for several hundred years. Ankerite in Indiana is often associated with sphalerite (Zinc sulfide) and millerite (Nickel sulfide), but none of the minerals occur in concentrations that are economic to mine.

Ankerite forms distinctive curved crystals like dolomite, but is almost never of a pink color. Most ankerite crystals are an off-white to pale brown to tan color. If ankerite is exposed to water that contains dissolved oxygen, it will develop a coating of iron oxides that can range in color from a brownish to a reddish shade. This is usually a fairly diagnostic feature of ankerite. If you have a mass of crystals that you suspect may be ankerite, a simple test can be performed. Take a small amount of the mineral and crush it to a powder. You can do this by simply using a hammer and hitting the sample a few times to reduce it to powder. Put some of this powder in a container and add vinegar. The powdered sample will dissolve slowly so give it some time to react with the vinegar. After an hour or so, pour a small amount of the vinegar you have dissolved the ankerite in into another container. Slowly add clear household ammonia to the liquid and if the sample is ankerite, you should get a brown cloudy precipitate of iron oxides after you have neutralized the vinegar. Caution: add the ammonia slowly. If you add too much ammonia, the precipitate will dissolve and you will not see the iron oxides. Even simple chemicals like vinegar and ammonia should be used outdoors or with good ventilation.

Ankerite is most often found in geodes in the Mississippian formations of Central and Southwestern Indiana. It can be a minor component of any rock that also contains dolomite. Because of the similarity of ankerite and dolomite, it may go unrecognized in many areas. . If you are interested in collecting specimens of ankerite, then joining a local mineral club or organization such as Friends of Mineralogy is one way to find out about where and how to collect safely. Here are some URL's for mineral organizations:

Friends of Mineralogy, Midwest Chapter http://www.indiana.edu/~minerals/

American Federation of Mineral and Lapidary Societies (Club list) www.amfed.org

Roberts, W.L., T.J. Campbell and G.E. Rapp. 1990. Encyclopedia of Minerals, 2nd Edition. Van Nostrand Reinhold, New York. 979 p.

Pough, F.H.. A field Guide to Rocks and Minerals. Houghton Mifflin, Boston (check for the latest edition)

The Indiana Geological Survey has a web site where more information on minerals, fossils and Indiana Geology may be obtained. Publications and maps may also be ordered.

http://adamite.igs.indiana.edu/

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