Kutnohorite from Wessels and N’Chwanning mines (KMF). Specimens and photos by – J de Jongh
Crystal system:                Trigonal                                 Hardness:            3.5 - 4
Density:                               3.11 average                         Cleavage:            perfect
Streak:                                 white                                   
Colour:                                 white, light brown, light yellow, light pink.
Occurrence:                      Manganiferous sediments.
Habitat:                              Massive, granular, common texture observed in granite and other igneous rock. 
                                             Forms aggregates of bundled bladed crystals.
Composition:                    carbonate                                                                         Ca(Mn2+, Mg, Fe2+)(CO3)2
One would think that for a beautiful mineral such as kutnohorite there would be abundant information available in books and on the internet. This was not the case with the Mineral of the Month and I actually struggled to find enough information to write this article. I think the reason for this is two-fold:  firstly kutnohorite can be classified as a fairly “new” mineral and secondly it so closely resembles dolomite. The latter will be explained in more detail below. 
Kutnohorite is a rare calcium manganese carbonate mineral with magnesium and iron, which is a member of the dolomite group. It forms a series with dolomite and with ankerite. The end member formula is CaMn2+(Co3)2, but magnesium Mg and iron Fe2+ commonly substitute for manganese Mn2+, with the Mn content varying from 38% to 84%, so the formula Ca(Mn2+, Mg, Fe2+)(CO3)2 better represents the species. Therefore, when a large amount of iron is present in dolomite, the mineral ankerite forms and when excess manganese is present kutnohorite is formed. All the above mentioned crystals have the same internal structure, but differ chemically from each other. This is known as an isomorphous series of minerals.

Kutnohorite was named by Professor Bukowsky in 1901 after the type locality of Kutna Hora, Bohemia, in the Czech Republic. It was originally spelt “kutnahorite” but “kutnohorite is the current IMA-approved spelling.

Rigorous proof has been lacking that this mineral belongs in the dolomite structure type, AB(CO3)2 and was not merely a calcite-type solid solution, with the formula (Ca, Mn, Mg, Fe)(CO3)2, that fortuitously approached the dolomite ratios. It was only years later, after a differential thermal analysis and x-ray study was done on a specimen from its type locality in the Czech Republic, that it was scientifically established that kutnohorite is in fact of the dolomite type.

I could not find uses for kutnohorite specifically, but the dolomite group it belongs to is used in the manufacturing certain types of refractory bricks used in steel making. The magnesium and calcium oxides have very high melting points and are an excellent, inexpensive refractory material. Dolomite is also used as a source of magnesium oxide for making magnesium metal and for chemical uses, such as the common laxative milk-of-magnesia. Dolomite is also used in some cements, as source of magnesium. Of course dolomite and kutnohorite are also used as mineral specimens.

Kutnohorite is found in Australia, Austria, Belgium, Bulgaria, Canada , China, Czech Republic, France, Gabon, Germany, Ghana, Hungary, India, Indonesia, Italy, Japan, Kazakhstan, Mexico, New Zealand, Norway, Poland, Romania, Russia, Slovakia, Sweden, Switzerland, Turkey, Ukraine, USA and various countries in South America. More locally in Southern Africa it, however, appears to be quite a rare mineral. The only notable localities I could find are Rosh Pinah and Kombat mines in Namibia and in the Kalahari Manganese Fields (KMF) in South Africa.

At Wessels and N’Chwaning I and II mines in the KMF, pale-pink to beige masses commonly occur as divergent bundles of thick fibres, or occasionally as separate small crystals and balls of crystals. Individual crystals are seldom larger than 1-2 mm, but masses of kutnohorite of up to 20 cm or more have been found at the Wessels Mine. Some of these specimens are susceptible to fading after exposure to light , while others gradually exfoliate.

Kutnohorite is similar in appearance to manganoan calcite, but does not fluoresce. Positive identification, however, is difficult without the aid of an analytical technique such as x-ray diffraction, because the varied habits and colours of calcite, rhodochrosite and kutnohorite all overlap. It is commonly associated with calcite and quartz and less common associations are barite, rhodochrosite, hausmannite, andradite and galena- JDJ.
Cairncross, Bruce; Dixon, Roger (1995) – Minerals of Southern Africa.
Cairncross, Bruce, (1997) – The Manganese Adventure.