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MINERAL OF THE WEEK
#82485, Topaz, Agnus Dei Claim, Tribute Pocket, Colorado (t) $225
News and CommentsRecently I was asked about mineral nomenclature when identifying minerals in a series like calcite and rhodochrosite. Often minerals form in what is called a solid solution series. This means that most of the chemical formula is the same between two minerals, but two elements vary in proportion. In this instance:
calcite = CaCO3 You can easily see the CO3 part of each formula is the same, but one has calcium (Ca) and the other has manganese (Mn). But minerals are not perfect and seldom are pure "end members" of the series. More often the calcite may have Mn impurities or the rhodochrosite may have Ca impurities. If the specimen is tested and Mn is 51% and Ca is 49% then the mineral is identified as rhodochrosite. If it is 51% Ca and 49% Mn than it is identified as calcite (or possibly as Mn-rich calcite or calcite var. manganoan or the now obsolete name manganocalcite). Just a 2% difference is all it takes to change the identification. And some crystals vary by being zoned with varying composition - it may be calcite in the center and rhodochrosite in an outer layer. Here in New England the pegmatite quarries frequently have a green apatite that fluoresces yellow under UV illumination. Local collectors call this mineral manganapatite. There is no such mineral species. A researcher did a review of several specimens from several quarries and found that they were all fluorapatite Ca5(PO4)3F with trace impurities of Mn which caused them to fluoresce. None had Mn content over 3%. If they had over 3% (or is it 10%?) then they would qualify to be called Mn-rich fluorapatite according to the IMA. (I am sure I will get a correction on this last tidbit, but I have forgotten the exact threshold.) Some common mineral series are:
adamite - olivenite I hope this clarifies how minerals are identified when they are not pure end members. John H. Betts 9/26/2023 Click here to read past news and commentaries...
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