Mineral group
Mineral groups are the lowest order units in the classification schemes for minerals . The division of minerals into groups is based on chemical, structural and, more rarely, specific physical properties.
Group definitions are not uniform. In some mineral classifications, the criteria used to define a group vary within a classification and even more from version to version of the same scheme.
Group definitions are often not strictly hierarchical. At group level, minerals are sometimes summarized again, which are assigned to different families, departments or even classes at higher levels of classification.
Minerals are summarized in groups,
- with the same general structural formulas and structure types that belong to a mineral class (e.g. calcite group )
- with the same general structural formulas and structure types belonging to different mineral classes (e.g. monazite group )
- with the same general structural formulas and different structure types ( amphibole group ) also from different chemical classes ( apatite group )
- with similar general structural formulas and closely related structural types ( homeotype ) ( astrophyllite group )
- with certain, structurally controlled physical properties (e.g. zeolite group )
Definition of the International Mineralogical Association (IMA)
It was only in 2009 that the "Commission on New Minerals, Nomenclature and Classification" (CNMNC) of the IMA published a consistent group definition together with a standardized hierarchical classification scheme:
A mineral group consists of two or more minerals with the same or essentially the same structure, which are composed of chemically similar elements. |
The structures of the minerals in a group therefore belong to a structure type ( isotype ) or at least agree in all essential topological properties ( homeotype ), such as the environment of the cations with anions and the linkage of coordination polyhedra and larger assemblies. Some polymorphs such as B. monoclinic and triclinic feldspar thus belong to one group. Homologous series, polysomatic series (e.g. biopyribols ) and other modular structures allow further structural variations and no longer define mineral groups.
The second condition for assigning a mineral to a group is that minerals which are composed of elements with different crystal chemical behavior do not belong to a group even if they are isotypic. Lead luster , periclase and halite are assigned to different mineral groups.
Definition of the International Center for Diffraction Data (JCPDS)
The JCPDS published a consistent classification in 1999, which groups minerals primarily on the basis of the similarity of their X-ray diffraction patterns and thus according to their structure . A mineral group here only includes minerals of one structure type , but without any restrictions on the composition. Minerals whose structure can be described by distorting a group structure are counted as "related structures" in this group. Lead luster, periclase and halite are therefore combined into one mineral group. They are assigned to different subgroups based on their different anions.
literature
- EH Nickel (2001): Discussion Paper on Mineral Groups
- Commission on Classification of Minerals (2001): ICCD Mineral Group Classification
- Dmitry Pushcharovsky (2000): Draft Discussion Paper on criteria to establish mineral groups (or families)
- Commission on Classification of Minerals (2000): Mineral Groups
- SJ Mills, F. Hatert, EH Nickel, and G. Ferraris: The standardization of mineral group hierarchies: application to recent nomenclature proposals (PDF; 481 kB); Eur. J. Mineral. 2009, 21, pp 1073-1080
- Deane K. Smith, Peter Bayliss, Andrew C. Roberts: Mineral Classification in the Mineral Powder Diffraction File (PDF; 678 kB), JCPDS-International Center for Diffraction Data 1999