Mineral Classification in the Mineral Powder Diffraction File

The concept of the mineral classification in the Mineral Powder Diffraction File (MPDF) is somewhat different from that used in classical mineralogical references. The goal of the classification is to exploit the crystal chemistry characteristics to the fullest to assist in the identification of mineral phases. The primary criterion is the similarity of the diffraction pattern, which results from the isostructural nature of the group members. One example is the halite, NaCl-type, halide compounds being included in the same group as the periclase, MgO-type, oxide compounds and the galena, PbS-type, sulfides. All the phases in this group are face-centered cubic, and the diffraction patterns differ in the position of the diffraction peaks and somewhat in intensity but not in the sequence. Subgroups are defined on the basis of chemistry or other structural characteristic. Derivative structure types, which follow group-subgroup symmetry relationships, such as dolomite from calcite or nepheline from tridymite, are also the basis for many subgroupings. Such situations lead to similar patterns with extra weak peaks for the lower symmetry phases. Where the derivative structure results in major changes of symmetry and lattice distortions, subgroups of related structures are recognized. A scheme for writing structure-type formulas using a consistent nomenclature for site symbols has also been developed for representing the mineral group general structure formula. The 12 letters of the Roman alphabetic which are not used for element symbols (A, D, E, G, J, L, M, Q, R, T, X, and Z), are used to represent structure sites with specific characteristics such as element size and coordination, The ability to write a general structural formula and recognize minerals which tit the formula and crystallography leads to the groupings which are the most useful to users of X-ray powder diffraction. Group name nomenclature is based on the mineral in the group, which has been recognized for the longest time, i.e., the rule of historical priority. For example, corundum predates hematite and halite predates either periclase or galena, which defines the proper mineral group name, Subgroup names follow the same rule. Over 170 groups are recognized in the MPDF with some groups having as many as seven subgroups. Copyright (C) JCPDS-International Centre for Diffraction Data 1999