Crystal chemical structural formula

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The crystal chemical structural formula is a special chemical formula notation that is used in mineralogy in order to be able to represent the bond structure of the chemical components of a mineral in addition to the stoichiometric ratio (see also empirical formula ) .

The crystal chemical structural formula was developed by the German mineralogist Karl Hugo Strunz and is in the general notation

Meaning of the individual components:

  • In front of the square brackets are the cations with A, B and C , in order of decreasing ionic radius
  • Within the square brackets follows the entire anion group , consisting of the non-complex anions O , OH and F , separated by a vertical line , followed by the complex anion RO 3 or RO 4 .

Examples

In the carbonate Dundasit PbAl 2 [(OH) 2 | CO 3 ] 2 · H 2 O, the non-complex anion OH - is bound to the cations lead and aluminum, while the O 2− in the carbonate complex [CO 3 ] 2− both with the called cations as well as with the central carbon. To clarify the structural structure, the anions that are foreign to the complex are therefore placed in the first place within the square brackets and thus closer to the cations.

Likewise, in the case of the phosphate fluorapatite Ca 5 [F | (PO 4 ) 3 ], the fluorine that is foreign to the complex is only linked to the calcium, whereas the oxygen in the phosphate complex is linked to the cations and phosphorus.

The silicate mineral titanite has the formula CaTi [O | SiO 4 ]. In the compound, the O 2− separated from the SiO 4 tetrahedron is bound only to the cations Ca 2+ and Ti 4+ , while the O 2− of the SiO 4 tetrahedron is bound both to the cations mentioned and to silicon.

Supplementary information

Occasionally, the valency of the cations or their coordination number is also given, if this is necessary to differentiate between polyvalent elements or to understand the structure, such as magnetite Fe 2+ (Fe 3+ ) 2 O 4 or Åkermanite Ca 2 Mg [4] [Si 2 O 7 ].

If certain structure spaces are not fully occupied, this is indicated with a small square □ as a symbol, such as with Glaucophane □ Na 2 (Mg, Fe) 3 Al 2 [(OH) 2 | Si 8 O 22 ].

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