Beitrag
Kieserite, Mg(SO4)(H2O), a Titanite-Group Mineral
Hawthorne, F. C.; Groat, L. A.; Raudsepp, M.; Ercit, T. S.
Neues Jahrbuch für Mineralogie - Abhandlungen Band 157 Heft 2 (1987), p. 121 - 132
33 Literaturangaben
veröffentlicht: Sep 2, 1987
DOI: 10.1127/njma/157/1987/121
ArtNo. ESP154015702002, Preis: 29.00 €
Abstract
The crystal structure of kieserite, MgSO4 ⋅ H2O, monoclinic, a = 6.891(2), b = 7.624(2), c = 7.645(2), ß = 117.70(2), V = 355.6(2), Z = 4, space group C2/c, has been refined by a least-squares method to an R index of 2.3 % using 509 observed (I > 2.5σI) reflections collected on an automated four-circle diffractometer using MoKα x-radiation. Mixed tetrahedral-octahedral chains of the form [Mg (SO4)Φ3] (Φ: unspecified ligand) run parallel to the Z-axis. Tetrahedra bridge across to adjacent chains to form a tetrahedral-octahedral [Mg(SO4)(H2O)] framework that is isostructural with the [Ti-(SiO4)O] framework in titanite and graphically identical to the [Al(PO4)F] framework in amblygonite. Kieserite is an important rock-forming mineral in many marine salt deposits, and is frequently associated with other (alkali metal) magnesium sulphate minerals in very complex assemblages. A generalized paragenetic scheme is set up for the more common magnesium sulphate minerals, and the crystal structures of the various phases are examined in terms of the arguments of Hawthorne (1985). There is a gradual depolymerization of the structure module with progressive evolution through the general paragenetic sequence, facilitated by the incorporation of increasing amounts of (H2O) into the mineral as a component of the structure module. This depolymerization is also accompanied by a decrease in the module basicity of the minerals, which has the effect of decreasing the acidity of the extra-module cations until, in the latest parts of the sequence, there are no extra-module cations and the structure modules are neutral.
Schlagworte
Kieserite • crystal structure • sulphate • paragenesis