The article examines chemical composition, crystal structure, and infra-red spectrum of low-symmetry vesuvianite from Monetnaya Dacha (Middle Ural, Russia). Crystal structure is defined in the space group P4/n as R1 = 0,038 on the basis of 6091 independent reflections with I > 2σI. Attempts to define the structure in the space group P4/nnc resulted in signifi cantly higher R1 of 0,072, whereas 981 reflections violated systematic conditions of extinction in the group. Chemical composition was examined by using Cameca electron microprobe as following MS-46 (wt. %): SiO2 37,92; TiO2 1,23; Al2O3 16,72; MgO 2,39; CaO 37,05; FeO 4,17; MnO 0,52, total 100,00. Empirical formula calculated on the basis of Si = 18 is Ca18.84Mg1.69Fe2+ 1.65Ti0.44Mn0.21 Al9.35Si18O68(OH)9.41O0.59. In the IR spectrum, we have observed a shift of the v3 absorption band of the Si-O bond to 20 cm-1 (from ca. 990 to 968 cm-1), which is typical for low-symmetry vesuvianites. The structure is based upon grossular 1-dimensional rods linked to each other by corner-sharing of SiO4 tetrahedra in such a way that double Si2O7 groups are formed. Th e rods are centered around the [0,75-0,25 z] and [0,25-0,75 z] axes; in between the rods, structure channels are occupied by the M3a, M3b, Ca6, Ca7, OH18 and OH19 sites. Diff erent occupancies of the M3a and M3b sites (75 and 25%, respectively) results in dominant orientation of chains of the intra-channel sites along the c axis, which favors formation of ordered domains and reduction of symmetry from P4/nnc to P4/n.The crystal can be characterized as consisting up to 75% of parallely oriented domains with the P4/n symmetry. On this basis, it can be concluded that the mineral was formed at temperatures of ca. 300-500 °?, which correspond to the proposed conditions of formation of contact metasomatic rocks of Monetnaya Dacha as a result of serpentinization of primary ultramafi c rocks.
|Журнал||Vestnik Sankt-Peterburgskogo Universiteta, Seriya Geologiya i Geografiya|
|Состояние||Опубликовано - 1 янв 2013|
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- Планетоведение и науки о земле (все)