ISSN:
1432-0967
Quelle:
Springer Online Journal Archives 1860-2000
Thema:
Geologie und Paläontologie
Notizen:
Abstract Lamellar nigerite is found in Zn-rich spinel from a sample that contains chiefly anthophyllite + spinel + cordierite, lesser amounts of quartz and chlorite, as well as sphalerite, pyrite, pyrrhotite, and galena, and rare cassiterite and rutile. Nigerite can be described as interlayering of spinel-like (R2+Al2O4) and nolanite-like ((Sn, Ti)Al4O8) structures. In nigerite, the spinel-like part is also compositionally related to spinel, but in the nolanite-like part only the structural analogy exists. Stoichiometric assumptions that relate the anhydrous cation sum to the amount of R4+ cations present, allow Fe3+ estimates from microprobe analyses, and a representative analysis gives the following anhydrous formula: Mg1.30Fe 0.65 2+ Zn3.03Mn0.03Al11.65Fe 0.35 3+ Sn0.32Ti0.18O24. The nigerite is Zn-rich with a Zn ratio (Zn/(Zn+Mg+ Fe2+)) of about 0.59, and the Sn ratio (Sn/(Sn+Ti)) that ranges from about 0.63 to 0.41. The Fe3+ content in these samples ranges from 0.35 to 0.52 (24 oxygen basis). Textures suggest that the nigerite could have formed by the breakdown of R 2 2+ (Sn, Ti)O4 and R2+Al2O4 spinel components during more complex reactions. An experimental investigation of the MgAl2O4-Mg2SnO4 join indicates that the solubility of Mg2SnO4 component in spinel over the T interval 500 to 900° C is about 0.5 to 3.0 mole %. This, coupled with the increased solubility expected from the presence of Ti, gives good agreement with the 2.4 to 2.6 mole % R 2 2+ (Sn, Ti)O4 component in spinel that is estimated to be the maximum necessary to form the compositions and amounts of observed nigerite.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1007/BF00381267
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