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Mineralogical ClassificationReinvestigation of mayenite from the type locality
10th Jul 2012 09:32 UTCUwe Kolitsch Manager
Galuskin, E.V.; Kusz, J.; Armbruster, T.; Bailau, R.; Galuskina, I.O.; Ternes, B.; Murashko, M. (2012): A reinvestigation of mayenite from the type locality, the Ettringer Bellerberg volcano near Mayen, Eifel district, Germany. Mineralogical Magazine, 76, 707-716.
New electron-microprobe analyses of mayenite from the Ettringer Bellerberg volcano near Mayen in the Eifel district, Germany have high Cl contents and Raman spectroscopy indicates the presence of OH groups. Neither of these components is included in the generally accepted chemical formula, Ca12Al14O33. A refinement of the crystal structure by single-crystal X-ray methods reveals a previously unrecognized partial substitution. The O2 site which forms one of the apices of an AlO4 tetrahedron (with 3 × O1 sites) is replaced by 3 × O2a sites, which change the coordination of the central Al atom from tetrahedral to octahedral. This substitution is related to partial hydration of Ca12Al14O32Cl2 according to the isomorphic scheme (O2- + Cl-) ↔ 3(OH)-. The revised composition of Eifel mayenite is best described by the formula Ca12Al14O32-xCl2-x(OH)3x (x ∼0.75); the original formula, Ca12Al14O33, is inadequate. The analysed mineral can be considered to consist of endmember Ca12Al14O32Cl2 (62.5 mol.%) and endmember Ca12Al14O30(OH)6 (37.5 mol.%).
13th Jul 2012 10:01 UTCF.P. Glasser
I am not expert in mineral nomenclature but both oxy- and hydroxy phases have the same basic structure. However In the old days they might therefore be deemed to deserve special names to indicate chemical differences. But with improved structural knowledge, would it be better to term the dehydroxylated phase mayenite- O (for oxygen) and mayenite- H (for hydroxide).? The chloride phase would be mayenite -Cl.
Readers may be inerested to know that synthetic mayenite made in oxidising atmospheres is also reported to show a positive test for peroxide. The underbonded oxygen clearly can exhibit a range of interesting substitutions and reactivities, as is shown by its incorporation in anti- ageng skin cosmetics where , presumably, the peroxide mops up free radicals. The use of minerals in beauty preparations is not new (realgar and orpiment come to mind) but this application of mayenite advances mineralogy into the high- tech era!
25th Aug 2012 17:16 UTCPeter Haas
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