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Reinvestigation of mayenite from the type locality

Posted by Uwe Kolitsch  
Uwe Kolitsch July 10, 2012 08:32AM
The last sentence of the abstract ("The analysed mineral can be considered to consist of endmember Ca12Al14O32Cl2 (62.5 mol.%) and endmember Ca12Al14O30(OH)6 (37.5 mol.%).") suggests that the sample studied is in fact brearleyite (IMA 2010-062).

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.%).
F.P. Glasser July 13, 2012 09:01AM
While I have complete confidence in the revised analysis, it is perhaps too son to be conclusive about the composition range of "mayenite". The synthetic mayenite can be made and chloro-, hydroxy and anhydrous variants are known. Thus for example if anhydrous mayenite is made at ~1300*C and allowed to equlibrate in air at lower temperature, ca 1000*C, it gains "water", presumably by spontaneous transformation of underbonded oxide to hydroxide . The hydroxide- oxide reaction is apparently reversible. So I would expect that if a sufficient range of samples were available, and depending on conditions of formation, the range of compositions between water free and hydroxylated phases might be encountered.

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!
Peter Haas August 25, 2012 04:16PM
Another (and more likely) possibility is that the mayenite acts as a UV absorber. Conventional cosmetic preparations contain combinations of an inorganic pigment (e.g. TiO2) and an organic compound for UV absorption (the pigment reacting to shorter and the organic compound to longer wave UV radiation). All preparations contain redox active compounds anyway. The peroxide absorbs UV radiation below ca. 280 nm (it's not a conventional, i.e. reversible, absorption, but a UV induced decomposition) and as such, an inorganic compound with peroxide anions could make the organic compound superfluous.
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