A reinvestigation of mayenite from the type locality, the Ettringer Bellerberg volcano near Mayen, Eifel district, Germany

Abstract

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, Ca 12 Al 14 O 33 . 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 AlO 4 tetrahedron (with 36O1 sites) is replaced by 36O2a sites, which change the coordination of the central Al atom from tetrahedral to octahedral. This substitution is related to partial hydration of Ca 12 Al 14 O 32 Cl 2 according to the isomorphic scheme (O 2À + Cl À ) $ 3(OH) À . The revised composition of Eifel mayenite is best described by the formula Ca 12 Al 14 O 32Àx Cl 2Àx (OH) 3x (x~0.75); the original formula, Ca 12 Al 14 O 33 , is inadequate. The analysed mineral can be considered to consist of endmember Ca 12 Al 14 O 32 Cl 2 (62.5 mol.%) and endmember Ca 12 Al 14 O 30 (OH) 6 (37.5 mol.%).

Raman imaging as a new approach to identification of the mayenite group minerals

“…4). The low-wavenumber range (A) indicates a group of low-intensity bands associated with the lattice mode – deformational ones of calcium in octahedra 39 or vibrations within Ca[AlO 4 ] units 3 . The range (B) includes vibrations of the fluormayenite structure backbone such as stretching and bending vibrations of Al-O in (AlO 4 ) 5− tetrahedra.…”

“…Unfortunately, the mineral composition was determined incorrectly 2 . In last decade, a re-investigation of holotype specimen from Eifel provided an opportunity to redefine and rename this mineral as chlormayenite with a crystal chemical formula of the end-member Ca 12 Al 14 O 32 [▯ 4 Cl 2 ], where ▯ means vacancy 3,4 . Description of a few new mineral species related to chlormayenite recently demanded an elaboration of a new classification of the mayenite supergroup, which combines two isostructural mineral groups – wadalite group (silicates) and mayenite group (oxides) 4 .…”

“…Description of a few new mineral species related to chlormayenite recently demanded an elaboration of a new classification of the mayenite supergroup, which combines two isostructural mineral groups – wadalite group (silicates) and mayenite group (oxides) 4 . The general crystal chemical formula of the mayenite supergroup minerals is as follows: X 12 T 14 O 32−x (OH) 3x [ W 6−3x ], where x–polyhedral site occupied by Ca; T – tetrahedral site occupied by Al 3+ , Fe 3+ , Mg 2+ , Si 4+ ,Ti 4+ ; W site in the centre of structural cages occupied by OH − , F − , Cl − as well as neutral molecules like H 2 O, 0 ≤ x < 0.75 3–7 . In synthetic mayenites W site can be occupied by e − , O 2 − , O 2 2− ,O − , S 2− , OH − , N x − , CN − , F − , Cl − , Au− 8–15 .…”

“…The porous nature of the mayenite enforces its economy meaning as a potential water clathrate or modifier of Portland cement 16–18 . This is due to the tendency of its relatively easy hydration as a result of replacing oxygen by hydroxyl group or water molecules 3,5,7,19,20 . However, the presence of H 2 O was described only in natural phases 5,7 .…”

“…The identification of mayenite-type minerals, using electron micro-beam apparatus with energy- (EDS) or wavelength-dispersive (WDS) X-ray detectors, is difficult and uncertain due to different types of water which may incorporate in crystal structure as hydroxyl group 4–6 , “zeolitic” or absorbed water 3,7 . The newest studies reported that water in minerals of mayenite group can also be presented in form of hydrogarnet defects 21 .…”