Neues Mineral Be1-x(Cr,Ti)3O6-y(OH)y Verbier, Schweiz

[Stefan]

Widmer, R., Meisser, N., May, E., Armbruster, T. (2014) Be1-x(Cr,Ti)3O6-y(OH)y anunusual newmineral from Verbier (Switzerland). 21st General Meeting of IMA, South Africa 2014, Abstract volume, 379.

http://www.ima2014.co.za/documents/IMA2014%20ABSTRACT%20VOLUME_2.pdf

The  new  mineral  Be0.47(Cr1.57Ti1.02Fe0.27Al0.08V0.05)O6-y(OH)y  (a  =  9.933(2), b= 8.458(2), c = 4.5106(8) Å, space group Pnma), is the Cr-dominant analogue of the kyzylkumite-like  mineral  Be1-x(V,Ti)3O6  from  Norway  [1]  It  occurs  near  Verbier, Valais  (Switzerland)  as  a  primary  phase  associated  with  Cr-bearing  muscovite  in quartzite, crosscut by mm-sized quartz and albite veins. Other primary minerals include quartz, apatite, monazite-(Ce), pyrite, a tennantite-tetrahedrite group mineral, and a new Th, Ca-rich mineral. These late alpine-stage veins, ~10 million years old [2], are located in  small  quartzite  lenses  embedded  in  Triassic  gypsum  and  tectonically  admixed carboniferous  black  shale.  The  new  mineral  probably  formed  due  to  retrograde decomposition of alpine greenschist facies Cr-bearing muscovite associated with late stage Na-rich hydrothermal activity. Results of fluid inclusion microthermometry and oxygen-isotope fractionation in albite veins, in a close-by similar geological setting, suggest crystallization conditions between 270 °C and 320 °C under a pressure of 1 kb to 2.5 kb [3].

The structure of Be1-x(Cr,Ti)3O6-y(OH)y is of the norbergite type and was refined from single-crystal X-ray data to R1 = 0.0338. In contrast to Be1-x(V,Ti)3O6 from Norway, Be1-x(Cr,Ti)3O6-y(OH)y from Verbier occurs as idiomorphic crystals, is untwinned, but displays severe structural disorder indicated by strongly streaked reflections parallel to b*. Transition metal ions (Cr, V, Fe, Ti) occupy octahedrally coordinated positions. Be partially  occupies  tetrahedral  sites  comparable  to  Si  in  norbergite sensu  strictoMg3(SiO4)(F,OH)2.  The  amount  of  Be  vacancies  and  possible  hydroxylation  in Be1-x(Cr,Ti)3O6-y(OH)y is related to the octahedral M3+/M4+ ratio and the availability of Be in the fluid.

[1] Raade G. and Balic-Zunic T. (2006). The crystal structure of (Be,?)(V,Ti)3O6, a mineral related to kyzylkumite. Canadian Mineralogist, 44, 1147-1158. 

[2] Marshall D., Meisser N., and Taylor R.P. (1998). Fluid inclusion, stable isotope and Ar-Ar evidence for the age and origin of gold-bearing quartz veins at Mont Chemin, Switzerland, Mineralogy and Petrology, 62: 147-165.

[3] May E. (2008). Albites et albitisation dans l'unité de la Pierre Avoi (Zone de Sion Courmayeur) à Verbier (Valais, Suisse): étude combinée des inclusions fluides et des isotopes stables. Unpublished Master Thesis, University of Lausanne