SCHÄFERITE, A NEW VANADIUM GARNET FROM THE BELLBERG VOLCANO, EIFEL, GERMANY

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Werner Krause, Günter Blass,  Hertha Effenberger

 

Neues Jahrbuch für Mineralogie - Monatshefte 3(3): March 1999, pp.123-134

 

Abstract:

Schäferite, a new member of the garnet group, has been discovered in a fragment of a silicate-rich xenolith from the Bellberg volcano near Mayen, Eifel, Germany.

The new mineral forms small euhedral crystals up to 0.15 mm and crystal aggregates, which overgrow magnesioferrite. Associated minerals on the type specimen are sillimanite, quartz, enstatite, sanidine, cordierite, corundum, mullite, hematite, and augite. Schäferite has an orange-red colour and a vitreous lustre, the streak is yellow. No distinct cleavage was observed; the fracture is conchoidal, and the Mohs' hardness is approximately 5. The crystals are non-fluorescent.

The dominating form is the deltoid-icositetrahedron {112}, additional forms are {100} and {110}. Schäferite is isotropic with n = 1.94(1) (λ = 589 nm). D(meas.) = 3.40(5) g/ cm3, D(calc.) = 3.43 g/cm3.

Electron-microprobe analyses (in wt.-%) gave CaO 23.44, MgO 16.46, Na2O 4.52, MnO 0.37, FeO 0.08, SiO2 0.12, P2O5 0.23, As2O5 0.10, and V2O5 53.95, sum 99.27. The empirical formula based on 12 oxygen atoms is Na0.73Ca2.09Mg2.04Mn0.03Fe 0.01(VO4)2.96(PO4) 0.02(SiO4)0.01 and the simplified formula as derived from chemical analyses and crystal-structure investigation is Na-Ca2Mg2(VO4)3.

The space group is Ia3̄d, a = 12.427(2) Å, V = 1919(1) Å3, and Z = 8. The structure investigation based on single-crystal X-ray data confirmed that schäferite is a member of the garnet group; the refinement on F2 converged at wR2(F2) = 0.0355 and R1(F) = 0.0175.