Schreyerite

'Schreyerite' (V₂Ti₃O₉), is a vanadium, titanium oxide found in the Lasamba Hill, Kwale district in Coast Province Kenya. It is a polymorph of kyzylkumite.

The ore mineral occurs as exsolution lamellae and particles in rutile, coexisting with kyanite, sillimanite, and tourmaline in a highly metamorphosed gneiss. It was named after German mineralogist and petrologist Werner Schreyer, for his research on mineralogy of rock-forming minerals and petrology of metamorphic rocks both in nature and by experiment.

Introduction

Investigation of deposits of green vanadium-bearing kornerupine, revealed the presence of a new vanadium mineral through observations in reflected light. Schreyerite, named after Professor Dr. Werner Schreyer, Professor of mineralogy at Ruhr University, Bochum, Germany, for his mineralogical and petrologic work on high grade metamorphic rocks, was first discovered in the Kwale district, Kenya. A polymorph of kyzylkumite, occurs in highly twinned unmixed grains in vanadium-bearing rutile that occurs as idiomorphic crystals in kornerupine-bearing quartz-biotite-sillimanite gneiss. It also occurs in a pyrite deposit at Sartra, Sweden, in a Pb-Zn ore deposit at Rampura Agucha, India, and recently in metamorphic rocks of the Ol’khon complex on the western shore of Lake Baikal, Russia. Instead of the usual intergrowths with rutile, single crystals of schreyerite were found, associated with titanite.

Composition

Analysis of the mineral through electron-microbe dissolution revealed that the main components of the mineral are Ti and V, and minor amounts of Cr and Fe. The ratioTi: R3+ (Cr, V, Fe, Al) is nearly 1:1, which is not consistent with the refined ratioR4+:R3+ = 3:2. This can be explained by the fact that Ti4+ is partly replaced byV4+.The V4+:V3+ ratio was calculated to maintain charge balance and to be consistent with the refined composition (Table 2). Usual substitutions are Ti4+ ← V4+ and V3+ ← Cr3+ (up to 5.2 wt% Cr2O3,or up to 0.27 a.f.u.), and minor Fe3+ (up to 1.13 wt% Fe2O3, or0.05 a.f.u.). Contents of other analyzed elements (Si, Al, Mn, Mg) are insignificant or not detected. The grain selected for X-ray investigation is relatively uniform in composition (three determinations, mean composition TiO2: 51.83, V2O3: 44.41, Cr2O3: 3.04 wt %) and corresponds to the average composition of schreyerite as determined by electron-microprobe analysis.

Crystal Structure

Schreyerite is anisotropic, biaxial (n=2.7), with X-ray powder diffraction of 2.737 (vs), 2.874 (s), 4.075 (m), 3.381 (m), 2.432 (w), 2.518 (vvw), 2.310 (vvw). Crystal system is monoclinic, with unknown point group and space group. Bireflectance is white to brownish, and has twinning: polysynthetic, universal.The chemical composition was determined with an electron microprobe giving the composition (V1.785Cr0 157Fe0.036)(Ti2.536V0.469)O-9. A peculiarity of this schreyerite sample is the partial substitution of V4+ for Ti4+. The crystal structure was determined by single-crystal X-ray diffraction and was refined in the monoclinic space group C2/c [a = 17.102(2), b = 5.0253(5), c = 7.0579(8) angstrom, beta = 106.636(10) degrees] to R, = 2.84%. The structure of schreyerite may be considered as a 1: 1 polysome composed of slabs of berdesinskiite, V2TiO5, and Ti2O4 (a high-pressure phase of TiO2 with alpha-PbO2 structure).The three cation sites Ti1, Ti1, Ti2, and V3 are each coordinated by six oxygen atoms within rather similar bond distances: Ti1-O = 1.987.2.025, Ti2-O = 1.913.2.070, V3-O =1.851.2.191 Å. The oxygen atoms bonding to Ti1 form an almost perfect octahedron with bond angles close to 180°and 90°, respectively, whereas the Ti2 and V3 octahedra are slightly distorted. These octahedra are connected to two different types of chains of edge-sharing octahedral usually referred to as aberdesinskiite-type chain and α-PbO2-type chain. The unit cell used to describe the revised structure is illustrative in that the monoclinic angle describes the slope of the P and Q slabs (Fig. 3). However, their cell (a = 7.06, b = 5.01, c = 25.06 Å, β = 139°) has a rather obtuse monoclinic angle and is A-centered, and can be transformed (by 201, 010, .100) to a reduced C-centered setting, as used in the present study. The original setting by Grey and Reid (1972), a = 7.06, b = 5.01, c = 18.74 Å, β = 119°, is also A-centered and can be transformed (201, 010, .100) to the standard setting for space group C2/c. The end-member phase berdesinskiite V2TiO5 (P slab) of the polysomatic series was described by Bernhardt et al. (1983) as a mineral. However, the structure has not been published as a crystal of composition V2TiO5, but it is assumed to be analogous to Fe2TiO5, based on corresponding unit-cell dimensions and symmetry. The structure refinements of berdesinskiite (from the same Baikal locality as schreyerite) confirm this assumption.

Physical Properties

Schreyerite is a reddish brown, opaque mineral with metallic luster. Its reflectivity is slightly lower than rutile, and as a result, it’s mostly gray. Pleochromism is weak: yellow-brown to reddish-brown. When immersed in oil, the contrasts between rutile and schreyerite become clearer, and the color becomes more intense.With crossed polarizers, moderate anisotropism becomes evident, so that the very fine lamellar twinning becomes distinct.It has hardness of 7, no known measured density, but its calculated density is 4.46. The minerals crystals can only be seen on the microscopic level. Internal reflectivity has never been observed because the mineral is opaque.

Biographic Sketch

Professor Dr. Werner Schreyer, of Ruhr University, Germany, was the firstGerman to join the Carnegie Institution in Washington after the War. He was an outstanding scientist of international stature. The presentation of the Roebling Medal by Peter J. Wyllie and the acceptance by Werner Schreyer provide wonderful and eloquent testimony to this. Originally trained in Munich as a hard-rock petrographer in the classical German tradition, Werner became one of the pioneers of experimental petrology in Germany after his fellowship at the Carnegie Institution. He combined his uncanny powers of perception and intimate knowledge of field relationships with precisely planned laboratory experiments to open new avenues of research. Werner was a member of the IUGS Commission on Experimental Petrology at High Pressures from 1971 to 1992 and its chairman from 1971 to 1984. He was a member (1972–1984) and chairman (1976–1984) of the German National Committee for IUGS and IMA national representative for Germany (1990–1994). Werner took on the role of IMA councilor in 1994. His key role in the establishment of the Bayerisches Geoinstitut in Germany can be viewed as his tribute to this institution and its importance in the geosciences.

References

#Medenbach, O. and K. Schmetzer (1978) Schreyerite, V2Ti3O9, a new mineral. American Mineralogist, Volume 63, pages I182-l186, 1978
#Bernhardt, H.-J., K. Schmetzer, and O. Medenbach (1983) Berdesinskiite, V2TiO5, a new mineral from Kenya and additional data for schreyerite, V2Ti3O9.
#Grey, L E. and A. F. Reid (1972) Shear structure compounds (Cr,Fe)2Tin-2O2n-1, derived from the a-PbO2 structural type. J. Solid State. 4, 186-194
#Dobelin N, Reznitsky L Z, Sklyarov E V, Armbruster T, Medenbach OAmerican (2006) Schreyerite, V2Ti3O9: New occurrence and crystal structure.American Mineralogist, (91) 196-202