American Mineralogist, Volume 63, pages I182-l 186, 1978

Schreyerite,VzTisOe, a newmineral

Orep MnoaNBACH

Institut tilr M ineralogie, Ruhr-Uniuersitiit D-4630 Bochum, West Germany

ANDKARL Sctrunrznn

I nsti tut fitr M ineral ogie, Rupp recht - Karl - Uniu e rs itiit D-6900 H eidelberg, l(est Germany

Abstract

Schreyerite,a new vanadiumtitanium oxide with the composition(Vo.rrCroo.Alo.0r)2Tisoe givingidealized VrTirOr, is describedfrom the Kwale District, southof Voi, Kenya.The ore mineraloccurs as exsolutionlamellae and particlesin coexistingwith ,sillima- nite, ,and kornerupinein a highly metamorphosedgneiss. The reflectivityof schreyeriteis 2l percent,and the microhardnessafter Knoop is l100-1200kp/mm,.The d valuesof 4.075(m ), 3.381(m), 2.874(s), 2.737 (vs), and 2.a32(w) suggesta directanalogy to an Anderssonphase (Cro.ruFe6.,u)2TisO" with a monoclinicunit cell.

The investigationof a depositof greenvanadium- overlyingweathered zone were collected by Dr. H. bearingkornerupine from Kenya (Schmetzeret al., Krupp, Heidelberg,who visitedthe mine in 1974.The 1974)has revealedthe presenceof a new vanadium strongly weathered,highly metamorphosedgneiss mineral through observationsin reflectedlight. The contains quartz, biotite, tourmaline, diopside,and mineralis calledschreyerite in honour of Professor epidote.The most abundantopaque minerals are Dr. Werner Schreyer,Professor of Mineralogyat graphite and rutile, although traces of pyrrhotite, Ruhr University,Bochum (F.R. Germany),distin- chalcopyrite, and pentlandite are also found. guishedfor his mineralogicaland petrologicalwork Schreyeritealways occurs in intergrowthwith rutile. on -and sapphirine-bearingrocks. The The rutile grainsin the gneissreach a diameterof up mineral and its name have been approvedby the to 0.5mmand are partly idiomorphic.The ratio of Commissionof New Mineralsand Mineral Names of schreyeriteto rutile is stronglyvariable. In one sec- the IMA. Type materialis depositedin the Instituteof tion, rutile crystalsdevoid of schreyeritecoexist with Mineralogyand Petrology,University of Heidelberg, others containing predominantlyschreyerite. Inter- D-6900Heidelberg (F.R. Germany). growths of the latter range from finest exsolution lamellae(Figs. I and2) throughcoarser lamellae to prominentcompact sections (Figs. 3a and 3b). Even Occurrenceand paragenesis the finest lamellaeshow strong polysynthetictwin- Around 1970emerald-green kornerupine crystals ning. In some casesthe intergrowth has a definite of gem quality were discovered6km southeastof orientationto the rutile. In the cyclic rutile twin LasambaHill (4'12'S,38o40'E) in the Kwale Dis-' shown in Figure 2, two exsolutionlamellae of trict, south of Voi, Kenya. The kornerupinecame schreyeriteoriented parallel to the rutile twin plane from a deeplyweathered layer about 2m thick, which {l0l} or {301}may be observed.From this andother wassystematically searched for materialof gemqual- observationsin different sections(for exampleFig. ity. Exploratory excavationsshowed that the host l), one can argue that parts of the orientedinter- rock consistsof alternatinglayers of gneissand growthsare parallelto {l0l} in rutile. In the more quartzite.Further informationon the geologyand stronglyweathered parts of the gneiss,schreyerite is petrologyof this particulararea is not available.The alteredto a spotty, inhomogeneoussubstance with samplesof schreyerite-bearingcountry rock and the lowerreflectivity (Figs. 3a and 3b, phasex). 0003-004x/78/I I l2-1182$02.00 I 182 MEDENBACH AND SCHMETZER: SCHREYERITE I 183

Fig. l. Euhedral rutile crystal elongated parallel to the c axis lamellaeof schreyerite.Reflected lisht, oil fr,l"lrT":-rtution Fig. 3. Rutile intergrowth with schreyeritewhich is partly altered into an unknown phasex. The unusually coarse appearanceis due to an orientation effect. Reflected light, oil immersion; (a) in plane Physical properties polarized light, rutile (R) (white), schreyerite (S) (bright grey)' polarizers The reflectivityof schreyeriteis only slightlylower alteration product (X) (dark grey); (b) under crossed showing anisotropy and twinning of schreyerite. than that of rutile;the color is reddish-brown.There is a weakpleochroism from yellow-brownto reddish- brown. With oil immersion the contrastsbetween rutile and schreyeritebecome clearer, and the red- small size of the untwinnedareas, no reproducible dish-browncolor is more intense.With crossedpo- measurementsof the spectralreflectivity could be larizersa moderateanisotropism becomes evident, so obtained.The meanvalue for 546nm (Na light)is 2l that the very fine lamellar twinning is very distinct percent.From this, neglectingabsorption, an index (Fig. 3). Internal reflectionswere never observed,a of refractionof 2.7 canbe calculated.Microhardness fact that points to an opaquebehaviour of schreye- is slightlyhigher than that of rutileand gives Knoop rite. This was confirmed by investigatingpolished hardnessnumbers between ll00 and 1200kp/mm2. thin sectionsin transmittedlight. The sectionswere Solubility testswith inorganicacids were negative' coveredby a thin aluminumfoil with a smallhole grainsrich in schreyeritethat directlyabove the rutile ChemicalcomPosition had been selectedfor investigation.The foil effec- tively blocks transmittedlight from the surrounding Chemical analyseswere made with an Anl-Etvtx Nb silicates.The reflectivitywas measuredwith a Leitz microprobe.The standardsused were synthetic synthetic OrthoplanPol togetherwith an MPV2.Owing to the and Ta oxide for Nb and Ta respectively, rutile for Ti, analysedchromite for Al, Cr, Mg, and Fe, analysedpyroxmangite for Mn, and analysed vanadinitefor V. Six representativeanalyses (Nos. l- 6) are shown in Table 1. The mean composition yields the formula (Vo.rrCro.ouAlo.ol)2TisOe,giving idealizedV2TisOe, which corresponds to 61.53weight percent TiOz and 38.47 weight percent VrOs. The coexistingrutile is vanadium-bearing.Three rutile analyses(Nos. 7-9) arealso given in Tablel. Quan- titative analysesof the alterationproduct (phasex) could not be made becauseof the high content of volatileelements (HrO?) and the rapid decomposi- tion underthe electronbeam. Energy-dispersive anal- yseswith a Si(Li) detectorgave as main elementsTi, V, Al, and Cr. Figure 4 comparesthe energy-dis- twin of rutile with orientedexsolution lamellae of Fig. 2. Cyclic persivespectra of the unknown phasex, schreyerite, schreyerite(S) parallel to the rutile twin plane {101}or {301}. Reflectedlight, oil immersion. and the coexistingvanadium-bearing rutile. MEDENBACH AND SCHMETZER: SCHREYERITE

Tablel. Microprobeanalysis of schreyerite(anal. nos. l-6) and coexistingrutile (anal. nos. 7-9)

urti3og No. 3 No. 5 No. 5 No. J No, 8 No. 9 caIcul. Nb^0_ n.d. n.d. 0.06 0.09 Ta^O- o.01 n.d, O. Ol+ 0.03 n.d. n.d. Tin 62.o Ai < 60.9 61.7 97.o 98.0 A1^0- o.56 0,)+1 o.52 0, 1+3 n.d. n.d. 1J v^0- 35.9 35.3 1.90 1.79 1J 2.28 2.22 2. ).18 1 05 0.08 0.08 Meo 0.1]+ 0.13 0.1)+ 0.o7 n.d, n.d, Mn0 0.03 0.01 n. d. n.d. O.O2 n,d. FeO 0. 10 0.06 0. 05 0.18 n.d. 0.05

Sm 100.83 99.73 99.66 100. 00 99.O5 100-oi

Cataons based on 9 oxy Cations based. on

ti 2.966 2.972 2.975 3.O00 0.986 0.981 0.98Ir o.o3t o.o3o 0. 0\0 0.000 0.oo0 0.000 1.882 1.881 1.85\ 2.O00 0.017 0.020 0.019 Cr o. 117 0.112 o.127 0.001 0.001 0.001 0.012 0.011 0.014 0.000 0.000 0.000 Fe 0. 00j+ 0. 002 o.001 0.000 0.000 0.000 Sm 5.O12 5.001 5.008 ++ --1M' 2 .030 1.999 2.O23 ttt "2M 2.982 3.002 2.985

t not detected tt V+Cr+A1 +++ Ti +Mo+M.+I'a

X-ray crystallography the syntheticphases FezTisOg and (Cr..uFeo.ru)2Ti3Or. The latter are membersof the Anderssonphases with Due to the intergrowth with rutile and the limited the generalformula M2M(o-2)O(2,-r;(Grey and Reid, amount of material available,neither single crystals 1972).From the similarity of the chemical formula nor concentratesofschreyerite could be separatedfor and the d values betweenschreyerite and those An- X-ray investigations. Rutile grains with a high derssonphases with n : 5, we assumethat the crystal schreyeritecontent were picked out of the polished structuresare the same.The structuresof the Anders- sectionsand cleaned from silicateswith HF. The d son phasescan be clearly derivedby meansof crystal- values measured on these powdered grains with a lographic shear from the rutile structure, or as ori- Debye-Scherrercamera (ll4.6mm diameter, FeKa) ented intergrowths of VrOu and PbO, structure types. werecontrolled by further films of singlegrains made A detailed discussionof these shear structure com- with a Gandolfi camera (57.3mm diameter). All in- pounds is given in Grey and Reid (1972), Grey et al. vestigatedgrains similar to the one in Figure 3 show (1973), Hyde et al. (1974), and others. Electron dif- predominantly rutile in the powder patterns, thus fraction patterns show that the primitive unit cell implying that the coarseappearance of the schreye- which Grey and Reid (1972) derived for Fe2TisO, rite is due to the subparallelorientation of thin la- may also be describedas a body centered unit cell mellae to the plane of the section. All films gave with a crystallographicshear plane (132) (Grey et al., identical X-ray patterns without any addilional lines 1973).The two alternative indexing schemesas well except for rutile. The d values of schreyerite (with as the lattice constants of FezTirO, and schreyerite rutile lines eliminated) are listed in Table 2. A com- are given in Tables 2 and 3 (see the discussionin parison of thesevalues with those of oxides with the Hyde et al., 1974). We conclude therefore that general formula MrO, from the literature shows a schreyeriteis the first natural Andersson phasewith very good correlation of d values and intensities with shearplane structure. MEDENBACH AND SCHMETZER: SCHREYERITE I 185

Table 3. Lattice parameters of synthetic (CrorrFeo 15)2Ti3Oe o Si (Li) DETECTOR J FerTirOr,and schreyerite o RESOLUTI0Nl'65eV sYnthetic n { (cr6.65F.g. F.2Ti3o9 schreyerite r5)zTi3o9 U o a Y J J A+A3+tAB N F '(.ar1 '(.a71 ao (R) 7.031 7.06 7.a6 j+ bo (R) l+.9'f )1.99't \.997 5.o1 5.01 o (R) 18.TBB rB.B62 25.08 18.71r 25.06 J "o t39.35" @ B 119.724 tr9.t,6o t39.tio rt9.)+o @ { v tR3) jja.I B 519.69 j79.$ t77.\B 57'( \3

U o (19T2) Y J t calculateal as prinitive unit cell after GREYmd REID (1973) ft calculated as boily-centered unit cetl after GREYet al' I U Y the gneiss. Unfortunately no fresh material for a :< whole-rock analysiswas available. unknown phose X The presenceof ,kyanite, and korneru- pine shows that pressure and temperature during metamorphism were high. Seifert (1975) determined that the stability field of boron-free kornerupine lies above 4.5 kbar and 700'C. The stabilityof boron- bearingkornerupine can be somewhat lower (Werd- schreyeri te ing, personal communication), but the presenceof kyanite and sillimanitegives minimum conditionsof more than 5kbar and 600"C (Richardson et al., 1969).The microscopicfeatures show that schreyerite has been exsolved from a homogeneousphase (see that at thesepressures and coexisting rutile Figs. I and 2), indicating temperaturesa titanium-vanadium-oxide of a com- rutile Fig 4. Energy-dispersiveX-ray spectra of V-bearing rutile, position intermediatebetween schreyerite and schreyerite,and its alteration product (phasex). must exist. Investigations on the stability of titanium-van- Discussion adium-oxides,especially in the pseudobinaryTiOr- rare. Kosuge and Kachi (1975)syn- Microprobe analysesof some silicatesfrom Voi VrO, system,are a seriesof Andersson phasesTin-2Y2O2o-1 show an unusually high VrO' content, e.g. korneru- thesized the integers20,30,40, 50, 100' 200) pine 0.22 weight percent (Schmetzer et al., 1974), (where n equals In addition, they found a seriesof solid tourmaline 4.03 weight percent, and kyanite 0.72 at 1200'C. T\o-rV2ozo-1and V,O2'-t for 3 < weigtrt percent(Schmetzer, 1978). Vanadium enrich- solutions between < representsthe endmember of one ment in bituminous sedimentsis common, but these n 7 . Schreyerite with r = 5. Nothing valuesimply an unusuallyhigh vanadium content of suchseries Til*Vt+Or-Vl*V8*O' is known on the stability of theseshear plane struc- tures at lower temPeratures. Table 2. d valuesof svntheticFerTigO" and schreyerite ln the FezOr-TiO, system,Andersson phases have been synthesizedat high temperatures.These FerTiro, schreyerite also *i* ,not** decomposeat lower temperatureseutectoidally to ru- 4.1C1 l.t.o7t n OOI+ OO)+ tile * pseudobrookite. Nevertheless,at still lower 3.382 3.381 m 20\ 20\ with r : 5 and the 2.8i7 s 213 215 temperatures,a phase FqTirOr 2.738 2.737 vs 015 015 sameshear vector as schreyeriteexists (Bursill, 1974). 2.\91 +t+ 020 020 2,)+3i2 2.1132 w 211 219 The excellent analogy between this phase and schreyeriteyields a very probable explanationfor the I indexed by GREYand REID (1972)' prinitive unit ce11 existenceof the latter as a discrete phase: at high 1t indexed after GREYet aI' (1973), body centered unit, ce1l temperatures,a V3+-bearingrutile was formed be- (101) *-tf line superimposed by d=2.)+8? of rutile causeof the high vanadium supply in the rock. The tttt intensities for both, Fe2Tiioq ild schreyerite valence deficiency in the crystal is compensated by It86 MEDENBACH AND SCHMETZER SCHREYERITE crystallographicshear leading to an Anderssonphase Grey, L E. and A. F. Reid (1972) Shear structure compounds (Cr,FelTi,-rOr,-, with the general formula Tin-2V2O2n-,with n ) 5. derived from the a-PbO2 structural type. "/. With decreasingtemperature, this phasebecomes un- Solid State Chem., 4, 186-194. and J. Allpress(1973) stableand decomposesto rutile and schreyerite. G. Compounds in the system CrrOr-FerOr-TiOr-ZrO", based on intergrowth of the a-PbO, Another conceivablemode of formation is based V,O, and structural types. "/ Solid State Chem.,8,86-99. on the variable valencestate of vanadium. The van- Hyde, B. G., A. N. Bagshaw, S. Andersson and M. O'Keeffe adium could be incorporated as Vo+O, in solid solu- (1974) Some defect structures in crystalline solids. Ann. Reo. tion in the rutile (VO, has a structure comparableto Mat. Sci.. 4. 43-92. Kosuge,K and S. Kachi (1975)Electron-diffraction that of rutile). During metamorphism,the decreasing and electron- microscopic observation of the pseudo binary TiOr-V2O" sys- O, fugacity in the graphite-rich gneisscould have lem. Chemica Scripta,8, 70-83. caused a change from Va+ to Vs+, thus initiating Richardson,S. W , M. C. Gilbert and P. M. Bell (1969) Experi- reductionexsolution ofschreyerite from rutile. In any mental determination of kyanite-andalusite and andalusite-sil- case,at low temperaturesschreyerite seems to be the limanite equilibria; the aluminum silicatetriple poinl. Am. J. Sci., 267, 259-272. only stable Andersson phase in the compositional Schmetzer, K (1978) Vanadium III als Farbtriiger bei natilrlichen > region Tin-2Y2O2n-,with z 5. Silikaten und Oxiden-ein Beitrag zur Kristallchemie des Van- Acknowledgments adiums. Ph.D. Thesis, University of Heidelberg. -, O. Medenbach and H. (1974) We are indebted to Professor Dr. P. Ramdohr for comments and Krupp Das Mineral Kor- nerupin unter besonderer help during the microscopicinvestigations and Dr. Th. Armbruster Berticksichtigung eines neuen Vor- kommens im Kwale for discussionson the Andersson phases.The reflectivity measure- Distrikt, Kenya. Z. Dtsch. Gemmol. Ges., ments were carried out with the kind support of Dr. K. Medenbach 23,258-278. Seifert, F. (1975) in the laboratories of Fa. LeiIz-Wetzlat GmbH. We appreciate Boron-free kornerupine: a high pressure phase. Am. J criticalcomments on the manuscriptand assistancein the prepara- Sci., 275, 57-87. tion of the English text from Dr. W. V. Maresch. References Bursill, L. A. (1974) An electron microscopestudy of the FeO_ FerOr-TiO, system and of the nature of iron doped rutile. "I. Manuscript receiued, March 13, 1978, accepted Solid State Chem..10.72-94. for publication, June 22, 1978.