American Mineralogist, Volume 76, pages 1380-1388, 1991

Crystal chemistry of Fe- and Cr-rich warwickite

SrNroN.l Brcr. Mlml FRANcI Bntcltl:r, Srr,vro C.lpr'onr Istituto di Mineralogia e Petrografia,Universiti di Modena, Via S. Eufemia 19, 41100 Modena, Italy

AssrRAcr Warwickite is an orthorhombic oxyborate with the generalformula t6lM2Ot3lBOj (M : Mg2+, Mn2+, Fe3+,Ti4+, Al3+). The studied warwickite samplesare from lamproitic rocks (umillites) and associatedcarbonatite-like veins (Jumilla, Murcia province, south- east Spain). Their chemical compositions are quite variable and different from those of warwickite described in the literature: in particular, CrrO, varies between 0.0 and 13.5 wto/0,total wto/oFe as FerO, is as high as 50.6 Mo/oand total wto/oTiO, as high as 10.2 wto/0.Two main substitutions occur in Jumilla warwickite: Mg'* + Tia+ = 2Fe3+and Fe3* + Cr3+. The structures of seven crystals of different compositions were refined in spacegroup Pnma, giving R : 0.018-0.038.The structuralresults show that: (l) The Ml octahedron is smaller and more distorted than the M2 octahedron. (2) Distortion of the octahedral layer is greaterin Fe-rich warwickite. (3) Fe preferentially enters the Ml site. (4) The M2- 04 distance decreaseswith increasingTi, suggestingpreferential ordering at the M2 site. TEM studies show planar (100) defects.These defects causevariations in the octahedral chain length.

INrnooucrroN respectively, for MgScOBO, (Pnam) and M80,.- (P2'/a). The name warwickite is applied to an unusual family Mn,roOBO. of warwickite are rare; they have of orthorhombic oxyborate (spacegroup Pnma; Natural occrurences in marbles from Edenville (Tak6uchi et a = 9.20, b = 3.09, c = 9.36 A) of iOealcomposition been described (Moore and Araki, 1974),both t6lM2Ot3lBO3(M : Mg,+, Mn2+, Fe3+,Tia*, Al3*). The al., 1950)and from Amity (OrangeCounty, New York), and structure was first investigated by Tak6uchi et al. (1950) in the town of Warwick from Taigs (Southern Yakutia, U.S.S.R.) (Malinko et al., for warwickite of composition (Mg,Fe)',Tio 5OBO3. Compositions of other warwickite structuresrefined sub- l 986). Warwickite samples from the lamproites of Jumilla sequentlyinclude Mg, ,rAlo,Feo 12TL3,OBO3 (Moore and (Murcia province, Spain) are compositionally Araki, 1974) and the synthetic compounds FeCoOBO, southeast previously described; they are rich in Fe (Venkatakrishnan and Buerger, 1972) and MgScOBO, and unlike those (FerO, up to 50.6 wto/o)and contain significant Cr (CrrO, MgoruMn,.roOBO, (Norrestam, I 989). As composition of samples According to Hawthorne (1986), warwickite belongsto up to 13.5 wto/o). the Jumilla varies widely, they are suitable for examination of the a subgroup of the infinite framework t6lM,l3lTyozminer- associatedwith variation in composi- als, characterized by chains of edge-sharingoctahedra structural changes (rurMr)with a repeat distance that is an integral multiple tion. of3 A. The chains, canted at 60" to each other, are cross AND PETROGRAPHTC linked by (BOr;'- triangles in the plane perpendicular to OccunnsxcE oF wARwTcKITE the length ofthe chain and by sharing edgesto form rib- FEATURES OF THE HOST ROCKS bons four octahedrawide (Fig. l). These geometricalfea- Lamproitic rocks(7.2-7.6 Ma; Nobel et al., 198l) con- tures are common to other mineral groups;thus, Merlino stitute two outcrops near La Celia, about 13 km west of and Perchiazzi (1988) proposed structural analogiesbe- Jumilla (Fig. 2). The smaller outcrop is composedof por- tween oxyboratesand the minerals of the cuspidine fam- phyritic lavas only, whereasthe larger outcrop contains ily, in which two parallel (BOr)'- g.roupsare replacedby porphyritic and coarser-grainedsubvolcanic rocks as well one (SirO,)6- group. Moore and Araki (1974) consider as geneticallyrelated -like rocks (Venturelli et warwickite a member of the structural group denoted as al., l99l). The lamproitic rocks (umillites as defined by "3 A wallpaper structures" becauseof the two-dimen- Osann, 1906) have been extensively studied (e.g.,Fuster sional characterand the short length ofthe c-axis (ifthe et al., 1967 Venturelli et al., 1984). They are composed mineral is referred to the nonstandard spacegroup Pnam). mainly of phlogopite, olivine, apatite, and occasionalcli- According to Norrestam (1989), the ratio of divalent to nopyroxene as phenocrysts,whereas the groundmass is trivalent cations is l:3 and 3:l at the Ml and M2 sites. composed of the same phasestogether with sanidine or 0003-004v9l/0708-l 380$02.00 l 380 BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE l38l

a

Fig.2. Sketchmap ofthe geologicalrelations in the areaof the Jumillalamproite. I : Quaternarycontinental sediments; 2 : Mioceniccontinental sediments; 3 : Triassicevaporites; 4 : jumillites.The dottedarea refers to the carbonatiteJikeoutcrop Fig. l. polyhedral;;;"rwickite strucrure wherethe warwickiteoccurs (Venturelli, personal communica- vieweddown b. Ruledoctahedra : M I dashedoctahedra : M2; ; tion). stippledtriangles : B; O atomsat y : r/r areblack circleslO atomsat y :3/c, -Va areopen circles. brittle and show good {010} .In thin section, they are deep yellow to light brown and show slight ple- analcime, K-rich richterite, subordinate carbonates,and ochroism with maximum absorption parallel to the length; accessorypseudobrookite, , magnetite, and war- they are uniaxial positive. wickite. Brown glass is occasionally present in the ExpnnrpmNTAL TEcHNTeuES groundmass.In the subvolcanic rocks, the groundmassis composed mainly of sanidine oikocrysts (up to 5 mm) Chernical analyses containing phlogopite, olivine, clinopyroxene, analcime, Warwickite samples were analyzed (Tables I and 2) pseudobrookite,and opaques.The interstices defined by using an ARL-SEMQ electron microprobe at the Istituto the large sanidine crystals are filled either with richterite, di Mineralogia e Petrografia,IJniversiti di Modena. The analcime, phlogopite, sanidine, oxides, and glassor with wavelength-dispersivemethod was used, operating at 15 magmatic carbonatesand Na-rich clinopyroxene, phlog- kV acceleratingvoltage, 20 nA sample current, 3 pm beam opite, sanidine, potassium amphibole, apatite, pseudo- diameter, and 20 s counting time on peak and 2 s on , hematite, and warwickite. backgrounds.Natural minerals were used as standards; Carbonatitelike veins permeate lamproites in a small the data were reducedby the method ofZiebold and Ogil- area (150 m long, 40 m wide) in the larger outcrop. The vie (1964) using the correction factors of Albee and Ray carbonatiteJike rocks probably crystallized in the range (1970). B could not be determined quantitatively oC on sin- 700-600 and under high -fo, (=HM buffer). They are gle crystals; it was qualitatively analyzed by atomic ab- composed mainly of granular calcite (with subordinate sorption on concentratesofwarwickite. Warwickite, both globular dolomite that replacescalcite) with fluorapatite, in polished thin sectionsand as crystalsused for structure hematite-ferrian ilmenite crystals, subordinate Na-rich refinement, was analyzed. Average chemical composi- clinopyroxene, phlogopite, sanidine, pseudobrookite, very tions ofhomogeneous grains and selectedrepresentative rare zircon, and quartz. Warwickite is frequently an ac- analysesobtained on zoned crystalsare reported in Table cessoryphase and occasionallya main phase.This is the l. The crystals used for structural refinement are chemi- case for a carbonatite-like veinlet (l cm across) which cally homogeneous, and their compositions (averaged may be seen along the pathway cut into the lamproites from 6 to l0 analysesper grain) are reported in Table 2. leading to the southern entranceof the old hematite quar- The chemical formulas reported in Tables I and 2 have ry, the location of most of the warwickites reported here. been obtained, starting from the general formula Warwickite constitutesabout l0o/oby volume and is con- t61M8t3lT4Or6(Hawthorne, 1986). Only those analysesfor centratedalong the walls ofthe veinlet in sheaflikeaggre- which the total octahedral content (tulZ)was in the range gatesof black shiny prismatic crystals (0.6 mm long with 7.99-8.01 apfu (atoms per formula unit) were used for square cross section) extending toward the center of the further investigation. All Fe was calculated as Fe3+ as vein. The luster is pearly to adamantine. The crvstalsare sr ggestedby the high oxidizing conditions ofthe carbon- l 382 BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE

o NnFCr)F FF(oNOO qEN$q€8€s$Eol rgqqaq rqqolCCC (o000alotNooNo t I OOrNO lO(!aDOrt(O

aD(r)t@Ftt@rO

ONNC)rr)OlO aONF OFF+ONN FF("rNOO o olqra?qqu? | qq\ a?qqqqnq I qColqqq N oOOOFNO I ONO rtOOOO6lO I OcO(DO*@ N F(t N ,il oNts

(oNO) +F* \to, o N ro6l(D rroloQQ E (9 @O- rQlo! | rta \ rq r\\\ l rgolqqq Atoo looo I loo * lo loFo I l@c)o*@ (t, N6l E

-t@OO O++6NOONF.+(', N FtAl(o$ N ol NO-(",c)

TABLe2. Chemicalcompositions of the warwickite crystals used were made +250lo beyond the peak scan range. Three for structure refinement standard intensities and orientations were monitored ev- ery 2 h and 200 reflections, respectively; no sigrrificant S1 S3 S4 J/ deviation was noted during the data measurement. MgO 28.26 30.82 31.43 32.58 31.31 32.35 30.02 The equivalent pairs (hkl) and (hkl) were measured in CaO 0.06 0.03 0.04 0.04 MnO 0.05 0.14 0.05 0.0s 0.11 0.03 0.04 the 2" < 0 < 33'range, k and / starting from zero. The At,o3 0.94 0.36 1.19 0.55 0.21 0.79 0.52 resulting intensities were corrected for background, Lo- Cr,O" 0.84 11.88 9.06 8.20 2.O0 0.80 11.71 Fe"O" 44.48 26.21 27.54 25.57 34.58 33.00 28.91 rertz-polaization, and absorption effects following the Tio, 2.o2 6.62 6.64 8.96 8.05 9.16 5.09 semiempirical method of North et al. (1968), and equiv- ZtO2 0.03 0.05 0.09 0.07 0.05 0.01 alent pairs were averaged (discrepancy factor R""- : Nb2o5 0.06 0.25 0.11 0.o2 0.10 B,O" 22.45 23j9 23.15 23.37 23.24 23.29 22.96 0.013-0.041).A reflectionwas consideredas observedif sio, 0.86 0.42 0.85 0.64 0.30 0 50 0.62 I > 3oI. Mg 4.27 4.55 4.61 4.75 4.62 4.73 4.46 Structure refinement was carried out using SHELX-76 Ca 0.01 (Sheldrick, Mn 0.01 0.01 1976) and starting from the atomic coordi- Ar 0.11 0.04 0.14 0.06 o.o2 0.09 0.06 nates and isotropic displacement factors of Moore and Cr 0.07 0.93 0.71 0.63 0.16 0.06 0.92 Araki (1974), modified for the difference in orientation Fe+ 3.39 1.95 2.04 1.88 2.58 2.44 2.17 Ti 0.15 0.49 0.49 0.66 0.60 ojr (Pnma). Scattering and anomalous dispersion factors were Zr Tt taken from the International Tablesfor X-ray Crystallog- Nb 0.01 raphy (1974). Neutral B, half-ionized O, and fully ionized r6rt 9.00 9.00 8.00 8.00 8.00 8.01 8.00 B 3.92 3.96 3.93 3.95 3.97 3.94 3.94 M cation scattering factors were used; as the scattering si 0.09 0.04 0.08 0.06 0.03 0.05 0.06 factors for Ti4*, Fe3+,and Cr3+are similar, only Fe3+and r3,4r> 4.01 4.00 4.01 4.01 4.00 3.99 4.00 o 16.00 16.00 16.00 16.00 16.00 16.00 16.00 Mg2+ scatteringcurves were used at the Ml and M2 sites. The final difference-electrondensity map showed only Note.'Sample 56 from jumillite;the remainingsamples from carbonatit+ like veins. random variations, with maxima between t0.5 e/A3 and +0.3 e/A3 for all samples.The atomic coordinates and displacement factors for the seven refined samples are listed in Table 4; Table 5 reports selected interatomic atite-like rocks and by the general indications of the distancesand Table 6,' the observedand calculatedstruc- structure refinement; Mn was assumedto be Mnr+. Silica ture factors. Mean atomic numbers at the octahedralsites was assignedto tetrahedralcoordination (for explanation, (estimatedby structure refinement and by electron probe seethe crystal chemistry section). microanalysis)are listed in Table 7.

X-ray data collection and refinement TEM study Seven chemically homogeneous warwickite crystals Finely ground warwickite crystals were dispersed on from the carbonatite-like vein were selectedfor the in- holey carbon films supported on a Cu grid and coated tensity data collection. Systematicabsences are consistent lightly with C; they were examined with a Philips 4007 with spacegroup Pnma. The crystalswere mounted on a transmission electron microscope operated at 100 keV CAD4 diffractometer operating at 52kV and 40 mA with with C" : 2.8 mm and a resolving power of + A. High- graphite-monocromated MoKa X-radiation. Unit-cell resolution images were obtained with underfocus condi- parameterswere obtained by least-squaresrefinement us- ing 25 reflections 18' < 0 < 25 (Table 3). Starting from I To the results of an o/0 plot basedon ten selectedreflections obtaina copy of Table6, orderDocument AM-91-465 from the Business Mineralogical (Einstein, Ofrce, Societyof America,1130 1974), all intensities were measuredusing the SeventeenthStreet l.IW, Suite 330, Washington,DC 20036, o-scantechnique (scan width 0.8-1.8). Backgroundcounts U.S.A.Please remit $5.00in advancefor the microfiche.

TlaLE 3. Crystallographicdata and parameters of the studied warwickite samples

Dimensions R*^ P6 D v Sample (mm) /V,. N.* x100 x100 IA1 tA1 tAl tA.1 s1 0.05x0.06x0.09 470 455 2.12 2.46 9.246(1) 3.0927(2) 9.384{1) 268.3 s2 0.02x0.03x0.15 349 348 3.06 1.88 9.22612) 3.0860(7) 9.369(1) 266.8 s3 0.03x0.04x0.13 403 400 1.30 2.32 e.230(s) 3.0906(8) 9.378(3) 267.5 S4 0.02x0.02x0.13 328 328 2.23 1.87 9.228(1) 3.0840(5) 9.371(1) 266.7 JC 0.02x0.03x0.1.| 343 343 2.54 1.79 9.246(3) 3.0e93(5) 9.378(2) 268.7 S6 0.01x0.01 x0.06 288 268 4.07 3.84 9.242(4) 3.098(1) 9.383(1) 268.7 S7 0.03x0.04x0.08 432 430 2.40 2.63 9.25s(1) 3.0941(8) 9.402(1) 269.3 )"-?I' tt- - t^"t. Note.'samples as in Table2; R"r = Shownare crystal size, number of uniqueand observed reflections, agreement factors, 2*2i' t*' andlattice parameters. I 384 BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE

Tlsle 4. Atom coordinates and equivalent isotropic and anisotropic displacementfactors [4'?]for the studied warwickites

B4

Sample51 -0.12(6) 01 0.0212(2) 0.25 0.8648(2) 1.03(5) 0.93(6) 1.02(7) 1.14(7) 02 0.2469(21 0.25 0.7465(2) 1.1 0(6) 1.10(7) 0.92(8) 1.28(8) 0.06(5) 03 0.2387(21 o.25 0.0064(2) 1.05(6) 0.s6(6) 1.25(8) 0.95(7) 0.02(s) 0.3870(2) 1.00(5) 0.e0(6) 1.1 8(7) 0.eq6) 0.04(6) 04 0.0091(2) 0.25 -0.08(2) M1 0.11564(5) 0.25 0.5688s(5) 1.08(1) 1.',t7(2) 1.02(21 1.04(2) M2 0.10278(7) 0.25 0.18992(6) 1.19(2) 1.12(3) 1.1 7(3) 1.29(3) 0.03(2) B 0.1678(3) o.25 0.8741(3) 0.90(7) 0.75(8) 0.s(1) 1.02(9) 0.06(9) Sample52 -0.36(8) 01 0.0202(21 o.25 0.8680(3) 1.39(6) 1.46(7) 1.50(8) 1.21(8) 0.7469(2) 1.29(7) 1.36(8) 1.3(1) 1.21(9) 0.20(6) 02 0.2463(21 0.25 -0.34(6) 03 0.2386(2) 0.25 0.00s7(2) 1.35(7) 1.41(8) 1.8(1) 0.81(9) 1.21(6) 1.44(71 1.26(8) 0.s3(7) 0.0s(7) 04 0.0098(2) 0.25 0.3841(3) -0.06(2) M1 0.11569(6) 0.25 0.56890(6) 1.27(21 1.58(2) 1.04(2) 1.19(2) M2 0.10218(9) o.25 0.19020(7) 1.17(3) 1 41(3) 1.o2(4) 1.07(4) 0.07(3) B 0.1658(4) o.25 o.87s2(4) 1.5(1 ) 2.3(1) 1.6(1) 0.6(1) 0.2(1) Sample53 -0.24(8) ol 0.0208(2) 0.25 0.8674(3) 1.42(6) 1.s8(8) 1.24(81 1.44(e) 1.38(7) 1.64(8) 1.3(1) 1.15(9) o.24(7) 02 0.2465(21 0.25 0.7471(2) -0.38(7) 03 0.2376(2) 0.25 0.0054(2) 1,29(71 1.64(8) 1.11(9) 1.13(9) 1.35(6) 1.80(8) 0.93(7) 1.31(8) 0.13(8) 04 0.0101(2) o.25 0.3835(3) -0.05(2) M1 0.11s67(6) o.25 0.56875(6) 1.21(21 1.66(2) 0.66(2) 1.31(2) M2 0.10266(9) 0.25 0.19032(8) 1.13(3) 1.42(3) 0.60(3) 1.36(4) 0.00(3) B 0.1674(4) 0.25 0.8750(4) 1.3(1) 1.9(1) 0.9(1) 1.1(1) 0.1(1) Sample54 -0.14(8) 01 0.0203(2) o.25 0.8669(3) 1.37(7) 1.1 1(7) 1.88(s) 1.12(9) 0.7475(2) 1.41(8) 1.3e(s) 2.0(1) 0.8(1) o.22(7) 02 0.2468(2) o.25 -o.28(71 03 0.2388(2) o.25 0.00s3(2) 1.44(8) 1.42(s) 1.s(1) 1 0(1) 04 0.0090(2) 0.25 0.3842(3) 1.26\7) 1.24(71 1.40(9) 1.1 5(8) 0.18(8) 1.16(2) 1.1 5(2) 1.15(3) 1.16(2) -0.06(2) M1 0.115s6(6) 0.25 0.56885(6) -0.06(3) M2 0.10245(9) 0.25 0.19035(8) 1.06(3) 1.02(3) 0.e1(4) 1.25(41 B 0.1672(4) o.25 0.8748(4) 1.5(1) I Qflt 1.9(1) 0.7(1) 0.1(1) SampleSs 0.46(9) 0.6(1) 0.70) -0.1(1) 01 0.0209(2) 0.25 0.8676(3) 0.68(6) -0.30(7) 02 0.2467(2) o.25 o.7481(2) 0.73(7) 0.54(7) 0.77(8) 0.74(s) 0.s5(8) 1.1(1) 0.63(s) -0.10(6) 03 0.2371(2) o.25 0.0050(2) 0.75(7) -o.'t2(7) 0.3834(3) 0.66(6) 0.61(7) 0.83(8) 0.53(7) 04 0.0101(2) o.25 -0.06(2) Ml 0.11495(6) o.25 0.56951(6) 0.s2(21 0.56(2) 0.48(2) o.52(21 0.41(3) 0.33(3) 0.43(3) o.47(4) -0.02(3) M2 0.10218(9) 0.25 0.190s8(8) -0.1(1) B 0.1673(3) 0.25 0.8755(4) 0.60(e) 0.46(9) 0.6(1) 0.7(1) Sample56 01 0.0192(5) 0.25 0.8662(8) 0.7(1) 02 0.2478(5) o.25 0.7484(6) 0.4(1) 03 0.2372(6) o.25 0.00s9(6) 0.6(1) 04 0.0099(s) 0.25 0.3824(8) 0.ss(s) 0.38(5) 0.41(6) o.29(7) 0.43(7) -0.01(6) Ml 0.11s0(2) 0.25 0.5696(2) -0.20(7) M2 O 1024(2) 0.25 0.1901(2) 0.50(8) 0.26(8) 0.6(1) 0.6(1) B 0.1672(9) 0.25 0.873(1) 0.8(1) Sample57 -0.01(8) o1 0.0210(2) 0.25 0.8671(3) 0.66(7) 0.37(8) 1.0(1) 0.6(1) 0.62(8) 0.43(e) 0.9(1) 0.5(1) 0.06(7) 02 0.2470(3) 0.25 0.7470(3) -0.14(7) o3 0.237s(3) o.25 0.00s3(3) 0.63(8) 0.33(9) 1.2(1) 0.40) 0.66(7) 0.75(e) 0.8{1) 0.42(9) 0.04(8) a4 0.0095(2) o.25 0.3843(3) -0.o7(2) M1 0.11 ss2(7) o.25 0.s6856(7) o.62(2) 0.57(2) 0.64(3) 0.65(3) M2 0.1025(1 ) 0.25 0.19040) 0.57(3) 0.41(4) 0.66(5) 0.66(4) 0.00(3) B 0.1663(4) 0.25 0.8740(5) 0.6(1) 0.5(1) 0.7(1) 0.5(1) 0.2(1)

Note.'SamDlesas in Table2. -Bl, : 'The form ol the anisotropic displacementfactor is expl-1/4(Prlta'2 + . + zhla'dBnll. Symmetryrestraints for temperaturefactors: fl"": 0.

tions using a 30 pm objective aperture. The microanal- Fe,O,(22.6V50.63wto/o; 1.66-3.94 aptu), and TiO2(0.21- yseswere done with the energy-dispersiveX-ray detector 10.15wto/o; 0.02-0.74 apfu). MgO is in the range25.68- systemEDAX 9100; this instrumental configurationmade 33.20wto/o(3.95-4.82apfu), and Mn, Ca,Zr, Nb, Al, and possiblethe detailed correlation of chemistry and micro- Si are minor elements.These variations occru within in- structure. dividual samplesand even within individual zoned grains. In most zoned crystals,Cr, Ti, and Mg increasefrom core CHnurcar, RELATToNS to rim, with Fe decreasingdramatically. This composi- The warwickite samples show wide compositional tional variation could be related to the chemical changes variation (Tables I and 2, Fig. 3). The greatestchemical induced in the differentiating carbonatite-like magmasby variations are in CrrO. (0.00-13.45 wto/o;0.00-1.07 apfu), the opaque oxides. These crystallized earlier than or con- BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE l 385

Taele 5. Selected data from structure refinementof warwickite samples

S1 DJ s4 JC S6 s7 B-o1[A] 1.358(3) 1.34s(4) 1 3s6(4) 1.3s8(4) 1.357(4) 1.369(10) 1.347(4) B-O2tAl 1.404(3) 1.413(41 1.404(4) 1.401(4) 1.402(4) 1.401(1 2) 1.408(5) B-O3tAl 1.404(3) 1.394(4) 1.383(4) 1.390(4) 1.375(4) 1.390(12) 1.401(s) (B-o)tAl 1.389 1.384 1.381 1.383 1.378 1.387 1.385 M1-o4(x1)tAl 1.970(2) 1.988(2) 1.992(2) 1.991(3) 1.997(3) 2.007(7) 1.se1(3) M1-o4(x2) tAl 1.973(1) 1.978(1 ) 1.984(1 ) 1.973(1) 1.983(1) 1.983(3) 1.982(2) M1-o2(x1) tAl 2.062(21 2.058(2) 2.063(2) 2.066(2) 2.o71(21 2.079(5) 2.072(31 M1-O3(x2) [A] 2.133(1 ) 2.130(1 ) 2.139(1 ) 2.131 (1 ) 2.1s3(1 ) 2.150(4) 2.142(21 (M1-O)[A] 2.O41 2.044 2.050 2.044 2.057 2.059 2.O52 OQE 1.0196 1.0166 1.0160 1.0167 1.0158 1.0158 1.0166 M2-o1(x2) tAl 1.992(1) 1.988(1) 1.99s(1 ) 1.987(1 ) 1.998(1) 1.985(4) 1.998(2) M2-o4(x1) [A] 2.042(2) 2.007(2) 2.003(2) 2.010(3) 1.s99(3) 1.997(7) 2.016(3) M2-o3(x 1)tAl 2.132(2) 2.139(2) 2.136(2) 2.142(2) 2.142(2) 2.130(6) 2.14s(3) M2-o2(x2) tAl 2.1464't) 2.149(1) 2.147(2\ 2.145(2) 2.155(2) 2.14s(41 2.149(21 (M2-O)[A] 2.075 2.070 2.07',| 2.069 2.O75 2.066 2.076 ooE 1.0124 1.01 10 1.0108 1.0112 1.0104 1.0114 1.0110 Nofe.'Samples as in Table 2. Bond distances and octahedralquadratic elongation(OOE) as in Robinsonet al. (1971).

temporaneouslywith warwickite, removing Fe from the most samples,Ml and M2 have lower valueswith respect melt and causing an increasein concentration of Cr and to O and B. probably Ti (at a slower rate) in the melt. The warwickite samples from the jumillites have very low Cr (Table l, Triangular and tetrahedrally coordinatedsites samplesl-7; Table 2, sample56). The B-Ol bond distance is always smaller than B-O2 Two main substitutions occur in the studied warwick- and B-O3 in Jumilla samples.Previous studies on war- ites: (l) Mg,* + Tio* = 2Fe3*and (2) Fe3* + Cr3*. The wickite related the smaller B-Ol distance to electrostatic flrst substitution characterizesthe crystal chemical rela- bond strength (Venkatakrishnan and Buerger, 1972; tions of all the analyzed crystals (Fig. 3a); it seems to Moore and Araki, 1974;Norrestam, 1989).Ol is linked prevail in warwickite with low Cr3+ content (Cr3+ < 0.2 to B and M2, whereas 02 and 03 are linked to B, Ml, apfu) (Figs. 3b and 3c). Substitutions I and 2 both occur and M2. Furthermore, the octahedralcomposition seems in sampleswith Cr3+content in the range 0.20 < Cr3+ < to affect mainly B-Ol and B-O3 individual bond dis- 0.65 apfu. The substitution mechanism Mgz+ * Tia+ + tances(1.345 A < S-Ol < 1.369A; 1.375A . B-O3 = 2Crr+ seems to prevail in warwickite with Cr3+ > 0.6 1.404 A), whereasB-O2 remains almost unchanged. apfu and Fe3+ ( 2.3 apfu (Fig. 3c). Si in warwickite has been assigned to the tetrahedral sites by analogy with cuspidine. By substituting a (SirO,)6- Cnysru, cHEMTcAL RELATToNS group for a (BrOu)6-group, the sequence. . . B ! B tr Btr The seven crystals for which structures were refined . . . along the [010] direction is changedto the sequence cover a wide range of octahedral(M) composition (Table . . . Si O Si tr Si O Si . . . as for cuspidine (Merlino and 2). The M cations occupy the Ml and M2 octahedra(Fig. Perchiazzi, 1988). It was not possible to determine if the l), which share edgesto form chains of four units M2- (SirO,)6- groups are randomly distributed or if they form Ml-Ml-M2 rotated at 60. to eachother in the (010) plane. cuspidine-typedomains within warwickite, as the Si con- The chain interstices are occupied by (BOr)3- triangles tent is too low. which sharethe O atoms (Ol, 02, 03) with the Ml and M2 octahedra.04 is shared by octahedra only. The iso- Octahedrally coordinatedsites tropic displacementfactors of samples l-4 arequite high, The mean atomic number at each cation site, calculat- and this may be ascribedto some positional disorder. In ed from the electronprobe analysis,agrees quite well with

TABLE7. Mean atomic number for octahedral sites, determined by structure refinement and microprobe analysis for the studied warwickitesamples

s1 s2 s3 s4 s5 s6 s7 M1 Xref 10.28(3) 10.15(3) 10.06(3) s.82(4) 10.00(4) 9.50(10) 10.1 3(5) M2 Xref 7.92(2) 7.27(3) 7.17(31 7.14(3) 7.4o(4) 7.36(s) 7.43(4) Ml + M2 Xref' 36.40 34.84 34.46 33.92 34.80 33.72 35.12 Ml + M2 EPMA"- 36.45 34.95 34.50 34.08 35.02 34.44 3s.29 Note.'Sampfes as in Table 2', )kel: X-ray refinement; EPMA: '2x(M2+M1). electron microprobeanalysis (Z: 4). '* Sum of octahedral cation electrons. l 386 BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE

2.08 Ti >!t a I 0.6 i1h s6l o | 2.06 s60 s5tr a O s70 oo s3tr S.o s2ttrs4 '.018 Mg 'o OGE Fig. 4. Mean (M-O) [A] vs. OQE for Ml (open squares)and 1.2 M2 (filled squares)octahedra. Cr b M2 octahedra(Ml-O :2.041-2.059 A, M2-O :2.066- 2.076A, OQE,, : 1.0160-1.0196,OQEM' : 1.0104- 1.0124:'Table 5). With increasing (Ml-O) and (M2-O) distances, the distortion parameter decreases(Fig. a). Furthermore, in o Fe-rich warwickite (sample Sl), the average distance is o^,% smaller and the distortion is higher than in Mg-, Ti-rich rao warwickite. ooloo^ o a oilo The individual bond distances M-Ol and M-O4 are Fe smaller than the mean (M-O) distance at both sites.The increasein Ti in the Jumilla warwickite samples causes the decreasein M2-O4 (r : -0.898, Fig. 5) and a cor- distance.Thus, it seems Ti ooo c respondingincrease in the M1-O4 o oo o.o! o that the occurrence of Ti at M2 induces the substitution a 0.6 'ot -.:.... of Fe for Mg at Ml. Moore and Araki (1974) suggestthat o Ti in Tirich warwickite preferentially substitutesat Ml. o' o gll. ^e MrcnosrnucruREs aea Although the low R values obtained for structure re-

1O finements support a well-ordered atomic arrangement, many crystals examined during the preliminary investi- t do. gations showed structural disorder and chemical zoning. oa Bovin et al. (1981), in a study of structural defects in 1'2 Cr 0.E Fig. 3. t, *. t, Cr vs. Fe (b);Ti vs. Cr (c) (atomspfu). Opencircles refer to the profileof a zonedcrystal (sample M6). "r, Ti s6l s4t Filled circles refer to unzonedwarwickite crystalsand to the s5t meanvalue of zonedwarwickite crystals. ll 5J 5 Z

that from the X-ray refinement (Table 7). The agreement is not as good for sample 56, perhapsbecause the chem- ical analysis is the averageofdeterminations on several crystalsfrom the same rock sample.The site refinements indicate that Ml is enriched in (Fe,Cr) relative to M2, in agreementwith the results of Norrestam (1989). 0.0 2.03 t* M2_ 04 The octahedraldistortion OQE (Robinson et al., 1971) was used to describe the deformation of octahedra.The Fig. 5. Ti (atoms pfu) content vs. M2-O4 individual bond Ml octahedra are smaller and more distorted than the distance [A]. BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE r 387

t+

t+

-+

--+

t+ Fig.6. Selectedarea electron diffraction pattern (a) andelec- tron micrograph(b) of a warwickitecrystal oriented with b par- allel to the electronbeam. Distances between the white spots correspondto the cell parametersa andc. One isolateddefect Fig.7 . Schematicrepresentation of thepair of defectsshown and onepair of defectsare marked by arrows. in Figure 6. The structureof warwickite is viewed down the b axis.Black and opencircles are as in Figurel. The omissionof two consecutivetwin operations,t, generateschains eight octa- oxyborates with the general formula tel\{rtr1lQr, de- hedrawide, linked by chainsof two octahedra. scribed the structuresas a sequenceof n octahedrallayers (n : 2, 4, 6 for ludwigite, orthopinakiolite, and tak6u- chiite, respectively) (s) between slip or twin (t) planes. tents. The occurrence of Cr-rich varieties is unique for The structure of warwickite may thus be indicated as . . . the warwickite family and is also rare in the natural oxy- 2t2t . .. in which the (or , twin axis rotation axis) is per- borates (Hawthorne, 1986). It is reasonableto hypothe- pendicular to (100) passes and through the center ofa B size the existenceof natural Cr and Fe end-members of triangle. The most common structural defects of the the warwickite family. The warwickite samples studied warwcikite samples examined (100), occur in changing show the following main substitutions: Mg2+ * Tia+ + the sequencefrom the type .. .2t2I ... to the type . . . 2Fe3+and Fe3+ + Cr3+. Coupled substitutions occur at 2t22t . .. . Figure 6a shows a typical selectedarea electron both Ml and M2 octahedral sites; Fe and Cr are concen- diffraction (SAED) pattern ofwarwickite crystalsoriented trated in Ml, whereasMg and Ti dominate at M2. Pre- with b parallel to the electron beam. It is characterized sumably the occurrenceof Ti at M2 is locally coupled to by well-defined spots, indicating a low proportion ofde- Mg at Ml. fects. The correspondinghigh-resolution electron micro- Many crystals are chemically zoned and show planar graph is shown in Figure 6b, in which periods the of white (100) defects,causing variations in the octahedral chain dots define the a and c cell translations. The defects ob- Iengths. In oxyborates having the general formula served in a number of crystals are of the same kind as t51M3l3lBO5,different structuresare generatedthrough the illustrated in Figure 6b and occur randomly in the se- regular repetition of similar structural defects (Bovin et quence.They are produced by the shift ofa row ofwhite al., 1981).The possibility of generatingcomparable struc- dots (Yza+ Y+c)with respect to an adjacent row of the tures through the repetition of such defects also seems undisturbed structure. In places, this defect is repeated reasonablefor warwickite. These defects are not related twice (Fig. 6b), generatinga sequence ofthe type . . .2t222t to variation in pressure and temperature conditions of . . . . As shown in Figure 7, this gives sequence rise to crystallization, as the analyzed crystals come from the chains eight octahedrawide, linked by chains oftwo oc- same veinlet and they seem to be independent of the tahedrathat retain their original orientation. pos- It is not chemical composition for the crystals.Probably thesede- sible to discern diferences in the topologies of the Ml fects are induced in the growing crystals by some impu- polyhedra and M2 in the new arrangement. Apparently rity. the appearanceof the structural defectsis not controlled by the chemical composition (asrevealed by electron mi- AcxNowr,nocMENTs croprobe analysesmade on diferent zones of the same We wish to thank S. Merlino of Pisa University and G. Venturelli of crystal); it is probable that it is induced in the growing Parma University for useful discussion and critical reading of the manu- crystalsby someimpurity (Bovin et al., l98l). script. Thanks are also due to P.B. Moore and F.C. Hawthorne for their helpful srrggestionsand stylistic improvements. The Centro Interdiparti- CoNcr,usroNs mentale di Calcolo (CICAIA) and Centro Grandi Strumenti (CGS) of Modena University are acknowledged The warwickite for their technical support and the samples from Jumilla are chemically CNR for the use of the ARL-SEMQ electron microprobe at the Istituto variable and are characterizedby high Fe and Cr con- di Mineralogia e Petrografia, Universite di Modena. Financial support I 388 BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE was given by Ministero della Universita della Ricerca Scientificae Tec- Rondeel, H.E. (1981) Isotopic dating ofthe post-Alpine neogenevol- nologica and by CNR (grant 89.00360.05). canism in the Betic Cordilleras, Southem Spain. GeologischeMijnbouw, 60,209-214. RrrnnnNcns crrED Norrestam, R. (1989) Structural investigation of two synthetic warwick- ites: Undistorted orthorhombic MgScOBO, and distorted monoclinic Albee, A.L., and Ray, L. (1970) Correction factors for electron micro- Mgo,uMn,rOBOr. Zeitschrift fiir Kristallographie, I89, I-1l. analysisofsilicates, oxides, carbonates,phosphates and sulphates.An- North, A.C T., Phillips, D.C., and Mathews, F.S. (1968)A semi-empirical alytical Chemistry, 42, 1408-1 4 14. method of absorption correction. Acta Crystallographica,424, 351- Bovin, J.-O., O'Keeffe,M., and O'Keefe,M.A. (1981)Electron micros- 359 copy of oxyborates. I. Defect structures in the minerals pinakiolite, Osann,A. (1906)Uber einige Alkaligesteineaus Spanien.H. Rosenbusch ludwigite, orthopinakiolite and tak6uchiite. Acta Crystallographica, A37, Fortschrift, Stuttgart, 283-30 l. 28-35. Robinson, K., Gibbs, G.V., and Ribbe, P.H. (1971) Quadratic elongation: Einstein, J.R. (1974) Analysis ofintensity measurementsofBragg reflec- A quantitative measure of distortion in coordination polyhedra. Sci- tions with a single crystal equatorial plane diffractometer. Journal of erce. 172.567-57O. Applied Crystallography,7, 331-344. Sheldrick, G.M. (1976) SHELX-76. Programme for crystal structure de- Fuster,J.M., Gastesi,P., Sagrado,J , and Fermoso,M.L. (1967) I-as rocas termination. University of Cambridge,Cambridge, England. lamproiticas del SE de Espafla.Estudios Geologicos,23,35-69. Tak6uchi, Y., Watanab6,T., and Ito, T. (1950) The crystal structues of Hawhome, F.C. (1986) Structural hierarchy in "rMrrIITriD, minerals. Ca- warwickite, ludwigite and pinakiolite. Acta Crystallographica,3, 98- nadian Mineralo etst,24, 625- 642. 107. International Tables for X-ray Crystallography(1974) Kinoch Press,Bir- Venkatakrishnan,V., and Buerger, M.J. (1972) T}Ie crystal structure of mingham. FeCoOBO,. Zeitschrift fiir Kristallographie, I 35, 321-338. Malinko, S.V , Yamnova, N.A., Pushcharowskii, D.Y, Lisitsin, A.E., Venturelli, G., Capedri,S., Di Battistini, G., Crawford, A., Kogarko, L.N', Rudnev, V.V, and Yurkina, K.V. (1986) -rich warwickite from and Celestini,S. (1984)The ultrapotassicrocks from southeasternSpain. the Taiga ore deposit (southern Yakutia). Zapiski VsesoyuznogoMi- Lithos. 17. 37-54. neralogicheskogoObshchestva, ll5, 7 l3-7 19 (in Russian). Venrurelli, G., Capedri, S., Barbieri, M., Toscani, L., Salvioli Mariani, Merlino, S., and Perchiazzi,N. (1988) Modular mineralogy in the cuspi- E.,andZrrbi, M. (1991)The Jumilla lamproite revisited: A petrological dine group of minerals. Canadian Mineralogist, 26, 933-943. oddity. EuropeanJournal ofMineralogy, 3, 123-145. Moore, P.B., and Araki, T. (1974) Pinaloolite,MgrMn3*Or[BO,]; war- Ziebold, T O., and Ogilvie, R.E. (1964)An empirical method for electron wickite, Mg(Mg,Ti",) O[BOI; wightmanite, Mg, (O)(OH),[BOI nH,O: microanalysis Anal)tical Chemistry, 36, 322-327. Crystal chemistry of complex 3 A wallpaper structures.American Min- eralogist,59, 985-1004. Mar.n;scnrvrRECEIVED Aucusr 10, 1990 Nobel, F.A., Andriessen,P.A.M., Habeda,E.H., Priem, H.N.A., and MeNuscrrrr AccEPTEDApnrl 6, 1991