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The Crystal Structure of Loveringite-A New Member of the Crichtonite Group

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The Crystal Structure of Loveringite-A New Member of the Crichtonite Group American Mineralogist, Volume 63, pages 28-36, 1978 The crystal structureof loveringite-a newmember of the crichtonitegroup BnveN M. Glrruouss Chemistry Department, M onash Uniuersity Clayton, Victoria 3168,Australia IeN E. GnEv CSIRO, Diuisionof MineralChemistry,P.O. Box 124 Port Melbourne, Victoria 3207, Australia InN H. CeNrpssLL'ANDPATRIcK KEU-v Earth SciencesDepartment, Uniuersity of Melbourne Parkuille, Victoria 3052, Australia Abstract Loveringite,a new calcium iron titanate,was discoveredin the JimberlanaIntrusion near Norseman, Western Australia. The unit-cell composition of the type sample,normalized to 38 oxygens, is Caoz2REE' $(Y,Th,U,Pb)o ouTi, orFe. ,"Cr, ,oMgo bsAlosrv, ,,Mno orOrr, ""Zro which may be abbreviatedas AMro"nOr",A: 2large cationsand M: ) small cations (including some REE). The mineral containsabout 0.2 percenturanium and is metamictin its natural state.The crystal structure of the type grain was reconstitutedby heating in air at 800"C. Single-crystalX-ray diffractionstudies showed it to haverhombohedral symmetry, RJ, with unit-cellparameters a,n : 9.1l7(4)A, a,n : 69.07(I )o. The structurewas solvedusing 398 observed [l > 2o(l)l symmetry-independentreflections, refined to an R value of 0.043. Loveringiteis isostructuralwith senaiteand crichtonite,with a structurebased on a nine-layer (hhc. ) close-packedanion lattice in which the I cation occupiesan anion site and the M cations are ordered into 19 octahedraland 2 tetrahedralsites per unit cell. Microprobe analysesfor a number of loveringitesamples are reported, and the observed compositionalvariations are discussedin relation to the structure. Introduction cations occupy both octahedraland tetrahedralinter- We have recentlyreported the crystal structuresof sticesin the close-packedlattice. the minerals senaite (Grey and Lloyd, 1976) and As part of a generalstudy on minor phasesoccur- crichtonite (Grey et al., 1976). These minerals are ring in the Jimberlana Intrusion near Norseman in isostructuraland have compositionsdescribed by the Western Australia, we identified a mineral with a generalformula AM2tOa8,where M : small cations, composition very similar to that of crichtonite and predominantly Ti and Fe, and A : large cations senaite,but with calcium as the major large cation. (mainly Pb in senaite and Sr in crichtonite). The An X-ray diffraction study showed that the mineral compounds have rhombohedral symmetry, and the was metamict, due to radiation damage from the structuresare based on a close-packedanion lattice uranium present (approximately 0.2 weight percent with a stackingsequence (hhc. ..) and with the large UOz). Crystals reconstituted by careful heat treat- cations occupying one of the anion sites.The small ment exhibited rhombohedral symmetry with unit- cell dimensionsvery similar to those for senaiteand crichtonite. Its isostructural relationship to these I Present address: The Department of GeologicAl Sciences, minerals was confirmed by a single-crystalX-ray Queen'sUniversity, Kingston, Canada structure analysis,which is the subjectof this paper. 0003-004x/78l0r02-0028$0200 2g GATEHOUSE ET,4L.: LOVERINGITE 29 Analytical data for a range of samples are also re- werealso observed,although these elements showed ported. no obvious replacementcorrelations with other ele- ments. Occurrenceand compositionaluariation The iron-chromium substitution correlateswith The mineral loveringite2was found during a study the positionof the mineralin the layeringsequence. of uranium-bearingphases of the Jimberlana Intru- Loveringitesamples taken from the plagioclase-hy- sion near Norseman, Western Australia (Campbell persthenelayer had negligiblechromium contents ( S and Kelly, in press).The phaseswere located by the 0.1 weightpercent). At the contactof this layerwith lexan print method (Kleeman and Lovering, 1967), the underlyingbronzite cumulate layer, the chro- and identified using a scanningelectron microscope mium contentof the loveringitesshowed a sudden equipped with a multichannel analyser. jump to 3 or 4 weight percent,whereas the iron Jimberlana is a layeredintrusion which containsa contentsshowed a concomitantdecrease. All samples repeatedsequence of olivine and bronzite cumulate taken from the bronzite layer had appreciablesub- layers overlain by a thick plagioclase-augite-hyper- stitution of chromium for iron, up to a maximum sthene cumulate layer (Campbell et al., 1970). Love- replacementofabout 50percent ofthe iron present. ringite is most abundant in bronzite cumulatelayers, Calcium-rareearth substitutionshowed a similar but even in theserocks it rarely exceeds5 grains per trend to iron-chromium,with the samplestaken slide. The mineral is also found in the lower half of from the plagioclase-hypersthenelayer generally dis- the plagioclase-augite-hypersthenecumulate layer, playingthe highestcalcium-lowest rare earth analy- but has not yet been recorded in the olivine cumu- ses.Sample Cl87-31 in Table I is an exceptional lates.It occurs as isolatedcrystals which are usually exampleof this. It is an almostpure calciumend- anhedral but occasionallyform needles.The grain memberphase, with 94 atompercent contribution of size rarely exceeds 100 X 50 pm. The mineral is calciumto the large-cationsgroup. This samplewas closely associatedwith either quartz and K-feldspar alsounique in havingthe highestvanadium and low- intergrowthsor phlogopite.Other uranium and rare- estzirconium contents of all the specimensanalysed. earth bearingaccessory minerals in the cumulatelay- Unfortunately,this sample was too small(0.03 mm X ers are baddeleyite,apatite, zircon, sphene,and ru- 0.01 mm) for X-ray study,and was also damaged tile. Ilmenite and chromite are also common. duringexcavation from thesurrounding silica matrix. The compositions of more than thirty grains of The compositionalvariations are discussed in rela- loveringitewere determinedby microprobe analyses. tion to the structureof loveringitein a latersection. The detailed interpretation of these results will be published elsewhere(Campbell and Kelly, in press). Experimental To illustrate the wide ranges of metal-atom sub- Naturalloveringite has an interestingtype of meta- stitutions observed,the resultsof microprobe analy- mict structure,in which thereis a breakdownin the seson five grains are given in Table l. SampleCl l6-5, metalatom orderingbut the close-packedanion lat- Table 1, was used for the single-crystal X-ray diffrac- tice remainscoherent enough to give a sharpdiffrac- tion study, after restorationofthe crystalstructure by tion pattern.Individual grains of the mineralgave heatingin air at 800"C. The analysison this grain was single-crystalX-ray diffractionpatterns consisting of performed after reconstitution,and so the elements sharp spotswhich all indexedon a hexagonalcell, a are consideredto be in the valencestates pertaining : 2.87,c : 20.67A. This correspondsto the strong to the oxidizing conditions used, r'.e.Feg+,Y5+ etc. pseudo-cellobserved for crichtonite(Hey et al., These are shown as oxides in Table L The analyses 1969),and is due to diffractionfrom a close-packed illustrate two main types of isomorphous replace- anion arraywith a (hhc...) stackingsequence; i.e. ment, i.e. iron-chromium and calcium-rare earth. ch"* 9 X anion layer repeat.The atom dis- Significant variations in the amounts of titanium placementsinduced by radiationare apparentlyonly (54.6-64.1weight percent),vanadium (0.5-4.5weight slight,and a singleheating for onehour at 800oCwas percent), and zirconium (1.2-6.1 weight perceni) sufficientto reconstitutethe mineral. The resulting single-crystalX-ray patternswere similar to thosefor senaiteand crichtonite. The rhombohedral symmetry 'The and the proposal for system- name for this new mineral the was confirmed from precessionand Weissenberg atic nomenclature of the crichtonite mineral series has been ap- proved by the IMA Commissionon New Minerals and New Min- photographs. eral Names. For the structuredetermination, a reconstituted 30 GATEHOUSEET AL,,,LOVERINGITE Table l. Microprobe analyseson loveringitesamples from Norseman, Western Australia, and davidite from Olary, South Australia c115-5 c 193-2 N138-17 c187- 3I RRD4-291-5 Davidite fron (Data crystal) o1ary, South Australia rio 58. 34 59. 15 64. LO 55.35 5r.'t 4 2 54,67 olzo: o.94 o. 58 1. 54 o.22 '-2- 20. 25. 30 3 15.77 24. L5 26.16 20.a4 58 MnO 0. r7 o. 13 0.03 o.2L o. l3 0.04 Mqo 2. 18 r.22 0.63 o.6'l 0. r0 Cao 2.37 2.50 3.43 4.54 r. 45 o.23 ct203 9.94 0. t0 0. 06 0, 15 6. 90 I.2I s j.o2 0. o7 0.57* t le Na2O 0. 06 o. 13 K2o o. 04 Nio 0.08 0. o7 0. 11 0.03 uo2 o. 18 0. 17 o. 34 0. 18 6.42 Pbo o.22 o.23 0. 1s 0.05 0.69 Hfo2 0. 35 o.20 0.03 0. 11 0. 16 o.51 ZrO2 4. l8 6.r2 I.2T 4. 24 0. 07 Tho2 0 .09 o.52 o. 53 0,09 0, 34 o. 05 CeO2 r.2a I.12 t. 21 o.26 2.75 3. O6 I.22 0. 84 0. 78 0. 08 2.43 ""2o3 *d2o: o,24 o. 15 o.42 o. 05 o. 35 uzo5 l. 10 o.94 0.37 4.40 1.4r 2.AA Y^O^ 0. 09 0. 10 0. r4 o -2'7 t. 86 R.E.E. o.26 0. 21 0, 19 0. 03 0.30 0. 88 SrO 0.0I o.02 Total 99.2L 98.28 97.L2 100. 10 99.2I 98.98 Samples N138-17 ild C187-31 measured 0.005 m x O.05 m ild O.Ot m x O.03 m respectively, si md Na @ntribution fron surrouding natrix for these very small crystals. **other rare earth contrilutions estinated frm chondrite nornalized rare earth patterns. crystal measuring0.03 X 0.05 X 0.07 mm was trans- 12.18'.A 0-20scan, 3-250, was usedwith a variable ferred to a Phillips PWll00 4-circle automatic dif- scan width given by L0 = (1.2 + 0.3 tan d) and a fractometer. Least-squaresrefinement of 20 values speed of 0.05o sec 1. Two background measure- obtained for centred high-anglereflections gave the ments, each for half the scan time, were made for rhombohedralcell parametersdrh : 9.ll7 (4)A, drh : eachscan, one at the lower and one at the upper limit.
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