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Classification and Nomenclature of the Pyrochlore Group

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Classification and Nomenclature of the Pyrochlore Group American Mineralogist, Volume62, pages403410, 1977 Classificationand nomenclatureof the pyrochlore group D. D. HoclnrHl Departmentof Geology,Uniuersity of Ottawa Ottawa,Canada Kl N 6N5 Abstract The IMA Subcommitteeon Nomenclatureof the PyrochloreGroup recommendsthe following classificationand nomenclature: Group pyrochlore[Ar-.B,O6(O,OH,F)r-".pHzO] Subgroups pyrochlore,microlite,betafite Species pyrochlore,kalipyrochlore, bariopyrochlore, yttropyrochlore, ceriopyrochlore, plumbopyrochlore, uranpy- rochlore(pyrochlore subgroup); microlite, stannomicrolite, bariomicrolite, plumbomicrolite, bismuto- microlite,uranmicrolite (microlite subgroup); yttrobetafite, plumbobetafite, betafite (betafite subgroup). Subgroupsare divided according to B-atoms(Nb, Ta, Ti) andspecies according to,4-atoms (K, Sn, Ba,REE, Pb,Bi, U). Forty-eightnames related to the pyrochloregroup should be dropped,and fivetype specimens should be reinvesti- sated. Introduction burg et al. (1960),van der Veen (1963),and Bon- The pyrochloregroup comprisesa seriesof cubic shtedt-Kupletskaya(1966). Adding to this growing oxidescontaining essential amounts of niobium,tan- complexity,the literaturecontinued to perpetuate "species" talum,or titanium.Pyrochlore itself was described by rnineral of questionablevalidity, such as Wdhlerin 1826,microlite by Shepardin 1835,and blomstrandite,ellsworthite, and chalcolamprite,and "hatchettolite"by Smith in 1877.The remaining synonymssuch as mendeleeviteand betafite, neo- memberswere all describedin the presentcentury. tantalite and microlite, koppite and pyrochlore. Prior to the late 1950's,Dana's Systemof Miner- Clearly a need existed for a universallyaccepted, alogy(Palache et al.,1944,p.747-757)provided the rationalclassification and a revisednomenclature. acceptedclassification of the pyrochloreminerals. In In viewof thisneed, the IMA Commissionon New thisclassification pyrochlore-microlite was described Mineralsand Mineral Names,at the requestof the as a series,and ninesimilar minerals were appended chairmanDr. MichaelFleischer, established a Sub- as"likely members"of thisseries. Betafite and "djal- committeeon PyrochloreNomenclature. A. H. van maite"(ibid, p.803-805) were excluded, but with the der Veen, Arnhem, Netherlands, was appointed cautionarystatement that "considerableuncertainty" chairmanin August, 1966,and he, in turn, selected existsconcerning their relationshipwith pyrochlore the followingmembers: and microlite.These two mineralswere later shown, chemicallyand structurally,to retainthe pyrochlore (l ) Subcommittee,uoting members structurewith manyof the largercation sites unfilled E. M. Bonshtedt-Kupletskaya,Moscow, (Borodinand Nazarenko,1957; Hogarth, l96l). U.S.S.R.' In the 1960sand 1970smany newanalyses of py- T. Deans,London, England (Secretary)g rochloreminerals were published, revealing a wide M. Gasperin,Paris, France rangeof compositionsand leading to manynew min- D. D. Hogarth,Ottawa, Canada eral names.In the sameperiod severalschemes ofl Akira Kato, Tokyo, Japan classificationwere proposed including those of Ginz- L. Van Wambeke,Brussels, Belgium 'For the IMA Subcommitteeon Nomenclature of the Pvro- 'zDiedJuly, 1974 chlore Grouo. 3 Retired Feb., 1976 403 404 HOGARTH: THE PYROCHLORE GROUP (2 ) Commissionobseruers, non-uoting MicroliteSubgroup in whichNb * Ta ) 2Ti and > M. Fleischer,Washington, D.C., U.S.A. until Ta Nb, in which2Ti > Nb t Ta. Feb.,1968 BetafiteSubgroup limits for the subgroups C. Guillemin,Orl6ans, France The abovecompositional of a naturalclustering of com- M. H. Hey,London, England since Feb., 1968 wereadopted because positionsand a relativescarcity of titanium-richanal- The Subcommitteefunctioned almost entirely by yses(see Fig. I ). Titanium-richspecies range from 33 correspondence.In general,their criteriawere based to 55 percentTi, whereNb * Ta + Ti : 100atom on publishedinformation, but in two cases,members percent.Analyses reporting larger amounts of tita- re-examinedtype specimens.Certain decisionsre- nium may representmixtures. In contrast,most py- quireda vote,and in the fewinstances when opinions rochloresand microlitesfall in the range70-100 per- of memberswere equally divided, the chairmancast cent Nb and Ta, respectively.Subdivision of the the decidingvote. pyrochloregroup accordingto the predominanceof At the outset,members were faced with the choice Nb, Ta, and Ti atomswould thereforerestrict the of recommending"orthodox" names,often well-es- titaniumsubgroup to fewexamples, and would cause tablishedand of deservingorigin, or appropriate wide variationsof compositionsin the pyrochlore chemicalnames. After detailedconsideration, the subgroup.Accordingly, it was eventuallyruled that Subcommitteeand later the Commissionruled in any mineralwith 2 Ti > Nb * Ta belongsto the favor of a chemicalnomenclature. betafitesubgroup. The Subcommitteesubmitted its first completere- Within the subgroups,individuals species are de- port to Dr. Fleischerin December197 1. He, in turn, finedwith respectto,4-atoms (uiz Na, Ca, K, Sn,Ba, distributedcopies to affiliatedsocieties for comments REE, Pb, Bi, U) in the followingmanner: and suggestions,which wereforwarded to the Sub- (a) Na-Ca members.Sodium or calcium,but no committeein November1973. A revisedreport was otherl-atom, shallexceed 20 percentof the totall- thensubmitted in Augustl9'74, and its recommenda- atomspresent, and tions were ratifiedby the Commissionin 1975and (b) other members.One or more l-atoms other 1976.The presentreport summarizesthe reportsof thanNa or Ca shallexceed 20 percentof the total l- the Subcommittee(unpublished, prepared by T. atomspresent. Deans),incorporates suggestions and decisionstaken The figure 20 percentcorresponds favorably with by the Commission,and representsthe final con- recentpractice in describingbetafite, uranpyrochlore, clusionsof the Subcommittee. and uranmicrolite,the most common speciesafter pyrochloreand microlite. Classification Theproposed classification is basedon totalI -ions The pyrochloregroup comprisesthose multiple (excludingoxonium), not on l-sites available.Thus cubicoxides having the followingcharacteristics: deficiencyof l-cations or filling of vacantl-sites by (a) essentialamounts of niobium,tantalum, and oxonium doesnot affectthe classification. titanium,either individually or in combination, Specialrules apply in the caseof lanthanidesand (b) the spacegroup Fd3m, yttrium. Although no exampleis known in the py- (c) the pyrochlorestructure as defined by Gaertner rochlore group of a singlerare-earth element ex- (1930)and Brandenberger (1931), and ceeding20 percentof thel-atoms, severalrare earths (d) the general formula Ar--B2O6(O,OH,F),-o may occur togetherin significantamounts. When 'PH,O' their total exceeds20 percentof the total l-atoms, In the caseof metamictminerals, compositional the Subcommitteerecommends that the mineral be equivalentswhich producethe pyrochlorephase on given separatespecies status. The well-knownten- ignition(preferably in vacuumor inert atmosphere) dencyfor the rareearths to be predominantlyof the are admittedto the scheme.Isostructural minerals, cerium group (light lanthanidesLa - Eu, styled suchas antimonates of thestibiconite series and tung- ECe), or theyttrium group(Y * heavierlanthanides statesrelated to ferritungstite,are excluded. Ga - Lu, styledtY), alsoprevails, and has created a Threesubgroups are recommended,based on the furthertwofold subdivision. atomicproportions of the B-atomsNb, Ta, and Ti. and species The recommendedsubgroups are: Nomenclature PyrochloreSubgroup in whichNb * Ta ) 2Ti and The current 16 speciesof the pyrochloregroup Nb ) Ta, recommendedin this studvare listedand definedin HOGARTH: THE PYROCHLORE GROUP 405 Table 1. Their namesreflect the decisionto adopt chemicalnames in preferenceto "orthodox" names. Thus uranpyrochlorereplaces hatchettolite, uran- microlite replacesdjalmaite, etc. Applicabilityof the Levinson nomenclature(Levinson, 1966) was re- ferredto, but not recommendedby theCommission. The root namesfor speciesof the Nb- andTa-rich subgroups,as alsothe namesof the subgroupsthem- selves,are derivedfrom the "orthodox" namesof the Na-Ca members.However, as no Na-Ca member has been definitely establishedfor the Ti-rich sub- group,the Subcommitteerecommends that betafite, the mostcommon member, be usedas root namefor this subgroup. The bismuthomicroliteof Zalashkovaand Kuk- harchik(1957) was considered as a new speciesbut rejectedby the Subcommitteebecause it contained Nb Ta pyrochloregroup. insufficientBi(4 atom Voof the ,{-ions). The Sub- Fig l. The threesubgroups of the Composi- tions are plottedatomically from data in Bonshtedt-Kupletskaya committeerecommends this name be used for the ( 1966).Minerals no longerincluded in thegroup (minerals with the bismuthmember of the microlitesubgroup as defined samiresitephase, "hatchettolite" from Hybla, Ontario, and "ti- in the precedingsection and first described(as west- tanpyrochlore"from Tangen,Norway) are not plotted. grenite)by Knorring and Mrose(1963). The use of additionaladjectival prefixes is op- tional, and should normally be restrictedto the with the approvalof the Commission,six newnames A-atomnext in abundanceafter thb principalcon- are introduced: kalipyrochlore,bariopyrochlore, stituent,following Palache et al. (1944,p. 43).Thus yttropyrochlore,ceriopyrochlore, stannomicrolite, bariopyrochlorefrom the type localitycan be called and bariomicrolite.Bismutomicrolite is redefined. strontianbariopyrochlore (Ba 44Vo;Sr 32Voof theA- atomspresent). Speciesof
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