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The Definition of a Mineral

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The Definition of a Mineral 689 The Canadian M ineralo gist Vol. 33, pp. 689-690(1995) THEDEFINITION OF A MINERAL ERNESTH. NICKEL* Divisionof Exploraion& Mining,CSIRO, Wembley, WA 6014, Australia IxrnorucnoN In responseto suggestionsfrom the mineralogicalcommunity that a new definition of a mineral, compatible with recenttechnological advances, should be promulgated,the IMA Commissionon New Minerals andMineral Names(CNMMN) has taken the initiative in producingthe definition embodiedin this paper.The paperis the end-productof active discussionof the subjectwithin the CNMMN over a period of severalyears, and represents a generalconsensus of the CNMMN membership. Although the main purposeof this definition is to provide internal guidelinesfor the work of the CNMMN, it is hopedthat it will be alsobe generallyaccepted by mineralogistsand other earthscientists when facedwith the problem of decidingwhether a particularsubstance should be classedas a mineral.The definition is not intended to be retoactive; i.e., substancesthat fall outside the scopeof the definition but which have been acceptedas mineralsin the past are not to be automaticallydiscredited by this publication. Gmmnal In generalterms, a mineral is an elementor chemicals6mpound tlat is normally crystallineand that hasbeen formed as a result of geologicalprocesses. This statementsuffices to include the vast majority of substancesthat are generally acceptedas minerals.There are some substances,however, that do not conform entirely to these requirements.It is, therefore,necessary to considerwhere the dividing line betweenmineral 41d aoa-mineral should be drawn, and what exceptionsto the general statementshould be permitted. The remainder of this documentis devotedfs an sxamin6tionof theseaspects. Cnvsreu,nrrv The term "crystalline", as generallyused in mineralogy,means atomic orderingon a scalethat can producean "indexable" (i.e., with Mller indices)ditlraction patternwhen the substanceis traversedby a wave with a suitable wavelength(X-ray, electronsoneutons, e/c.). However, somenaturally occurring substancesare noncrystalline. Such substancescan be divided into two categories:(1) amorphous,substances that have never beencrystalline and do not ditfract X rays or electrons,and (2) metamict,those that were crystalline at one time, but whose cry$tatlinity has been destoyed by ionizing radiation. Some mineralogistsare reluctant to accept amorphous substancesas minsnls fecauseof the difftculty in determiningwhether the substanceis a ffue chemicalcompound or a mixtureo and the impossibility of characterizingit completely; some prefer to call such substances 'mineraloids'. However, some amorphoussubstances (e.9., geotgeite.calciourauoite) have been acceptedas mineralsby the CNMMN. With modem techniques,it is possibleto study amorphousphases more effectively than was possiblein the past.Specfioscopic methods associated with a completechemical analysis commonly cap identify an amorphous phaseunequivocally. In fact, appropriatespectroscopies (e.g.,IR, NMR, Raman,EXAFS, Mdssbauer)can reveal the three-dimensionalshort-range structural environment of eachelement (chemical bonds). Of course,without the possibility of obtaining a completecrystal-structure analysis, which can give coordinatesand nature of the atomsothe necessityof a complete chemical analysisis more stringent with amorphousmaterial than with a crystallinephase. * Vice-chairman,IMA Commissionon New Minerals and Mineral Names.Publication approved by the Commissionon New Minerals and Minerals Namesof the IntemationalMineraloeical Association. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/3/689/3420613/689.pdf by guest on 23 September 2021 690 TIIE CANADIAN MINERALOGIST The basesfor acceptinga naturally occurringamorphous phase as a mineral could be: (1) a seriesof complete quantitative chemical analysesthat are sufficient to reveal the chemical composition of all the grains in the specimen, (2) physicochemical(normally spectroscopic)data that prove the uniquenessof the phase, and (3) evidencethat the materialcannot produce an ooindexable"diflraction pattem,both in the naturalstate, and after teafinent with somephysicochemical solid-state process (e.9., heating). Metamict substances,if formed by geologicalprocesses, are acceptedas mineralsif it canbe establishedwith reasonablecertainty that the original substance(before metamictization)was a crystalline mineral of the same bulk composition.Evidence for this includesthe restorationof crystallinity by appropriateheat-treatrnent and the compatibility of the diffraction pattern of the heat-treatedproduct with the extemal morphology (if any) of the original crystal (e.9.,fergusonite-Y). A special case of noncrystalline naturally occurring substancesare those that are liquid under ambient conditions. Water, in its liquid form, is not consideredto be a mineral, but its solid form, ice, is. Mercury, however,is recognizedas a mineral evsa thsngh it doesnot occur in a crystalline stateon Earth. Petroleumand its noncrystallinebituminous manifestations are not regardedas minerals. Srarnrrr UNDERAIImENT CotnrnoNs Many minsl6ls were forrned under conditions of high temperatureor pressure(or both) and are metastable underambient conditions; others may tendto hydrateor dehydratewhen removed from their placeof origin. Such mineralsmay require specialprocedures to preventtheir decompositionbefore the investigationis complete.The use of special proceduresin the investigation does not preclude the acceptanceof a metastableor unstable substanceas a mineral if it can be adequatelycharacterized and if it meetsthe other criteria for a mineral. ErrnareRREsrRIALSussrANcEs Extraterrestrialsubstances (meteorites, moon rocks, etc.) were apparentlyproduced by processessimilar to thoseon Earth,and therefore such processes are now called geological,even though the term "geology" originafly meantthe study of rocls on this planet. Consequently,naturally occurring componentsof extraterrestrialrocks and cosmic dustsare regardedas minerals(e,g.,the lunar mineral tranquiilityite). ANTHRoPocENTcSussrANcEs Anthropogenicsubstances are those produced by Man, and arenot regardedas minerals. If suchsubstances are identical to minerals,they can be referredto as "synthetic equivalents"of the mineralsin question. Ggoloctcany Moormo ArqtrnopocBNrcSussrANcEs Chemical compoundsformed by the action of geological processeson anthropogenicsubstances haveo on occasion,been accepted as minerals (e.g., the Laurium oominerals"formed by the reactionof seawaterwith ancient metallurgicalslags). However, in the modemer4 when many exotic materialsare produced the possibility arises that suchsubstances can be placedin a geologicalenvironment to producereaction products that might otherwise qualify as new minerals.The CNMMN has thereforeruled that, in future, chemical compoundsformed by the action of geologicalprocesses on antbropogenicsubstances c4nnot be consideredas minerals. Somechemical compounds formed by the action of geologicalprocesses on rocks or mineralsthat havebeen exposed to such processesby activities of Man (e.9., mine openings, ore dumps, road cuts, etc.) have beenacceptedasmineralsinthepastand,iftheexposurewasinadvertenti.e.,notdonewiththeexpresspurpose of creatingnew minerals,then suchproducts can be acceptedas minerals.Chemical compounds caused by mine fires are consideredto be a specialcase, as it is not always clear whetherthere has beenhuman involvement in initiating the fue, and suchsubstances are thereforenot acceptedas minerals. BIocH\IIcSunsreNcss Biogenic substancesare chemicalcompounds produced entirely by biological processeswithout a geological component(e.9., urinary calculi, oxalate crystals in plant tissues,shells of marine molluscs, etc.) and are not regardedas minerals.However, if geologicalprocesses were involved in the genesisof the compound,then the product can be acceptedas a mineral. Examplesof acceptableminerals of this kind are substancescrystallized from organicmatter in black shaleor from bat guanoin caves,and the constituentsof limestonesor phosphorites derived from marineorganisms. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/3/689/3420613/689.pdf by guest on 23 September 2021.
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