American Mineralogist, Volume 64, pages 1133-1155, 1979 Temperature-compositionrelationships between naturally occurring augite, pigeonite,and orthopyroxeneat one bar pressure MeIcoTu RoSS AND J. SrepHEhr HUBBNBR U.S. Geological Survey Reston, Virginia 22092 Abstract Natural orthopyroxene crystals containing augite exsolution lamellae were heated at one bar and controlled oxygen fugacity to determine the temperature at which orthopyroxene + augite first react to form pigeonite + melt. Pigeonite was detected by X-ray precession pho- tography. The reaction to form pigeonite is initiated at the orthopyroxene-augite boundary, probably on the augite lamellae, which are structurally similar to pigeonite and where the or- thopyroxene and augite are in chemical equilibrium. The upper temperature lirnit for the minimum stability temperature of pigeonite at one bar, determined from heating experiments on natural pyroxenes,varies from 1284"C at Mgl(Mg + )Fe):0.92 (atomic), to 90loC and 0.04. Orthopyroxene which does not contain augite exsolution lamellae, and remains unsatu- rated with respect to augite component, transforms to pigeonite at the higher temperatures of the orthopyroxene-pigeonite stability field. Partial melting accompanies formation of pigeon- ite for orthopyroxene compositions more magnesian than Mg/(Fe+Mg) : 0.55 (atomic). Introduction existenceof three pyroxenesat one bar and a particu- lar bulk composition.This work has previously The pyroxene minerals, becauseof their presence been reportedin abstractform (Rosset al., 1972; in a wide variety of igneousand metamorphicrocks, Rossand Huebner,l975a,b). are the subject of intense and continuous study by many petrologists, mineralogists,and crystallogra- The quadrilateral pyroxenes phers. An understanding of the stability fields of theseminerals in terms of temperature,pressure and Definitions chemicalcomposition is crucial to understandingthe For the purposesof this paper, we consider the crystallizationhistory of pyroxene-bearingrocks. Of "quadrilateral pyroxenes"to be augite,pigeonite, or- the seven common pyroxenes,jadeite, acmite, om- thopyroxene,and protopyroxene.The chemicalcom- phacite,aegirine-augite, augite, pigeonite, and ortho- position of these pyroxenesis given by the formula pyroxene,the last three are by far the most important M2T2O.. The M sites may contain Ca,Mg,Fe'*, as and are the particular subject of this paper. One or well as lesseramounts of Fe3*, Li, Na, Mn2*, Tia*, more of thesethree pyroxenesoccurs in many terres- Cr3*, and Al3* in natural pyroxenes.The T sitescon- trial and lunar igneous rocks and in metamorphic tain Sio*and Al'*. rocks of the pyroxene-hornfels, amphibole-gran- All augites have space-groupsymmetry C2/c. ulite, and granulite facies. Most augitesare in the compositionrange defined by In this paper we give the resultsof heating experi- the following occupancylimits of the M sites: ments at one bar on naturally-occurring ortho- (l) ca + Mg + Fe2"+ Na * Fe3*+ Al > 1.00 pyroxeneswhich contain augite exsolution lamellae. (2) l00Ca/(Ca + Mg + Fe'z*)> 20 From these experimentswe determined the temper- and aturesat which the transformation (3) 100(Ca+ Mg + Fe2*)/(Ca+ Mg + Fe2*+ Na + Fe3*+ Al) > 75 opx t augite-+ opx + augite * pigeonite+ liquid Pigeonitespossess space-group symmetry A/c, or is first observed. This temperature is equal to, or FLr/c at lower temperatures,and have M-site occu- greater than, the minirrrum temperaturefor the co- pancy fimits ot 00n.3-004X/79/ l l l 2- l l 33$02.00 l 133 I 134 ROSS AND HUEBNER: AUGITE, PIGEONITE, AND ORTHOPYROXENE (l) Ca + Mg + Fe'z*= 1.40 gaard pyroxene trend," now firmly establishedin and the geological literature, commenceswith copreci- (2) 100Ca/(Ca + Mg + Fe'*) < 20 pitation of magnesium-rich augite and ortho- Orthopyroxeneshave spacegroup symmetry Pbca pyroxene followed by coprecipitation of augite and and M-site occupancylimits of: pigeonite; the pyroxenesincrease in iron content as (l) Ca + Mg + Fe2*> 1.40 fractional crystallizationproceeds (Brown, 1957,Fig. and 2). This crystallizationtrend has also been observed such 12) l}}Ca/(Ca + Mg * Fe'z*)< 7 in numerousother volcanic and intrusive rocks, Protopyroxenehas not been found to occur natu- as BushveldComplex (Atkins, 1969)and the Hakone rally but has beensynthesized by many workers,usu- tholeiitic andesites(Nakamura and Kushiro, 1970b). ally has the compositionMgrSi'O", and has been re- Some of the most extensivepyroxene fractionation ferred to as protoenstatite.tto (1976)was the first to trends are observedin the lunar basalts(summarized prepare good single crystals of protopyroxene; the by Papike et al., 1976).Some individual pyroxene composition he synthesized is Li'ScrMgr-r*SirOu. phenocrystsin the lunar basalts are chemically and The spacegroup of Ito's phase is Pbcn, based on a structurally zoned so as to reflect the entire pyroxene full three-dimensionalcrystal structure analysis (H. crystall2ation trend, the phenocrystcores being mag- T. Evans,Jr., personalcommunication, 1977;Smyth nesium-rich pigeonite, the rims iron-rich augite and lto, 1977),which verifiesthe symmetryproposed (Boyd and Smith, l97l). by Smith (1959) on protopyroxeneof the composi- The association augite-orthopyroxene is found tion MgrSirOu. most typically in granulite-faciesmetamorphic rocks, The compositionsof pyroxenesare commonly ex- but also in some pyroxene-hornfelsand transitional pressed in terms of mole percent CarSirOu(Wo), amphibolite-granulite rocks. The chemical composi- MgrSirOu (En), and FerSirOu(Fs). The amount of tions of the coexistingpyroxenes in granulite-facies these three componentsdepends on the method of rocks having a variety of bulk compositions have normalization of the chemical formula; different been carefully describedby a number of geologists, methods can give slightly different values.To avoid including Binns (1962)on granulitesfrom the Broken inconsistenciescaused by the use of different pyrox- Hill district of New South Wales, Subramaniart ene nonns to calculate "Wo," "En," and "Fs" we (1962) on charnockitesand associatedgranulites have chosen to express"quadrilateral" pyroxene from various localities, Davidson (1968) on gran- compositionsusing the relationships ulites of the Quairading district, Western Australia, (l) [Ca]: lOOCa/(Ca* Mg + zFe) Immega and Klein (1976) on iron formation, and (2) lMgl: l00Mg/(Ca * Mg + )Fe) Rietmeijer (19'19)on iron-rich igneousrocks in Nor- and way. Coexistingorthopyroxene and augite also occur (3) [Fe] : 100Fel(Ca + Mg + )Fe) in lunar metamorphic rocks such as the Apollo 15 where Fe : Fe2** Fe3*and Ca,Mg,Fet*,and Fe'* anorthosite 15415(Stewart et al., 1972),tllLe Apollo are the numbers of cations per formula unit. For 17 troctolite 76535(Gooley et al., 1974),and the no- "quadrilateral" pyroxenesthat do not contain large ritic breccia 77215(Huebner et al., 1975).Petrogra- amounts of AlrOr, TiOr, MnO, FerOr, and NarO the phic and chemographicevidence suggests that the or- in these [Ca], [Mg], and [Fe] values are close to the values thopyroxene and augite were equilibrated Wo, En, and Fs, respectively,calculated by the vari- rocks. ous normalization methods. Equilibrium assemblagescontaining ortho- pyroxeneand pigeoniteare apparentlyquite rare, for Occurrencesin igneousand metamorphicrocks thesetwo phasesin the presenceof melt coexistonly Cooling magmas crystallize quadrilateral pyrox- over a very narrow compositional range. Virgo and enesin a generally consistentsequence of composi- Ross (1973) reported on coexisting orthopyroxene tions such that the first pyroxenesto crystallize are and pigeonitephenocrysts in Mull andesiteglass but magnesium-richand the last are iron-rich. The stud- could not demonstratetrue equilibrium relationships. ies of Wager and Deer (1939),Brown (1957),and The three-phaseassemblage orthopyroxene-pigeon- Brown and Vincent (1963) establish quantitatively ite-augite is also quite rare: basaltic or andesitic the pyroxene crystallization trend in rocks of gab- magmas are in equilibrium with three pyroxenes broic composition-specifically those of the Skaer- only over very restricted temperature-composition gaard Intrusion, East Greenland. The "Skaer- ranges.In strongly fractionatingliquids, however,the nOSS,4.ryD HUEBNER: AUGITE, PIGEONITE. AND ORTHOPYROXENE I 135 melt may intersectthe three-pyroxenefield over an appreciable temperature-compositionrange where orthopyroxene,pigeonite, augite, and melt are in re- action relation (Huebner et al., 1972).Examples of three coexistingpyroxenes have been reportedin an- desitesfrom Weiselberg,Germany and Hakone vol- cano,Japan by Nakamura and Kushiro (1970a,b). Protopyroxenehas not been reported as occurring naturally; however, evidence exists that a magne- sium-rich pigeonite ("clinoenstatite") found in por- phyritic volcanic rocks from Cape Vogel, Papua 1 (Dallwitz et al.,1966)and in the ShawL-group chon- I 1000 E (Dodd drite et al., 1975)is an inversionproduct from E high-temperatureprotopyroxene. E ,o qoo Subsolidusreactions An exampleof the subsotduspyroxene phase rela- tions is depictedin Figure l, a planar sectionthrough the pyroxene ternary phase volume, and coincident with an orthopyroxene-augitetie line. Augite and pi- geonite which coprecipitate above the three-pyrox- ene stability region and then cool slowly will unmix pigeoniteand augite,respectively. Pigeonite on cool- ing into the two-phaseaugite-orthopyroxene stabitty volume may transform to the stableassemblage aug- ite plus orthopyroxene, thereby forming