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Observations on Terrestrial Ureyite and Ureyitic Pyroxene

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Observations on Terrestrial Ureyite and Ureyitic Pyroxene American Mineralogist, Volume 72, pages 126-136, 1987 Observationson terrestrial ureyite and ureyitic pyroxene Groncn E. H.q.RLow Department of Mineral Sciences,American Museum of Natural History, New York, New York 10024, U.S.A. E. Pprnn Or,ns Department of Geological and GeophysicalSciences, Princeton University, Princeton, New Jersey08540, U.S.A. Ansrnlcr Ureyitic clinopyroxene with up to 87 molo/oureyite component (Ur, NaCrSirOu) and showing reaction textureswith chromite is found in jadeitites from Burma; from Mocchie, Susa,Italy; and in Mesoamericanartifacts from Mexico. Complete ureyite miscibility with jadeite and significant solid solution with omphacite are observed. Minor Ur content is found in jadeitic diopside from a chromite-bearing diopside rock from California and in jadeite and omphacite associatedwith jadeitites from Guatemala. The relatively uncom- mon amphiboles eckermanniteand nyboite and other sodic amphiboles,often with exten- sive replacementof octahedralAl by Cr, are common constituentsof the Burmesesamples. The reaction texture betweenureyite or Ur-rich clinopyroxeneand chromite and the ubiq- uitous presenceof eckermannite in the Burmese samplesare phenomena consistentwith the high-pressureinteraction of a sodic fluid on albitites in contact with chromite-bearing serpentinites.Although phaseassemblages vary for other occurrencesofureyitic pyroxene, comparable interactions of fluid with chromite during jadeitization are required. INrnooucrroN The major mineralogical references(e.g., Deer et al., with jadeite, chromite, and amphibole from the Burmese 1978; Cameron and Papike, l98l) do not acknowledge jades. So there is growing evidencefor the occurrenceof the terrestrial occurrenceof ureyite, NaCrSirOu;it is only terrestrial ureyite and ureyitic pyroxene. infrequently mentioned as a rare accessorymineral from Some confusion exists with regard to pyroxene com- the iron-rich Mexican meteoritesToluca (Iaspeyres,1897; positions resulting in greenjadeites, particularly with the Couper et al., l98l) and Coahuila (Frondel and Klein, loose use of the term "chloromelanite" for all dark-green 1965). Moreover, ureyite flJr) is not generallyviewed as jadeitic pyroxenes. Mineralogically, chloromelanite is a significant component in terrestrial pyroxene, and only normally used for a wide range of composition in the rare accountsare to be found (e.g., Sobolev et al., 1975; middle ofthe pseudoternarydiagram defined by the end- Mevel and Kienast, 1980; Carpenter,1981). Neverthe- members jadeite, acmite, and diopside + hedenbergite less, spectroscopistshave known that crystal-field split- (Esseneand Fyfe, 1967).However, Webster(1983), the ting of Cr3+ is responsiblefor the emerald-greencolor in authority for gemologists,has consideredchloromelanite preciousjadeite jade (Rossman,1980). Among the lapi- to be a mixture ofjadeite plus "oxides of iron." Clearly 'Jadeite" dary jades are rare dark emerald-greenrocks there is a nomenclatural problem betweenthe disciplines, that variously go by the names of Maw-sit-sit and Taw- but more significantly, these dark emerald-greenrocks mawite, referring to Burmese mine occurrences, and need analysis. chloromelanite. These rocks are poorly described in the In this paper we report on the pyroxene compositions geologic literature and are usually considered mixtures, and phaseassemblages ofa suite ofdark-greenjades from either in solid solution or as intergrowths, ofquadrilateral Burma and ltaly, severaljade artifacts from archaeolog- plus alkali pyroxene. Sometimes they have been labeled ical sites in Mexico (clarification has been neededon the as chromian epidote (e.g., Chhibber, 1934). lacroix nature of these jades for some time), and a chromite- (1930) describeda Maw-sit-sit from Burma that con- bearing diopside rock from California. An occurrenceof tained up to I I molo/oUr in the pyroxenewith the assem- minor Ur solid solution in pyroxene from jadeitites from blage of chromite, albite, nepheline, and an alkali am- Guatemala is also described.On the basis of these data, phibole. More recently, Gubelin (1965) inaccurately we have tried to develop a model for ureyite paragenesis described comparable material as chromian albite, and and to determine the constraintsthat this model plays on Manson (1979) analyzedand determined a true ureyite. jadeitite genesis.Other questionsabout comparisonsbe- Yang (1984) reported up to 86 mol0/oUr in association tweendifferent jades and sourcesofjadeitite, both in terms 0003-o04x/87l0I 02-0 I 26$02.00 t26 HARLOW AND OLDS:TERRESTRIAL UREYITE AND UREYITIC PYROXENE t27 Table1. Materialand sources Phases Dresent*. Sample' jd omp cm ab wm amp anl chl Locality AMNH 32734 uh" x X X X Banalsite Burma: iade mines region AMNH 35024 Urre XX Burma: iade mines region AMNH 37970 Ul"o X Burma:iade minesregion AMNH 97327 Ul"o 7 XX Burma:jade minesregion AMNH 97328 Ur, Ur" X Zoisite Burma:jade minesregion AMNH 98642 Ur* X Ur". X X Burma:jade minesregion BGS 16853 Urru X Ur16 X Burma:jade minesregion NMNHR3076-1 X Ur6 X Burma: Mogaungregion NMNHR3239 Ulra X X X Burma: jade mines region NMNH94784 X Ur1 X X Burma:jade minesregion NMNH104671 Uru" X X X Yunnan.Chinat NMNH104982 Uf,o Uf,, X X Unknown(probably Burma) NMNH152707 Uln X X X Burma:jade minesregion NMNH145011 Uf,o X Diopside,Pyrite California,U.S.A.: Williams Creek and Buttermilk Creek NMNHR8802 Urr X X Sphalerite Mocchie,Susa, ltaly MVJ-42-4 Ut", X X X La Palmilla,Motagua Valley, Guatemala R-15 UTuXXX X Na- and Al-bear- Rio La Palmilla,Guatemala ing silicate AMNH 30/5556 Ur, Pyrite Finca Pompeya,Guatemala: emerald-green pen- dant AMNH30/10818 XUT,XX Mixteca, Oaxaca, Mexico: emerald-greendisk NMNH106891 Ufr"XUrlsXX Xalitla,Guerrero, Mexico: bead (NMNH Mineral Sci.) NMNH391073 Ur" Ur, X Au, Cu alloy Chichenltza, Mexico:ring (NMNHAnthropology) NMNH407254 Ur* Uru X La Venta,Mexico: bead (NMNHAnthropology) NMNH419801 Ut" Guerrero,Mexico: figurine (NMNH Anthropology) - AMNH : AmericanMuseum of NaturalHistory; BGS : BritishGeological Survey; NMNH : NationalMuseum of NaturalHistory; R : Ridinger, Jade S.A.; MVJ : authors'field study, 1984. " Phasesare ur, ureyite(Ur":2 mol% ureyitein pyroxene);jd, jadeite;omp, omphacite;cm, chromite;ab, albite;wm, white mica; amp, alkali amphibole;anl, analcime;chl, chlorite. t Presumablyfrom the iade mines region in Burma of geological and archaeological interpretations, will be The archaeologicalspecimens presented a problem for probe handled in papers in preparation. analysis since the pieceshad unusual shapes,the surfaceswere not always smooth, and no modifications were allowed. To get MlrpnHrs AND ANALyTTcAL METHoDS around this, the following procedurewas used: First a relatively flat to convex surfacewith promising green color was selected. A seminal Maw-sit-sit specimenwas brought to our attention Then largecracks or flaws were filled with Ambroid glueto avoid and donated (AMNH 98642) by JosephSataloffof Philadelphia, uncleanablecontamination with evaporatedcarbon. The surface Pennsylvania. Further specimenswere obtained from mineral was lightly dry-buffed using l-pm alumina to remove film and collectionsat the American Museum of Natural History, Smith- contamination from the outermost surfaces.Finally all but the sonian Institution (National Museum of Natural History), and selectedarea was maskedwith adhesivetape before coating with the British Geological Survey. Several small Mexican artifacts evaporated carbon. These specimenswere mounted with low- were borrowed from the Foshagcollection overseenby the An- temperaturethermal adhesiveto 1-in.-diameter(2.54 cm) blocks thropology Department at the National Museum. A similar col- so that the selectedsurface was as near as possibleto horizontal. lection of Mesoamericanartifacts was loaned by the Department The surfacewas monitored with reflectedJightoptics, with back- of Anthropology at the American Museum of Natural History. scattered-electronscanning, and with an audio spectrometersig- One specimenwith emerald-greencolor has been analyzedfrom nal to determine locations for phase analysis.After probe anal- a field study of Guatemalanjadeitites made by the authors. All ysis, the artifact was cleanedby bufrng with a Kimwipe soaked specimensare listed in Table 1. Doubly polished thin sections ln water or acetone. were made from all samplesexcept for those from the anthro- pological collections. Sections were examined petrographically under the polarizingJight microscope. Artifacts were observed PnrnocupHy AND MTNERAL coMposrrroNs with an incidentJight binocular microscope since no modifica- Burmese samples tion of the artifacts was permitted. jades pyrox- Chemical compositions were determined with an ARL-sEMe Dark emerald-green from Burma contain jades microprobe using natural and sy'ntheticsilicate and oxide stan- ene grains with up to 83 molo/o Ur. The fall into dards. Analyses were obtained at 15 kV, 10-nA beam current, two groups with some overlap; the first consists of mostly and 20 s counting time for peaks and background. The matrix pyroxene-bearing rocks, and the second consists ofmost- correction technique ofBence and Albee (1968) was employed. ly amphibole-bearing rocks. t28 HARLOW AND OLDS:TERRESTRIAL UREYITE AND UREYITIC PYROXENE Fig. l. Micrographs of textures in ureyitic jadeitites from the jade nines, Burma. All are in plane-polarizedlight with a I -mm scalebar unlessotherwise noted. (a) A fractured, corroded chromite (black) with surrounding ureyite (dark gray) in a matrix ofless ureyitic omphacite (sample 16853).(b)
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