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 ubiquitous 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:Mogaung region 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 glue to 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) A chromite with a narrow ureyite rim is intersectedby a fracture (horizontal) producing a large zone of ureyite replacement(sample 98642). (c) A large zoned ureyitic omphacite crystal with partially skeletal terminations (black portion is chromite with very thin ureyite rim; cross-polarizedlight; sample 98642). (d) Subparallelto wheat-sheafaggregates of acicular ureyitic jadeite (sample I 6853).

Table2. Representativemicroprobe analyses of ureyite-bearingpyroxenes

10 11 12 13 14 Weightpercent oxides sio, 52.3 55 0 53.9 56.1 58.4 56.3 54.5 55.3 53.7 55.0 53.8 51.9 54.4 56.0 Tio, 0.02 0.17 0 08 0 13 0.01 0.43 0 00 0.11 0 02 0.06 0.02 0.08 0.00 Al,o3 4.01 9.63 6.26 163 21.7 9.77 4.19 8.22 4.08 s.80 5.24 5.50 4.08 Cr,O. 27.6 110 24.4 10.9 0.57 1.72 24.5 18.4 26.1 3.71 24.7 3.59 2.33 22.6 FeO 0.91 3.23 1.09 1.0 0.76 4.06 1.80 2.24 1.82 4.96 1.03 1.97 1.97 0.4' MnO 0.04 0.07 0.08 0.06 0.01 0.06 0.02 0.00 0.04 0.00 0.10 0.04 0.02 Mgo 0.55 4.21 0.44 0.52 1.58 7.14 0.18 0.68 0.38 9.08 0.99 12.1 14.O 5.4 CaO 0.83 5.97 0.57 0.57 2 41 10.0 0.34 0.72 0.48 13.7 1.44 21.2 20.9 3.7 Naro 13.3 11.2 13.9 14.7 14.0 8.61 13.6 14.8 13.8 6.35 12.9 3.0 2.70 11.6 Total 99.6 100.5 100.8 100.3 99.4 98.1 99.1 100.5 100.5 98.74 100.3 99.4 100.3 99.7 Cationsfor six oxygens Si 1.96 1.98 1.98 1.97 2.00 2.O4 2.04 2 01 1.99 2.02 1.98 1.91 1.96 2.07 Ti 0.001 0.005 0.002 0.003 0.00 0.012 0.00 0.003 0.001 0.002 0.001 0.002 0.00 AI 0.18 0.41 o.27 0.68 0.88 0.42 0.18 0.35 0.18 0.25 0.23 0.24 0.17 o.82 0 31 o 71 0.30 0.015 0.049 0.72 0.53 0.77 0.11 0.72 0.10 0.066 0.66 Fe 0.03 0.10 0.033 0.031 0.022 0j2 0.056 0.068 0.056 0.15 0.032 0.060 0.059 0.01 Mn 0.001 0.002 0.003 0.002 0.00 0.002 0.00 0.00 0 001 0.00 0 003 0.001 0.001 Mg 0.03 0.23 0.024 0.027 0.081 0 39 0.010 0.037 0.021 0.50 0.055 0.66 o.75 0.30 Ca 0.03 0.23 0.022 0.022 0.088 0.39 0.013 0.028 0.019 0.54 0.057 0.84 0.81 0.15 Na 0.97 0.78 0.99 0.99 0.93 0.61 0.98 1.04 0.99 0.45 0.93 0.21 0.19 0.83 Total 4.O2 4.04 4.03 4.03 4.O2 4.02 4.00 4.07 4.03 4.02 4.00 4.03 4.01 4.02 Nofe. Columns are as follows: (1) Ureyite, 98642, Burma. (2) Ureyitic omphacite, 98642, Burma. (3) Jadeitic ureyite, 32734, Burma. (4) Ureyitic jadeite,32734, Burma. (5) Jadeite,R307G1, Burma. (6) Omphacite,R3076-1, Burma. (7) Ureyite,R3239, Burma.(8) Jadeiticureyite, 104671 (China?). (9) Ureyite,R8802, ltaly. (10) Ureyiticomphacite, MVJ-42-4, Guatemala. (11) Jadeiticureyite, 106891, Olmec bead. (12) Ureyiticdiopside, 145011, California.(1 3) Diopside,14501 1 , California.(14) Ureyite,Toluca octahedrite (Frondel and Klein,1965). - Fe determinedas Feros for this analysis. HARLOW AND OLDS: TERRESTRIAL UREYITE AND UREYITIC PYROXENE 129

\roxene-rich rock. These rocks are dark green with black specksor blotches. They contain mainly pyroxene with a subordinateamount of chromite (< l0o/0)and light- colored veins of Na- and Mg-bearing amphibole (-20o/o). Minor veins of glaucophane,serpentine, chlorite, and in one casechlorite with banalsite (seeTable 5) are present. The rocks generally have a massive unequilibrated texture with frequent shearzones consisting ofvariable-sized grains of pyroxene and other phases. Shear zones are bounded by deformed pyroxene showing undulose extinction, but virtually all crystals are slightly deformed, not showing complete extinction in one orientation. Un- like earlierdescriptions (e.g., Lacroix, 1930),we did not find nepheline, epidote, or qvartz; even albite is rare. Ureyitic pyroxene occurs in severaldistinct types. The most Ur-rich pyroxene (reaching 83 molo/o)exists in fine radiating coronal eggregatesup to 0.5 mm thick Di+Hd surrounding and invading corroded chromites or as con- centric bands enclosing chromite (see Fig. la). Ureyitic pyroxeneappears to replacechromite grains, particularly where veins and shearzones intersect the chromites (Fig. lb). The highest Crl(Cr + Al) ratio of these pyroxene grains correlateswith the adjacent chromite (seeFig. 2). Most pyroxene grains have much lower Ur content (from 2 to 30 molo/o),and the range of composition for different grains in a single specimen may vary from a nearly binary Jd fiadeite) and Ur solid solution to a qua- ternary solid solution with the addition of usually less Fig. 2. Molar-componentternary plots (Di + Hd, Ur, Jd than 20 mol0/oDi (diopside) and smaller amounts of Ac projectedfrom Ac) ofureyitic pyroxenecompositions from dif- (acmite)(see Table 2 and Fig. 2). Larger ureyitic pyroxene ferentsources. Dashed lines indicate highest Crl(Cr + Al) for grains range from euhedral, including skeletal (see Fig. chromitein samples.(a) All Burmesesamples; 98642 (X), all lc), to anhedral and are 100 pm to over 5 mm in length. others(tr). (b) Italian sampleR8802. (c) Mexican(tr) and Gua- They range from homogeneousto erratically inhomoge- temalanartifacts (+). (d) Guatemalansamples. (e) Californian sample14501l. neous in composition, though frequently compositional zoning follows growth bands of original euhedral mor- phology that is usually absent in the final grain. Some large crystals in several samplesshow corroded interiors broken-up parallel to octahedral faces.Grain-size varies or lathlike structure (parallel to c) that appearsto be the from micrometer-sized particles mixed with ureyite to processof reaction to becomeamphibole, 35 srrggestedby >4-mm, partially fragmentedcrystals. Chromites may or Yang (1984). Smaller crystalsare manifestedas radiating may not show a reaction with ureyitic pyroxene. When clustersor "wheat sheafs" or as shearedzones. In many the reaction texture is absent, the associatedpyroxene is cases,colorless (in thin section)jadeite is juxtaposed as typical of the coarsergrains. Chromite compositions lie matrix next to green, euhedral, Ur-rich pyroxene grains. very close to the chromite end of the magnesiochromite- Some samples show several generations of pyroxene chromite join, containing almost no Fe3+(inferred from growth, starting with older large jadeite grains, to inter- stoichiometry), very little Mg and unusually high Mn and mediate "wheat sheafs" of ureyitic blades, to the ureyite Zn (see Table 3). The Fel(Fe + MC) and Crl(Cr + Al) growths around chromite. ratios range from upper limits for "alpine-type ultramaf- Sample 98642 stands out as being the only dark-green ic" and "stratiform" chromites toward pure endmember one that is substantially calcic, being primarily ureyitic chromite (e.g.,see Irvine, 1967). omphacite with lesser jadeite-ureyite; other specimens Amphibole rock. Sodic amphibole is ubiquitous in Bur- show occasional zones of omphacitic pyroxene (e.g., mese emerald-greenjades and is sometimes the major 32734,97328,I 8653,and I 04982)but fall into the main constituent (sample 35024 is primarily amphibole). La- categoryof being mostly jadeite-ureyite. Severalsamples croix (1930) attributed the name szechenyite(after Kren- with less-intensegreen color and no ureyite per se (e.g., ner) to this amphibole. The most common variety is eck- R3076-1, 94784), show greatersolid solution of Ur in ermannite, but compositions rangethrough solid solution omphacite than in coexistingjadeite. to glaucophane, nyboite, richterite, and magnesio-alu- Most chromites are fractured and corroded and show mino-katophorite (Leake, 1978). The amphibole can be little or no crystal form, though some appear to have slightly to markedly greenin thin section and contains as 130 HARLOW AND OLDS: TERRESTRIAL UREYITE AND UREYITIC PYROXENE

Table 3. Representativemicroprobe analyses of chromites

Weightpercent oxides sio, 0.63 0.13 0 40 0.53 0.15 0.03 7.30 7.23 Tio, o 02 0 03 0.01 0.004 0.26 0.25 0.008 0.00 At,o3 1675 9.46 4.83 4.71 11.44 12.01 3.94 3.98 Cr,O" 46-1 54.9 59.5 59.3 51.9 50.9 56.0 56.3 V.Ou o.o4 0.02 0.03 0.o2 0.05 0.04 0.03 0.02 FeO 24.0 22.6 25.9 26.7 24 6 25.8 23.0 23.0 MnO 4.02 6.24 374 3.79 0.20 0.18 4.19 4.32 MgO 3 53 278 1 23 1.52 11.46 10.71 1.10 1.11 CaO 0 08 0 07 018 0.04 0.22 0.19 0j2 0.11 ZnO 5.05 3.45 2.39 1.53 0j2 0.12 1.12 1.10 Na"O 0.35 0.26 2.43 2.45 Total 100.15 99.63 98.57 98.36 100.37 100.18 99.2 99.63 Cationsfor 32 oxygens Si 0.17 004 0117 0.16 0.04 0.01 2.02 1.99 Ti 0.004 0.007 0.002 0.001 0.052 0.05 0.002 0.00 AI 5.35 3.15 1.67 1.63 3.58 3.78 1.28 1.29 Cr 9.87 12.28 1382 13.76 10.88 10.72 12.25 12.27 0.008 0.004 0.01 0.006 0 011 0.01 0.006 0.003 Fe 544 5 35 6.37 6.56 5.45 5.75 5.31 5.30 Mn 0.92 1.50 0.93 0.94 0.04 0.04 0.98 1.01 Mg 1.43 1.17 0.54 0.66 4.53 4.26 0.45 0.46 Ca 0.o24 0.02 0.057 0.014 0.06 0.06 0.035 0.033 Zn 1.01 0.72 0.52 0.33 0.02 0.02 0.23 0.22 Na 020 0.15 1.30 1.31 I oral 24.21 24.24 24.23 24.22 24.67 24.69 23.87 23.88

/Vofe;Columns are as follows:(1 , 2) Chromitein ureyiticOlmec bead, 106891.(3, 4) Chromitein ureyiticomphacite, 98642, Burma.(5, 6) Chromitein diopsiderock, 145011, California-(7, 8) Chromite contaminatedwith ureyite in jadeitite, R8802, Mocchie, Susa, ltaly.

Table4. Representativemicroprobe analyses of amphibole much as -7 wto/oCrrOr, Cr replacing octahedral Al (see and chlorite Table 4). In hand specimen, these rocks vary in color from dark gray-greenwhere grains are large to a chalky emerald-greenwhere grains are small. Eckermannitemay Wdght percent oxides mm long surrounded by sio, 58.4 48.8 53.8 57.0 56.4 59.6 30.1 occur as single crystals over 6 Tio, 0.2s 0.02 0.04 0.19 0.04 0.01 0.00 pyroxene or intergrown in a massive texture enveloping Alro3 11.2 10.9 6.43 6.00 6 53 9.08 21.0 irregular clots or grains (<10-pm diameter) of ureyitic CrrO. 2.42 6 61 2.O4 1.42 0.18 4.63 1.7 FeO 2.65 5 08 4.32 3.s1 1.62 1.19 0.87 pyroxene. Besides the larger crystals, sodic amphibole MnO 0.02 0.02 0.00 0.02 0.62 0.11 0.09 often occurs as anhedral grains or as a fine fibrous mesh, Mgo 12.9 13.7 17.5 18 2 19.9 14.6 30.9 approaching the texture of nephrite, particularly along CaO 0.34 3.8 5.26 1.30 2.65 0.52 0.13 Naro 9.91 I 14 8.49 10.8 8.86 8.85 0.O2 shearedzones. KrO 0.24 0.20 1.00 Total 98.1 983 98.0 98.5 979 98.6 95.2 Cationsfor 23 O (amphiboles)or 28 O (chlorite) Italian occurrence si 7.84 6.91 7.50 7.78 7.71 7 94 5.76 SampleR8802 is a small dark-greenrock that contains 'Al 0.16 1.09 0.50 0.22 0.29 0.06 2.24 ri 0.03 0.00 0.004 0.02 0.00 0.00 0.00 in addition to Ur-rich pyroxene (up to 87 molo/oIJr, see ,Al 1.61 0.73 0.55 0.75 0.76 1.36 2.50 Table 2), chromite, glaucophane,chlorite, and accessory Cr 0.26 O.74 O.22 0.15 O.02 0.49 0.26 sphalerite.The high-Ur pyroxene occurs as lenses(up to Fe 0.30 0.60 0.50 0 40 0.19 0.13 0.14 Mn 0.00 0.00 0.00 0.00 0.07 0.01 0.01 3 mm wide) of micrometer-sizedgrains that appearto be Mg 2.59 2.89 3.62 37O 4.06 2.89 8.82 a replacementof chromite (Fig. 3). Chromites are so cor- SumC 4.78 4.96 4.91 5.02 5.10 4.89 grains Ca 0.05 0.58 0.79 0.19 0.39 0.08 0.03 roded that individual are too small to obtain ac- Na" 2.'17 1.47 1.31 1.79 1.51 2.03 0.01 curate analyses(see Table 3). The ureyitic lensesoccur in SumB 2.22 2.04 2.09 2.00 2.00 2.11 rock that contains less ureyitic jadeite and glaucophane. Na^ 0.41 1.04 0.99 1.05 0.83 0.25 K - 0 04 0.04 0.02 0.'t7 Pyroxene compositions are low in Ca but occasionally Total 15.41 16.08 16.03 16.07 16.10 15.25 19.77 acmitic, up to 26 molo/o(see Fig. 2). Large pyroxenegrains Note.'Columnsare as follows:(1) Chromianglaucophane, 37970, Bur- severalmillimeters long are dissectedand separatedinto ma. (2) Chromian nyboite, 32734, Burma. (3) Magnesio-aluminokato- 40-200-pm fragments.Locally the groundmasspyroxene phorite-eckermannite,32734, Burma.(4) Eckermannite,104671, China (?). (5) Eckermannite,R3076-1, Burma. (6) Chromianglaucophane, and amphibole fragments are enveloped with chlorite. R8802,Mocchie, Susa, ltaly.(7) Chromianclinochlore, R8802, Mocchie, Sphalerite occurs as small grains interspersed in the Susa,ltaly. groundmass. HARLOWAND OLDS:TERRESTRIAL UREYITE AND UREYITIC PYROXENE t3r

California occurrence A chromite-bearing rock from Williams Creek, Cali- fornia (14501l), consistsprimarily of diopsidethat contains small areaswith up to l0 molo/oUr. Ureyitic zones are always adjacent to the chromites and grade to almost pure diopside no more than 50 pm away from the chromite. Jd is a minor component in the pyroxene and ap- pears to be correlated with Ur, concentrated up to 20 molo/oin the Ur-rich zones (Fig. 2). The most ureyitic analysis also shows significant apparent tetrahedral A| whereasother analysesshow little or none (seeTable 2). The rock has a brecciated,vein-filled texture, but is nearly monomineralic, consistingof fine-grainedpyroxene laths of approximate dimensions 2 x 20 pm, interwoven in a Fig. 3. Micrographof lens of fine-grainedrelict chromite, felted texture. Some grains are much larger, x up to 40 largelyreplaced by ureyitein sampleNMNH R8802,Mocchie, pm, 800 and these larger grains are occasionallyaligned Susa,Italy. Scalebar is 1 mm. in vein-filling swaths.The chromites have compositions typical of alpine-typeultramafic chromites,containing less Cr, more Al, and much more Mg than the Burmesechro- mites describedabove. Chromite grains are equant, roughly Guatemalanoccurrence 0.7 mm across,and are severelybroken up. There is also In contrast to Burmesejadeitites, the Guatemalanjade- percentage a small of euhedral pyrite grains, usually less itites examined to date contain no ureyite, but two spec- pm than l0 but up to 200 pm across.Although Ur-bearing imens have been found that have a IJr component vary- pyroxene and chromites are in closecontact, the reaction ing up to 36 molo/o(Fig.2), which is consistentwith their texture seenin Burmesesamples is absent. pale to noticeably emerald-greencolor in hand specimen (details of the jadeitites are discussedelsewhere; Harlow, Mexican and Central American artifacts in prep.). Pyroxene compositions for the two specimens Of six small artifacts with obvious emerald-greencolor are quite different-one is nearly pure Jd, the other is all and easily measurablefJr content, two contain very urey- Omp. The primarily jadeite rock is reasonablytypical of itic pyroxene (Uro, and IJr, max., see Tables I and 2). Guatemalanjadeitites (McBirney et al., 19671,Harlow, in Since thin sectionscould not be made from archaeolog- prep.) in that is contains variable-sizedjadeite grains (up ical collection items, these were characterized only by to 6 mm long), some showing subhedralto anhedral out- microprobe analysisof a portion of their surfaces.Micro- lines with internal growth zoning. Paragonite (Table 5) probe analysis and backscatteredimages indicate that shows up as a minor (- 100/o)primary phaserimmed by theseartifacts are all predominantly jadeitic to omphac- a very sodic nepheline-like phase, whereas albite, anal- itic pyroxene (=900/o),with emerald-greenspots showing cime, and omphacite appearas secondaryminerals along 2 to 42 molo/oUr (Fig. 2), and minor albite and/or white grain boundariesand fractures and as small inclusions in mica (Table l). The Olmec bead (sample 106891),for heavily included, anhedral pyroxene.The ureyitic regions which we were able to obtain a thin section, consists of are minor and extremely pale green; they appear to be about 800/ojadeitic pyroxene, with accessoryphengitic correlatedwith late-stagecalcic (and more acmitic) zones muscovite (- 150/o),chromite, and ureyite. Large subhed- of the pyroxene growth. The omphacitic sample consists ral interlocking pyroxene grains (about I mm long) are of -95o/o intergrown lathy pyroxene up to 2 mm long pervasively fractured, often along cleavage surfaces, with minor broken or corroded grains of chromite and probably the result ofthe lapidary work done on the bead. some intergranular albite and chlorite. Ureyitic compo- Smaller euhedral to subhedralphengite grains (about 0.5 sitions are particularly associated with chromite (but mm across) are interspersed throughout the pyroxene. without any visible reaction zone) and often follow inter- Anhedral mica sometimes invades pyroxene. Emerald- nal euhedraloutlines within larger pyroxenecrystals. The green spots (about 0.2 mm across)with up to 72 molo/o ureyitic omphacite again shows an enrichment of Ac (up Ur enclosesmaller corroded chromites and exhibit nar- to Ac,o) relative to more jadeitic zones (Fig. 2, Table 2). row zonesof reaction texture. The chromites have com- Chromite compositions are similar to those from Bur- positionssimilar to thoseof the Burmesespecimens, being mese samples(Harlow and Barry, in prep.). very rich in Cr and Fe2* (seeTable 3). Combined pyroxene analysesdefine two compositional trends, one from DrscussroN Jd to Omp (omphacite)with minor Ur and the other from Ur toward the Omp-Jd field (seeFig.2). Among analyses Pyroxene crystal chemistry in the first trend, Ac content is correlated with Ur for All of the pyroxene with significant Ur content can be more omphacitic compositions, reaching about Acr. describedin terms of a solid solution of Ur, Jd, Di, Ac, 132 HARLOW AND OLDS: TERRESTRIAL UREYITE AND UREYITIC PYROXENE

Table 5. Representativemicroprobe dnalyses of micas and omphacitic compositions.The presenceof ureyitic om- feldspar phacite is very interesting,though not totally new (e.g., Sobolevet al., 1975).Experimental studies on the Di-Ur join show only minor to intermediatesolid solution of Weightpercent oxides 24 wto/oUr in diopside sio? 488 44.7 543 36.0 68.5 70.3 Ur in diopside-a maximum of Tio, 0.04 0.08 0 00 0.00 0.00 0.00 at 1 atm and I150'C (dry), decreasingto 13 wto/oat 20 Alro3 40.6 38.1 21.6 3144 19.6 18.8 kbar (Ikedaand Yagi, 1972).Under hydrousconditions, FeO O.27 0.34 0.43 0.07 0.00 0.03 MgO 0.10 0.41 s.98 0.04 0.00 0.00 the liquidus descends,and only l5 wto/oUr was found at CaO 0.15 0.15 0.02 0.07 0.02 0 11 850'C and 20 kbar (Vredevoogdand Forbes,1975). Nei- NarO 6.27 7.80 0.08 9.84 126 115 ther set of experimentsdiscovered a Ur-rich omphacitic K,O 0.58 0.38 10.5 0.00 0.00 0 04 BaO 0.10 0 47 20.2 0.00 0.00 phase,and only 2 wto/oDi was found in ureyite. The nat- Total 97.0 92j 93.s 97.8 1008 100.7 ural samplesare consistentwith little solubility of Ur in Cationsfor 22 O (micas) diopside(e.g., sample 145011from California)but show or 8 O (banalsiteand albite) substantialsolid solution of Ur into an omphacite-chlo- si 609 5.93 7 33 1.98 2.98 3.03 pyroxene, Ti 0 003 0.008 0.000 0 001 0.000 0.00 romelanite-like at least to 30 molO/oUr at Di.r. At 5.97 5.95 3.43 2.O4 1.01 0.96 Two Guatemalan samplesand some Mexican artifacts show Fe 0.028 0 036 0.05 0.003 0.000 0.001 trends of ureyite solution toward omphacite, whereasthe Mg 0.018 0 082 1.20 0.003 0.000 0.00 Ca 0.020 0 021 0.003 0.004 0.002 0.003 Burmese data additionally show a large cluster of com- Na 1.52 2 01 0.020 1.05 1 06 0.97 positionsin the middle of the Jd-Di-Ur field (dominated K 0.092 0.063 1.81 0.000 0 000 0 002 by sample 98642; seeFig. 2). The omphacite-jadeitegap, Ba 0.005 0 025 0.44 0.000 0.000 Total 13.73 14.0't 1395 5.52 5 05 4.97 which is obvious in both Burmeseand Guatemalan sample data, is tenuousfor more ureyitic compositions,par- Note:Columns are as tollows:(1) paragonite,391073 Olmec bead, La A number of samples Venta (2) paragonite,R-15, Guatemala. (3) phengiticmuscpvite, 106891 ticularly for the Mexican artifacts. Olmecbead, Xalitla. (4) banalsite,32734, Burma. (5) albite,94784, Bur- (e.g.,R3076-1,94'784, 104982,30i 10818) show greater ma. (6) albite,391073 ring, Chichen ltza Ur content in omphacitethan in coexistingjadeite, though this correlation'alsoexists for Ac, perhapsbeing simply a crystallization trend. However, this increasedsolubility may be related, in a simple-minded way, to the unit-cell and occasionallyHd (hedenbergite),as determined from volume of ureyite being more comparable to omphacite cation and charge(sometimes inferred) balancesfrom mi- (both ordered and disordered forms) than to jadeite (or croprobe analyses.In particular, Ca + Na cations usually diopside). Consideringthe relationship ofordering to the sum very close to I per 6 oxygens,and a total chargeof stability of omphacite, an interesting problem in the site 4 on Ml + M2 sitesis maintainedby Cr + Al + Fe - topology for the solid solution with ureyite (and acmite) (Ca - Mg) charge-balancingNa, leaving residual Fe2+ * is raised. Mg to balanceCa. Ti content is generallyvery small and can be described by a substitution such as Ca * Al : Na + Ti. There is alwaysenough Di component in jadeitic Paragenesis pyroxeneto allow this exchange,and this result is similar In meteorites, ureyite is presumed to have crystallized to the interpretation ofGasparik (1985), although no evi- from a very sodic silicate melt in equilibrium with me' dence for a tr * Ti substitution (l is a vacancy) was tallic iron (Frondeland Klein, 1965).Experimental work noticed. While Si cations frequently sum to less than 2 on pressure dependenceof Jd-Ur solid solution (Abs- per 6 oxygens, there is no indication that Ti or Al is Wurmbach and Neuhaus,1976)is consistentwith crys- replacing Si except in the case of a diopside from Cali- tallization of ureyite at very low pressuresin the metallic fornia (see section below). In some analyses,Si signifi- meteorites. Even though ureyite coexists with albite in cantly exceeds2 cations, but in these cases,Ac is usually meteorites,it containsno Jd component. inferred to be high such that correction for increasedoxy- In terrestrialrocks, the textural association-i.e., cor- gen from FerO, yields a reduction of total cations and Si. roded chromites surrounded by fine-grained ureyite in Some authors have reported significant deviations for most ureyite-bearingsamples-suggests a reaction in cation totals in jadeitic pyroxene(e.g., Carpenter, 1979); which chromite is consumed and ureyite produced. All however, we did not observe significant cation deflcien- describedjadeititesexcept for the Polar Urals occurrence cies,though some very fine grained mixtures of pyroxene are hosted in serpentinizedultramafic rock in highly tec- and amphibole produced cation totals similar to pyrox- tonized terranes.The Polar Urals occurrencemay be an ene but with high Mg content. exception; here jadeitites and albitites are contained in The total compositional range can be seenin Figure 2. serpentinepyroxenite bands that are part of a system of The data support complete miscibility between jadeite ultramafics, some of which are unaltered peridotites and ureyite, as shown experimentally by Abs-Wurmbach (Morkovkina, 1960;Dobretsov and Ponomareva,1965). and Neuhaus (1976), as well as some extension toward Chromitites also commonly occur in serpentinitesand in HARLOW AND OLDS: TERRESTRIAL UREYITE AND UREYITIC PYROXENE I JJ

Burma are describedas adjacent to jadeitites (Chhibber, Similar reactions can be written for the Fe2* endmem- I 934). Presumably, chromite-bearing ureyitic jadeitites bers.Though albite is often associatedwith jadeilites,we incorporated their chromite from surrounding serpentin- do not observe albite in association with ureyite-chro- ite. Therefore serpentine (e.g., chrysotile) and chromite mite-bearing assemblageson a thin-section scale. Fur- are possible reactants. Similarly, albite, described by thermore, in our samplesit is always the case that [Crl Chhibber as a primary constituent of the albitite-jadeitite (Cr + Al)]"n,"-,,.= [Crl(Cr * Al)],"o"',.-"..vircwhere the re- "dikes" of which the chromiferousjadeitite-amphibolites action texture is observed,an observation consistentwith are part, is a likely reactant. However, it is impossible to the consumption of an aluminous phasesuch as albite in write a reaction for a closed system involving only as the production of ureyitic pyroxene. This seemsto rule major speciesureyite, jadeite, chromite, eckermannite, out Reaction 2 in which ureyite and albite occur on the albite, and chrysotile. same side. In Reactions 3 and 4, ureyite and the Na-rich In igneousand metamorphicrocks, Na and Al are highly phaseeckermannite appear on opposite sides,so that eck- correlated or "tied" to one another. If, in our jadeitites, ermannite needs to be accounted for independently of Na is decoupledfrom Al to make ureyite, the appearance ureyite. The presence of eckermannite independent of of a ironsodic aluminous phase is demanded. The only ureyite can only be accountedfor via reasonablecandidate for this aluminous phase is spinel (i.e., in solid solution with the chromites); hence,we can SNaAlSirO,* 5MgrSi,O,(OH)o+ 6Na* postulatea solid-stateexchange reaction in which jadeite ab serp and chromite react to form ureyite and hercynite. How- : 3Mg2++ 5NaAlSi,Ou ever, aside from this being unlikely crystal-chemically, to the chromites associatedwith ureyite are among the least * 3Na.MgoAlSisO,,(OH),+ 7HrO. (5) aluminous of naturally occurring chromites. Further- eck more, there is no evidencefor zoning (in particular, zoning with respectto Al) in these chromites. This completes the set of possible reactions given the Allowing for mobility of Na+, Fe2*, and Mg2*, in the aboveassumptions. Adding the appropriatemultiples of presence of "water," i.e.,in an open system,it is possible Reaction 5 to either Reactions 3 or 4 to cancel ecker- to write four reactionsinvolving ureyite for the Burmese mannite resultsin Reaction I (i.e., there are only three (considering assemblage only the Mg2* endmembers): independentreactions). The main point is that, given the observed speciesand assumingthe presenceof albite (or MgCrrOo + 4NaAlSi.O, * 2Na* other similar Na- and Al-bearingsilicates) and chrysotile, sp ab an influx of Na+ and escapeof Mg2* is required to explain the chromite-ureyite reaction. A hydrous fluid might fa- : Mg2++ 4NaAlSirOu* 2NaCrSirOu (1) cilitate sucha reaction and is consistentwith the presence jd ur of serpentinite,so long as the pressureis high enough to 5MgCrrOo* l2NarMgoAlSi8Orr(OH),+ 7Mg2* prevent the formation of analcime. sp eck + 28HrO Geologic implications : l4Na* + l2NaAlSi,O, + l0NaCrSirOu Metasomatism. By the above argument, the chromite- ab ur ureyite reaction and the presenceofeckermannite in Bur- + 20Mg,SirOr(OHL (2) mesesamples imply sodic-fluidmetasomatism or some se!p similar sodic-fluid-dominated reaction and crystallization process.Metasomatism is also supported by the ex- 2MgCr,Oo+ 3Na.MgAlSi8Orr(OH),* Mg,t + 7H,O istenceof monomineralicand bimineralic zonesof jade- sp eck ite and jadeite-amphibole. In addition, the restriction of : 2Na* + 3NaAlSirOu + 4NaCrSi,Ou jadeitites to serpentinites or serpentine-bearingbodies jd ur suggestsa connection betweenserpentinization andjade- + 5Mg.Si,O,(OH)o (3) itization. Although this is a much larger questionbeing serp addressedelsewhere (e.g., Harlow, in prep.; Olds, in prep.), the important implication of ureyite paragenesisis the 3MgCrrOo+ 3NarMgAlSi8O,,(OH),+ 4NaAlSirO, necessityof a sodic fluid in forming the jadeitites. sp eck ab P, T, and conditions of pyroxene crystallization. The + THrO conditions under which jadeitites form are not well con- : TNaAlSizOu* 6NaCrSirOu strained:Coleman (1961) estimated conditions of 2 kbar jd ur and 250'C for New Idria jadeitites;Dobretsov (1968) es- * 5Mg.SirOr(OH)". (4) timated conditions with pressuresgreater than l0 kbar serp and temperaturesnear 500'C for jadeitites in the Soviet t34 HARLOW AND OLDS: TERRESTRIAL UREYITE AND UREYITIC PYROXENE

100 to be done on jadeitites in general and the rocks from Guatemalaand Burma in particular. Phasediagrams have been worked out for simple closed systems of dry and 90 uuRr^n water-saturatedjadeite and albite compositions,but these cannot be easily applied to open fluid-rock systems.(The data for eckermanniteand the 80 absenceof thermodynamic lack of models for aqueousionic activities at very high pressuresmay preclude a rigorous thermodynamic treat- 70 ment of the above reaction system.)We do know, however, that ureyite can be crystallized from a silicate melt at 1-atm pressure.This observation and limited experi- 60 mental work (Abs-Wurmbachand Neuhaus,1976) sug- GUAT E T L gest that ureyite is stable at lower pressurescompared to jadeite over a range of reasonabletemperatures. Hence 50 the presenceof ureyite in jadeitites probably places no additional restrictions on required P-lt conditions for jadeitite pro- 40 petrogenesis(whatever those may be for the posedfluid-rock system). The presenceof omphacite adds a neededextra dimen- 30 sion to the pyroxene complexity for determining crystallization and annealingconditions. The jadeite-omphacite gap in Burmesejadeitite pyroxene suggestsa crystalliza- 20 tion temperature below the crest of the miscibility gap (solvus) in the Jd-Di binary field. Carpenter and Smith (1981) have placed this between 500 and 600'C, at an 10 unspecifiedpressure probably from l0 to 20 kbar, with an Ac content of between 3 and l2o/o,comparable to the o Burmese values. Crystallization temperatures would be 100 60 40 20 well below this value, appropriately corrected for pressure, considering the measurablesize of the gap. Alter- natively, in a fluid-dominatedenvironment, a changeof crystallization conditions could causethe break in pyroxene compositions. The intimate intergrowth textures of pyroxenesin Burmesejadeitites makesthis latter possi- bility less likely than, for example, in the Guatemalan jadeitites where omphacite is an overgrowth and obviously a later phasethan jadeite. Crystallization stages.That Ur-rich pyroxeneoccurs in at least two distinct modes in many Burmese samples, i.e., as fine-grainedUr-rich pyroxeneaggregates radiating from chromites and as large euhedral crystals with less Fig. 4. Plotsofcation proportionsfor chromitesfrom three more quadrilateralpyroxene components, indi- ureyitesources (axes are selfexplanatory). Large open box and Ur and diamondrepresent general cluster of compositionsfor Burmese catesthe occurrenceof at least two Ur-forming events or and Italian samples,respectively. Enclosed regions denote the a seriesof crystallization steps.The ureyite-chromite re- normalrange of compositionsfrom alpine-typeultramafic bod- action texture must have been formed latest simply be- ies(Irvine, 1967). causeit is still there and did not have time to recrystallize and homogenizewith the rest of the pyroxene. The crys- tallizationofnearly purejadeite seenin rockscontaining Union. Carpenter(1981) suggesteda peak P and Z of both modes of ureyite may or may not have occurred 350-400'C and 6-8 kbar for Guatemalan occurrences simultaneously with a Ur-producing event. If so, ureyite basedon lawsonitein associatedrocks and experimental would have formed at conditions within the stability field estimates on the low-pressure breakdown of jadeite to ofjadeite, a possibilityconsistent with the abovediscus- form albite + nepheline.Actually, the lawsonite- and sion on P and T. The associationof ureyitic compositions jadeite-bearinginclusions occur in distinctly different ser- to acmitic omphacite in some artifacts and Guatemalan pentinites, separatedby the Motagua fault (a misinter- samplessuggests that the ureyite formed later than jadeite pretation of McBirney et al., 1967), so the former con- (ladeite often has omphacite overgrowths), but it is not straint is not appropriate.Obviously, more work needs yet clear why this should be. HARLOW AND OLDS: TERRESTRIAL UREYITE AND UREYITIC PYROXENE 135

Chromite compositions.The chromites associatedwith Guatemala:Composition limits of cation ordering. American all ureyitic samplesexcept 14501I from Californiafall in Mineralogist, 64, 102-108. - (198 l) Omphacite microstructures as time-temperature narrow composition range. Although it a similar and indicators of blueschist- and eclogite-faciesmetamorphism. seemsreasonable to assumethat these chromites came Contributions to Mineralogy and Petrology, 78,441-451. from adjacentultramafic rock, their compositions are not Carpenter,M.A., and Smith,D.C. (1981)Solid solutionand cat- within the range of normal podiform chromites from al- ion ordering limits in high temperature sodic pltoxenes from pine-type ultramafic bodies (Fig. 4; see Irvine, 1967). the Nybo eclogite pod, Norway. Mineralogical Magazine,44, 37-44. Either the chromites associatedwith ureyite are different Chhibber, H.L. (1934) The mineral resourcesof Burma. Mac- in origin from typical podiform chromites, or enrichment millan, London. in the chromite endmember has occurred by some later Couper,A.G., Hey, M.H., and Hutchison, R. (1981) Cosmo- process(perhaps related to jadeitization or ureyite for- chlore: A new examination. Mineralogical Magazine, 4,265- 267. mation).This subjectis beingconsidered more complete- Coleman,R.G. (1961)Jadeite deposits of the Clear Creekarea, ly in Harlow and Barry (in prep.). New Idria district, San Benito County, California. Journal of Petrology,2, 209-247. Deer,W.A., Howie, R.A., and Zussman,J. (1978)Rock-forming minerals, vol. 2A. Single-chainsilicates. Wiley, New York. CoNcr,usroNs Dobretsov,N.L. (1968) Miscibility limits and mean composi- jadeite We have presentedfurther data and documentation on tions of pyroxenes. Doklady Akademia Nauk SSSR, pyrox- 146,118-120. the terrestrial occurrenceof ureyite and Ur-rich Dobretsov,N.L., and Ponomareva,L.G. (1965) Comparative enes. The occurrencehas been expanded from the jade characteristicsofjadeite and associatedrocks from Polar Ural mines of Burma to Mocchie.Susa. in the westernItalian and Prebalkhashregion. Akademia Nauk SSSR,Trudy Insti- Alps; a sourceof Mesoamericanjade; and the Motagua tut Geologiya i Geofizika, no. 31, 178-243 (transl. Interna- Valley of Guatemala (the latter two possibly being relat- tional GeologyReview, 10, 221-279). Essene,8.J., and Fyfe, W.S. (1967) Omphacitein Californian ed or the same). Ureyite occurs in solid solution with metamorphic rocks. Contributions to Mineralogy and Petrol- jadeite, as shown by Yang (1984), and with omphacite ogy,15, l-23. and to a lesserextent diopside. The reaction textures be- Frondel, C., and Klein, C. (1965) Ureyite, NaCrSi'Ou:A new tween ureyite and chromite within jadeitite demonstrate meteoritic pyroxene. Science,149, 742. (1985) Titanium in ureyite: A substitution with produced Gasparik, T. that ureyite is at the expenseof chromite and vacancy.Geochimica et Cosmochimica Acta, 49,1277-1280. requiresan outsidesource ofNa*, most probablya meta- Gubelin,E.J. (1965) Maw-Sit-Sit proves to bejade-albite. Gems somatic fluid. The juxtaposition of chromite with jade- and Gemology,I l, 302-308. itite is probably the product of tectonic activity mixing Ikeda, K., and Yagi, K. (1972) Synthesis of kosmochlor and phase join Contri- adjacentserpentinite and jadeitite blocks. equilibria in the CaMgSirOu-NaCrSi'O.. butions to Mineralogy and Petrology, 36,63-72. Irvine, T.N. (1967) Chromian spinel as a petrogeneticindicator, Part 2. Petrologicapplications. Canadian Journal ofEarth Sci- AcxNowr,pncMENTS ences.4.7l-103. Thanksto BobColeman and Juhn Liou for reviewingan early Lacroix, A. (1930) La jadeite de Birmanie: ks roches qu'elle versionofthe manuscript,to Jim Shigleyand J. AlexanderSpeer constitueou qui I'accompagnent.Composition et origine. Bul- for critical reviews,to BruceBathurst for use of his reaction letin de la Soci6t6Frangaise de Min6ralogie, 53, 216-264. generatingprog.ram REAcrApL, to JosephSataloff, Mary Lou Ry- Laspeyres,H. (1897) Mittheilungen aus dem mineralogischen dinger,the British GeologicalSurvey, the Departmentsof An- Museum der Universitiit Bonn. VII. Theil. Zeitschrift fiir thropologyand Mineral Sciences at the SmithsonianInstitution, Krystallographie und Mineral ogie,27, 58 6-600. and the AmericanMuseum of Natural History for generously Leake, B. (1978) Nomenclature of amphiboles. American Min- providingsamples and to the Instituto Geogr6ficoMilitar of eralogist.63. 1023-1052. Guatemalafor assistanceand supportduring field studiesin Manson, D.V. (1979) Recent activities in GIA's researchde- Guatemala. partment; clarification of composition of Maw-sit-sit. Gems and Gemology,16, 217-218. McBirney, A.R. (1963) Geologyof part of the central Guatemala cordillera. University of California Publications in Geology Series,38, no.4, l'77-242. RrrnnnNcrs McBirney,A.R., Aoki, K.-I., and Bass,M. (1967)Eclogites and Abs-Wurmbach,I., and Neuhaus,A. (1976)Das SystemNa- jadeite from the Motagua fault zone, Guatemala. American Alsi,o6 (JadeitFNacrsi,Ou(Kosmochlor) im Druckbereich Mineralogist,52, 908-918. von I bar bis 25 kbar bei 800'C.Neues Jahrbuch fiir Miner- Mevel, C., and Kienast,J.R. (1980)Chromian jadeite, phengite, alogieAbhandlungen, 127, 213-241. pumpellyite, and lawsonitein a high-pressuremetamorphosed Bence,A.E., and Albee, A.L. (1968)Empirical correction factors gabbro from the French Alps. Mineralogical Magazine, 43, for the electronmicroanalysis of silicatesand oxides.Journal 979-984. of Geology.72. 382-403. Morkovkina, V.F. (1960) Jadeitites in the hyperbasitesof the Bonatti,E. (1976)Serpentinite protrusions in the oceaniccrust. Polar Urals. 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Chrome-rich clinopyroxenes from the kimberlites of Yakutia. Yang, C.M.O. (1984) Terrestrial source of ureyite. American Neues Jahrbuch fiir Mineralogie Abhandlungen, 123, 213-218, Mineralogist,69, I 180-1183. Vredevoogd,J.J., and Forbes,W.C. (1975) The systemdiopside- ureyite at 20 kb, Contributions to Mineralogy and Petrology, 52, t47-156. Webster, R. (1983) Gems, their sources,descriptions and iden- MaNuscnrrr RECETyEDJulv 29, 1985 tification. Archon Books, Hamden, Connecticut. M^q.NuscRrprAccEprED Ssprslrnnn 2, 1986