American Mineralogisl, Volume 66, pages 278-289, I98I
Carbonatesand pyroxenoidsfrom the manganesedeposit near Bald Knob, North Carolina'
Gnnv A. WrNrBn,2 ERrc J. ESSENEeNo Doxero R. Peecon Department of Geological Sciences,University of Michigan Ann Arbor, Michigan 48109
Abstract
Studiesof minslals from a metamorphosedmanganese deposit at Bald Knob, North Caro- lina, yield estimatesof temperaturesand local fluid compositionsat the peak of metamorph- ism. Electron microprobe analysesof Mn-Ca carbonatesdemonstrate that they span the rho- dochrosite-kutnahorite solvus without a break, suggesting metamorphic temperature >550oC. The adjacent lid-nmphibolite facies metamorphism implies temperaturesnot greatly exceedingthis value. Pressureis estimatedat 5tl kbar basedon nearby occurrences of aluminosilicates.Compositional data on coexistingrhodonite and pyroxmangite sriggests that a narrow miscibility gap separatesthe two phasesat theseP-Tconditions. The assem- blage rhodonite/pyroxmangite-rhodochrosite-quartz requires water-rich fluids during meta- , morphism. The common occurrencesof the accessorymins16fu cattierite (CoSr), alabandite (MnS), and pyrophanite (MnTiOr) at Bald Krob require a relatively reduced and sulfidizing local metamorphic environment. Occasional finds of rhodonitg,/pyroxmangite-alabandite- quartz and rhodonite/pyroxmangite-tephoite-alabanditerestrict fO2/fS2 to a narrow band between l0-'e/10-3 and l0-t6/100. Conditions were probably butrered throughout meta- morphism by reaction of a sulfur-rich, oxygen-poorprotolith.
Introduction sediment composed primarily of quartz, manganese The manganesedeposit near Bald Knob, North carbonates, sulfides, and oxides, which was sub- Carolina, is located in Alleghany County, in the sequently metamorphosedto yield the present assem- northwestregion of the state.Sparta, North Carolina, blages.This conclusion is supported by data showing fies about 3 miles southwest,and Bald Knob is a half that the temperature for the deposit is compatible mile northwest. The deposit is enclosed by Pre- with that inferred from the local metamorphic assem- cambrian-early Paleozoicamphibolites, misa schists, blages. and garnet-mica schistsof the Piedmont terrane. Two parageneses,carbonate-rich and quartz-rich, The deposit was first describedby Ross and Kerr exist at Bald Knob. The carbonate-rich rocks are prinarily (1932)who reported two new minerals, galaxite and now undersaturated in SiO, and consist of alleghanyite.More recently, Peacor et al. (1974) de- Mn-Ca carbonates, alleghanyite, sonolite, mangan- jacobsite, scribed another new mineral, kellyite, from this de- humite, tephroite, galaxite, and kellyite, posit. The idealized formulae of these minerals and with accessoryalabandite, cattierite, cobaltite, apa- pyrophanite. quartz-rich those mentioned below are given in Table l, along tite, caryopilite, and The py- with their structure types. Although Ross and Kerr rocks contain quartz, spessartine,rhodonite, ascribeda hypogenevein origin for the manganese- roxmangite, Mn-Ca carbonate, tirodite, with minor rich body, the depositprobably representsan original pyrophanite and galaxite.The rocks are banded,and the constituent grains show typical interlocking metamorphic textures. The banding results from con- tContribution No. 362 from the Mineralogical Laboratory, De- centrations of one or more of the phases,probably partment of Geological Sciences,University of Michigan, Ann Ar- bor, Michigan 48109. reflecting original sedimentary variations. These ob- 2Presentaddress: Tennessee Valley Authority, P.O. Box 2957, servations lend support for a metamorphic origin of Casper,Wyomng82602. the deposit, as opposed to the vein origin suggested w03-0o4x/ 8| /0304-0278$02.00 278 IryINTER ET AL.: CARBONATES AND PYROXENOIDS. NORTH CAROLINA
Table 1. Unusual Bald Knob minerals, their ideal formulae and at varioustemperatures at a total pressureof l0 kbar. structure types Their exploratory runs in the pressurerange 2 to 15 kbar indicate that the efect of pressure on the rela- l"1ineral Formula Stru cture tions is small, and in our study their results are as- sumedto be directly applicableto assemblageswhich equilibrated at -5 kbar. The solvus in the Mn-rich pyrophanite MnTi03 i lmenite part of the CaCOr-MnCO, system widens slightly galaxite MnAlr0O spinel with increasingMgCO, as it extendsinto the ternary jacobsite MnFe2 04 spinel system.At 600"C there is completesolid solution of alabandite MnS hal i te Ca-Mn-Mg carbonatesin the Ca-poorhalf of the sy- cattierite CoS, pyr i te tem for those compositionswith less than l7 mole cobaltite CoAsS arsenopyrite percent MgCOr. These systemsmay be used to esti- alleghanyite (MnrSi0O)rMn(0H,F),chondrodite mate the temperature at which natUral Ca-Mn car- manganhumite (Mnzsi04)3Mn(0H,F)2humite bonates equilibrated. Carbonates from the Bald sonoli te (Mn2si04)4Mn(0H,F)zcl inohumite Knob deposit were analyzed and compared to the tephroite Mn^Si0, ol ivine Z+ phase relations in order to restrict their temperature spessartine MnrAl i ga ,S ,0.,, rne! of equilibration. nhodonite MnSi0, rhodonite Exploratory X-ray diffraction studies of many pyroxma ng i te MnSi 03 pyroxmangite Bald Knob carbonatesshowed a wide range of Ca- ( kutna hori te CaMn COr ), dol omite Mn solid solution (Stein, written communication). ti rodi te MnrMSi (0H tremo US,0r, ), l i te Subsequentpartial analysesby electron microprobe yi ke11 te MnoAlOSir0.,O(0H), serpentine showed that Mn and Ca are the major cations in caryopi I i te MnUSiUOrU(0H)10* friedelite thesecarbonates, with Mg and Fe presentin only mi- nor amounts. Eleven thin sections with carbonate compositionsthat would span the general range of *Peacorand Essene(l 980) values found previously were chosenfor more com- plete analysis.A total of 67 grains from thesesections was analyzedon an ARL-EMXelectron microprobe at by Ross and Kerr. Our discussionis predicated on The University of Michigan. Details of the analytical the assumptionthat the Bald Knob deposit was in- proceduresare given in the appendix. deed equilibrated during the peak of regional meta- morphism. The experimental and thermodynamic data on manganesecarbonate, silicate, and sulfide Discussion systemsare applied to determine the metamorphic After converting the carbonate analyses to mole conditions at which the Bald Knob assemblages percent and extracting the carbonate components, formed. the carbonatesrange from CcrRcrr'to CcrrRc",with less than l07o (Mg,Fe)COr. These compositionsare Carbonatethermometry plotted on a ternary diagram (Fig. l), where FeCO, Quantitative electron microprobe analysesof car- is combinedwith MgCOr becausetheir effectson the bonateninerals from the Bald Knob depositsuggest solvusare approximatelythe sameat low concentra- that they can be approximated by the ternary system tions. Increasingthe MgCOr component of Ca-Mn CaCOr-MnCOr-MgCOr. Goldsmith and Graf carbonateswidens the solvusextending out from the (1957)investigated subsolidus relations in the system CaCOr-MnCO, binary and presumablyraises its up- CaCOr-MnCO, and found a solvus in the Mn-rich per temperaluls limi{ to someextent. Bald Knob car- half of the system.They found complete solid solu- bonate compositions are apparently unintemrpted by tion betweencalcite and rhodochrositeabove 550'C. the solvi in the system,and we infer that the mini- One limb of the solvusapproaches a l:l Ca:Mn mum temperatureof equilibration of these carbon- compositionat about 450oC,indicating a high degree ates was greater than 550'C, the peak of the kut- of cation ordering below this temperature corre- nahorite-rhodochrositesolvus (Goldsmith and Graf, sponding to kutnahorite, CaMn(COr)r. Goldsmith 1957). Country rocks are muscovite-oligoclase- and Graf (1960) also investigated subsolidus rela- tions in the ternary systemCaCOr-MnCOr-MgCO, 3 Cc = calcite (CaCO3),Rc = rhodochrosite(MnCO3). 280 WINTER ET AL.: CARBONATES AND PYROXENOIDS, NORTH CAROLINA
Table 2. Analyses of Bald Knob carbonates in weighr perc€nt oxide component and mole p€rc€nt carbonate end member
Sanple weight% li{ol % BK- Mno Cao lu1g0 Feo *C0Z !1nC03 CaC03 M9C03 FeC03
4-t 32.13 23.ll 53 I.13 40.42 49.29 44.85 4.14 l.7l 4-2 34.81 20.33 47 1.25 39.91 54.09 39.97 4.03 1.92 4-4 27.08 18.99 27 0.57 39.63 58.03 37.59 3.50 0.88 4-7 3t .10 23.62 77 0.78 40.22 47.95 46.06 4.81 1.18',l 77 0.87 39.4? 68.74 25.0t 4.89 .35 4-10 43.71 12.57 ',| 4-12 39.77 16.53 7't 0.16 39.62 62.24 32.72 3.87 .18
39.25 17.67 57 0.22 38.97 62.48 35.58 ',I.581.60 0.35 6-2 37.99 18.89 57 0.18 39 12 60.25 37.89 0.29 4t.88 t5.17 58 0 24 38.66 67.21 30.79 1.62 0.39 6-4 44.40 13.21 67 0.32 38.83 70.94 26.70 1.87 0.51
8-2 22.56 31.48 0.88 2.08 40.92 24.19 60.36 2.34 3.ll 8-6 55.45 3.57 0.61 1.21 38.5789.10 7.26 1.72 1.91 91.25 6.65 0.95 I .15 MqCq + ftCO3 Mn C03 8-7 56.61 3.26 0.34 0.72 38.45 8-l 2 54.96 3 .41 0.78 2.51 39.12 87.06 6.82 2.19 3.93 8-18 32.02 20.74 1.85 4.40 40.84 48.61 39.84 4.95 6.60 Fig. l. Triangular composition diagram superimposedon the 8-19 27.28 27.10 1.06 2.41 40.85 41.41 52.05 2.84 3.70 phase diagram of the CaCO3-MgCO3-MnCO3slstem, showing 10-2 the solvi at 500oC(after Goldsmith and Graf, 1960).Bald Knob 24.84 26.58 I .59 2.52 39.55 38. 96 52.75 4.39 3 . 90 l3-t 29.53 I 6.49 0.53 0.24 38.l8 64 2t 33.89 1.52 0.38 carbonate compositions are plotted. FeCO3 is combined with t3-2 39.73 16.60 0.55 0.21 38.41 64.17 33.92 l.s8 0.84 MgCO3 in this plot. Dashed boundariesshow the position of the '1 solvi at 600oC. 4-t 3t . 66 24.7 5 0.43 0.24 39-66 49.50 48.96 1.17 0.36 14-2 32.18 24.67 0.41 0.24 39.91 50 0t 48.49 t.l3 0.36 14-3 31.98 24.70 0.39 0.25 39 79 49.84 48.70 1.07 0.39 24.54 0.38 0.23 39.75 50.17 48.42 1.05 0.36 't4-5t4-4 32.16 3l .93 24.74 0.42 0.22 39.80 49.75 48.76 l.l5 0.34 14-6 3t .98 25.11 0.38 0.24 40.O9 49.46 49.14 1.04 0.37 quartz-garnet-mica schists and hornblende-ande- '17-3 sine+cuqrmingtonitetmagnetiteamphibolites, 14.00 38.76 I.36 1.50 41.50 20.93 73.29 3.56 2.22 repre- 17-4 15.74 37.43 l.l4 1.55 41.32 23 62 71.08 3.00 2.29 1.29 t.56 41.55 21.22 73.O9 3.38 2.30 senting medium-grade amphibolite facies meta- 17-6'17-7 14.22 38.71 15.09 37.82 l. l8 1.60 41.30 22 66 71.86 3.tI 2.37 1.64 41.37 32.31 71.34 2.92 2 .43 morphism. These assemblagessuggest maximum '17-12r7-r0 r5.55 37.61 1.il ts.0r 37.66 1.24 t.7t 41.26 22.56 71.62 3.28 2.53 temperaturesof about 550o-600oc during the meta- 17-20 15.22 37.64 Ll8 l.4t 41.13 22-96 71.82 3.13 2.09 morphism. The temperature during the metamorph- l7-21 ts.l9 37.94 1.28 1.85 41.73 22.58 71.34 3.35 2.72 ism of the Bald Knob deposit is estimated at l8-l 27.00 27.16 1.50 1.37 40.53 41.31 52.58 4.04 2.07 54.35 4.30 2,46 l8-l-A 26.10 28 83 ',l.441.64 1 .67 41.62 38.89 575+40"C. 18-2 43.15 14.35 1.39 40.42 66.t 8 27.83 3.88 2.1I Frondel and Bauer (1955) describedkutnahorite, 18-2-A 24.4A 28.87 1.78 2.08 4t.00 36.91 55.25 4 .74 3 .11 t8-3 24.86 29.23 1.52 1'I .7r 4l .05 37.56 55.86 4.03 2.54 the manganese 18-4 25.19 29.56 I .61 .90 4t .74 37.43 55.56 4.21 2.79 CaMn(COr)r, analogue of dolomite, t8-4-A 24.80 29.04 1.65 I .74 4t .03 37.49 55.53 4.38 2.60 from three localities. Carbonate analyses(Table 2) 18-5 24.12 29 78 1.42 1.54 40.81 36.65 57.24 3.80 2.31 t8-5-A 25.69 28.65 1.58 t.6t 4t l2 38.76 54.66 4.20 2.39 show that carbonateclose to this compositionis pres-
ent at Bald Knob. Single-crystalWeissenberg X-ray 18-6 25.43 28.20 'l.431.60 1.13 40.70 38.75 54.36 4.30 2.60 18-7 24.23 30.09 1 .67 41.23 36.46 58.26 3.80 2.48 photographs were obtained from two samples close l8-7-A 26.35 27.75 1.71 ].68 4t.01 39.85 53.09 4.55 2.5',1 l8-'t'l 24.57 30.26 r.38 l 65 4t 50 36.72 57.22 3.63 2.43 to the kuthahorite compositionto determineif order- 18-lI-A 29.r5 25.62 |.37 L3C 40.47 44 67 49.66 3.70 1.96 18-13 25.78 28.83 ].50 1.77 41.32 38.69 54.73 3.95 2.62 ing reflectionscould be observed.The resultsshowed l8-t5 25.70 28.82 1.57 1.79 41.35 38. 54 54.67 4 .13 2.65 no reflectionsof the type h0l, I : 2n + l, indicating 18-17 24 75 ?9.71 1.66 1.99 4t .69 36.82 55.92 4.34 2.92 group that the space is R3c, as for calcite, and sug- 22-1 37.66 20.18 0.24 0.29 39.6r 58.95 39.95 0.65 0.45 gestingno long-rangeorder for Ca and Mn. Unpub- 22-2 38.O4 20.48 0.?2 0.16 39.98 58.99 40.17 0.59 0.24 22-3 32.87 24.30 0.23 0.13 39.78 51.24 47.91 0.64 0.21 lished studiescited by Goldsmith and Graf (1960)in- 22-4 34.72 23.00 0.2r 0.r 6 39.89 53.97 4s.22 0.57 0.24 22-5 33.78 23.78 0.22 0.16 39.94 52.44 46.70 0.6\ 0.24 dicate that disordering in the compound 22-6 33.00 23.43 0.26 0.17 39.78 52.84 46.19 0.70 0.?6 22-7 37.72 20.68 0.23 0.00 39.86 58.68 40.70 0.62 0.00 CaMn(CO,), begins at approximately 450oC. Thus 22-8 3s.97 21.52 0.28 0.19 39.60 56.32 42.63 0.76 0.29 material having the kutnahorite composition should 50-2 25.24 30.22 0.36 0.06 39.81 39.34 59.56 0.99 0.09 be disordered at temperaturesof 550oC, the mini- 635-2-A 44.62 14.22 0.21 0.19 39.16 70.64 28.47 0.59 0.29 63s-5 45.67 13.36 0.25 0.18 39.r7 72.28 26.74 0.69 0.29 mum temperature indicated by the composition of 635-5-A 48.60 11.42 0.2s 0.r8 39.47 76.33 22.70 0.70 0.27 the Bald Klob carbonates.Nevertheless it is surpris- 635-6 43.95 14.36 0.24 0.'18 38.88 70.08 28.96 0.68 0.28 635-7 48.69 10.48 0.28 0. 15 39.08 71.22 2i.15 0.19 0.24 ing that thesecarbonates close to kutnahorite compo- 635-8 48.39 10.72 0.25 0.18 38.78 71.34 21.61 0.10 0.29 635-9 48.41 10.66 0.21 0.I 4 38.78 77.47 21.55 0.76 0.22 sition did not attain long-rangeorder upon sosling, 635-10 46.93 12.18 0.26 0.12 39.00 74.60 24.50 0.72 0.18 despite the relatively long cooling times following re- gional metamorphism. *C0^ calculated from stoichometry. WINTER ET AL.: CARBONATES AND PYROXENOIDS, NORTH CAROLINA 281
Table 3. AnalysesofBald Knob rhodonitesand pyroxmangites
Sample BK- 8-3 8-9 8-l 0 8-t 4 8-l 5 8-t6 8-17 8-20 l7-0
si02 45.05 44.71 44.75 44. 55 44.56 44.77 44.99 44.73 45.27 45.36 45.86 45.22 46.36 Alru? 0.06 0.06 0.07 0.06 0.07 0.06 0.07 0.07 0.07 0.05 0.06 0.04 0.06 Fe0*- 7.18 2.59 2.54 2.65 1.32 2.77 2.71 3.44 3.66 ll.61 1'].95 9.2\ 9.77 Mn0 4l .33 49.14 49.68 49.26 5].49 49.83 49.56 48.83 48.34 37.77 36.42 40.37 37.02 M9o t.Jt 0.52 0.50 0.54 0.34 0.50 0.5.l 0.73 0.79 .l.362.72 2.86 2.19 3.96 Ca0 3.60 1.47 1.32 I .35 1.06 1,25 1.60 1.44 1.83 2.07 1.78 2.77 Zn0 ***n.a. n.a, n,a. n.a. n.a, n.a. n.a. n.a. n.a. n.a. n.a. n.a. t0tal 98.59 98.49 98.86 98.41 98.84 99.18 99.44 99.24 99.96 98.87 99,22 98.81 99.94 (pm) (pm) **1rh) (rh) (rh) (rh) (rh) (rh) (rh) (rh) (rh) (pm) (pm) Catjons relatjve to 3 oxygens
5I 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.99 0.99 0.99 0.99 0.98 AI 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe* 0.13 0.05 0.05 0.05 0.02 0.05 0.05 0.06 0.07 0.21 0.2'l 0.,l7 0.17 Mn 0.76 0.92 0.92 0.92 0.96 0.92 0.91 0.90 0.89 0.69 0.66 0.75 0.65 Mg 0.04 0.02 0.02 0.0? 0.01 0.02 0.02 0.02 0.03 0.09 0.09 0.07 0.12 Ca 0.08 0.03 0,03 0.03 0.02 0.03 0.04 0.03 0.04 0.03 0.05 0.04 0,06 Zn ***n.a. n.a, n.a. n.a. n.a. n.a, n.a. n.a. n.a. n,a. n.a. n,a. n.a.
SampleBK- l7-l 17-5 17-8 17-9 I 8-3 I 8-5 I 8-6 I 8-8 t8-9 l8--10 l8-]2 I 8-14 I 8-16
Si02 46.21 45.39 45.53 45.60 47.85 47.77 47.58 47.41 47.45 47.55 47.56 47.06 47.43 AlrOa 0.06 0.04 0.06 0.06 0.05 0.06 0.05 0.07 0.05 0.06 0.07 0.09 0.07 Feb*- I 0.55 I 0.23 10.53 r0.35 7.46 7.50 6.88 6.69 5.12 6.84 6,78 5.60 8.07 MnO 35.67 36.78 36.49 37.00 40.26 39.79 40.66 4].05 41.50 40.79 40,62 40.94 38.27 3.6] MgO 4.31 3.29 3.17 3.36 3.34 3.15 2.94 1.78 3.07.l.83 'l.893.04 2.10 CaO 2,87 2.78 2.73 2.72 1.84 t:83 2.12 1.84 4.17 3.58 I .87 ***n. 0.06 0.13 0.,l7 ZnO a. n.a. n.a. n,a. 0.ll 0.00 0.ll 'l 0.08 0.07 'l 0.15 'l Total 99.67 98.53 98.63 98.90 100.93 100. 29 100. 55 00.08 100 .1 7 00.29 00.02 99.50 99.49 **(pm) ( pm) (pm) (pm) (pm) (pm) (pm) (pm) (rh) (pm) (pm) (rh) (pm)
Cations relative to 3 oxygens .l.01 si 0.98 0.98 0.99 0.99 1.00 r.0r 1.00 t.0r l.0l I .0] t.0t 1.0] Al 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe* 0.]9 0.'l8 0.19 0.19 0.13 0.13 0.12 0.12 0.09 0.]2 0.12 0.'l0 0.'l4 Mn 0.64 0.68 0.67 0.68 0.71 0.71 0.73 0.74 0.75 0.73 0.73 0.74 0.69 Mg 0.,l4 0.lt 0.ll 0.10 0.10 0.10 0.10 0.09 0.06 0.]0 0.10 0.07 0.ll Ca 0.06 0.06 0.06 0.06 0.04 0.04 0.0s 0.04 0.09 0.04 0.04 0.08 0.04 Zn ***n.a. n.a. n.a. n.a. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
xAll Fe assumedto be Fe2+, **(rh) = rhodonite, (pm) = pyroxmangite,***n.a. = not analyzed.
Pressureestimate for the Bald Knqb deposit The pyroxenoids A reasonableestimate of the metamorphic pres- Microprobe analyses of 26 pyroxenoids from 3 sure which prevailed during the metamorphism of samples are given in Table 3 and are plotted on a the Bald Knob rocks can be made by utilizing kya- MnSiO,-CaSiOr-(Mg,Fe)SiO3 ternary diagram in nite and sillimanite occurrencesin North Carolina Figure 2. The compositions fall into the fields for py- (Stuckey,1965). Kyanite and sillimanite are found in roxmangite and rhodonite respectively. Single-crystal separate occumencessouth and southwest of Alle- X-ray study of sampleBK-18 confirmedthe presence ghany County, and they occur in a trend which, if ex- of rhodonite and pyroxmangite in separate grains. tended,would passvery near the Bald Krrob deposit. Universal-stage optical data on anLalyzedcrystals Applyng the temperature data extracted from the f1e6 s.amplesBK-8, BK-17 and BK-18 fell within carbonatesto the kyanite-sillimanite phaseboundary two groups, corresponding to rhodonite and py- of Holdaway (1971)yields a pressureof 5 kbar, con- roxmangite, respectively. Averaged optical data val- sistent with the metamorphic assemblagesfound in ues for the two phasesare given in Table 4. Grains the enclosingmica schistsand amphibolites.Urlcer- with relatively low 2V (40-50') corresponded in tainties in this pressure estimate are estimated at f 1 every caseto pyroxmangites, and grains with high 2V kbar when erors in the experimental determination (60-70') to rhodonites, and these values allow a of the kyanite-sillimanite curve are combined with rapid distinction between rhodonite and pyroxman- uncertainties in our thermometrv. gite. Subsequentidentification of a particular pyrox- 282 WINTER ET AL.: CARBONATES AND PYROXENOIDS, NORTH CAROLINA
lvn Mn
Fig. 3. Compositions of coexisting pyroxenoids (dots) and carbonates(circles). The diagram representsMnSiO3-CaSiO3- (Mg+Fe)SiO3 for the pyroxenoids, and MnCO3-CaCO3- Fig. 2. Compositionsof rhodonite (circles)and pyroxmangites (Mg+Fe)CO, for the carbonates. (squares). Open circles and squares are data from the literature (Deer et al., 1978;'Mason, 1973;Hodgson, 1975;Dickson, 1975; Ohashi and Finger, 1975; Peters et al., 1978; Wall, personal (with over 50VoCaCOt comFonent)to exchangesome communication,1978; Tracy et al., 1980;Brownet al., 1980).Solid Ca for someMn, Mg, or Fe of the coexistingpyroxe- symbolsare from Bald Knob. noid. However, our data clearly show that the car- bonate strongly partitions the Ca over the coexisting rhodonite or pyromangite at Bald Knob P-Z condi- enoid was made by optical measurementsand by tions. Data of Peterset al. (1977) show a similar frac- noting in which field its analysisplotted on the ter- tionation of Ca between coexisting pyroxenoiil and nary diagram. These fields are well defined by plot- carbonate, and this may be a general effect at moder- ting natural rhodonite and pyroxmangite composi- ate gradesof metamorphism. tions on the ternary diagram (Peacoret al.,1978). When natural pyroxenoids are found in the system The low Ca content of the pyroxenoids is surpris- (Mn,Ca,Mg,Fe)SiOr,the coexisting pyroxmangite ing in view of the high Ca content of coexistingcar- and rhodonite pairs represent the mutual solubility bonates (Fig. 3). Most of the carbonatescoexisting limits of elementsin the two phases,and the width of with pyroxenoid contain more than 50 mole 7o the miscibility gap will dependon P-Zconditions, as CaCO, component, and yet the large Ca site in well as the pyroxenoid compositions. Although a rhodonite is less than 50Vofil7ed by Ca. The two Ca miscibility gap must occur between rhodonite and sites in pyroxmangite are less than 2OVooccupied by pyroxmangite (Sundius, l93l; Momoi, 1968;Ohashi Ca. Considering that one in five sites for rhodonite et al., 1975;Peacor et al., 1978} few reports of co- (Peacorand Niizeki, 1963)and two in sevenfor py- existing pairs are available in the literature. Ohashi roxmangite(Burnham, l97l) prefer to accommodate and Finger (1975)give analysesof pyroxmangiteand the large Ca ion, one might expect the carbonate rhodonite, and Ohashi (personal communication, 1977)informed us that one sample(AKl8) contained Table 4. Optical data for analyzed pyroxenoid grains from both phases.Wall (personal communication, 1978) samplesBK-8, BK-17, and BK-18 has analyzed two pyroxmangite-rhodonite pairs from Broken Hill, Australia. Peterset al. (1978) gave Rhodonite Pyroxmang i te analyses of two pairs from the Alps. Tracy et al. (1980)report on a pair from Massachusetts.A num- 2vz 660 460 ber of an4lyses of coexisting pyroxnangite and given cAZ 400 400 rhodonite from Bald Knob are in Table 3. The pyroxmangite-rhodonite miscibility gap appears to Y^1(llo) 500 ?no widen somewhatwith increasing(Mg,Fe)SiOr. Some y^r(ll0) 650 900 overlap may occur between pyroxmangite and rhodonite compositions, presumably due to forma- W'INTER ET AL.: CARBONATES AND PYROXENOIDS, NORT:H CAROLINA tion at diferent P-Z conditions. Higher-temperature Table 5. Calculated shifts in the rhodonite-pyroxmangite rhodonites appear to approach the MnSiOr- eouilibrium
(Mg,Fe)SiO, tieline more closely,overlapping in part '18-8 Sample 18-9 t8-l 0 l8-12,14 l8-16 with the compositions of more calcic, lower-temper- Pm Rh Pm Rh Pm ature pyroxmangites. More analyses of pyroxman- utlnsior(dis).tuu gite-rhodonite pairs are needed from metamorphic .76 -74 .75 rocks at a variety of P-T conditions to fully explore ^P (kb) -4.i -20.9 the relative effectsofP and 7. P (kb) +7.9 +3.8 +7.9 -8.9 Maresch and Mottana (1976) published a phase diagram for the rhodonite-pyroxnangite transforma- XMnin Ml .755 .85 .743 .83 .708 tion in the systemMnSiOr, using a combination of their experimental reaction reversals and those of M2 .755 .85 .743 .83 .708 Akimslo and Syono(1972). With reasonabletemper- "3 .755 .85 .743 .83 .7C8 ature and pressure estimates for the metamorphism M, .755 .70 .743 .66 .708 of the Bald Knob deposit, we calculated the dis- "5 .86 .55 .86 . 60 .86 placement of the transformation curve due to solid . 5',1 - .487 .416 pyroxenoids solutions in the at Bald Knob, in order M7 .86 - .86 .86 to test the applicability of the curve for extracting pressure useful data from natural rhodonite-py- ar..rn (ord) roxnangite pairs. The equation of interestis: '- - -3 .749 .729 .694 ^P (kb) -8.2 -21.O
- P (kb) +3.8 -8.9 LqP2,T,)- AG(P',T,): [' A^Sdf trr f,, + I LVd,P+ RZln(K,/K,) (l) two structures in assuming that a(MnSiOr) : Holding , and assuminE LV?nr: LW, X(MnSiO,). This model may be realistic if the cat- equation I reduces"orr.,"rr,to: ions become disorderedin the pyroxenoids at high temperature. Some support for this, simple model 4.p: (-RZlnK)/w (2) comesfrom Schwerdtfegerand Muan's (1966)exper- whereAG:0 both at P, with K, : 1 (the experimen- iments, consistentwith ideal mixing of Mn and Fe tal equilibrium point in pure MnSiO, system)and at for the simple systemMnSiO.-FeSiO, at ll50'C. P, (for equilibrium with impure pyroxmangite + Pressureestimates from this calculation for the Bald rhodonite in the rock). The molar volumesof rhodo- Knob rhodonite-pyroxmangite pairs are given in nite and pyroxmangite were taken from lto (1972).a Table 5. For the end-memberreaction: The second model considerscation distributions consistent with room-temperature structure refine- MnSiO.: MnSiO, (3) ments in thesestructures (Peacor and Niizeki, 1963; pyroxmangite : rhodonite Burnham, l97l; Ohashiand Finger,1975;Peacoret al., 1978).For rhodonite all Ca was placed in the the calculatedLV :0.23 + 0.1cc and P"q,u: 12kbar M(5) site with Mn alone filling the remaining por- (Mareschand Mottana,1976).The pressureshift was tion. For M(l)-M(3), Mn and Mg + Fe were parti- calculated at 575"C (848 K), but a shift of even tioned such that Mg * Fe : x where Mg * Fe : 2x +l00oc in the temperaturewill not materially afect in M(a) and : 0 in M(5). For pyroxmangite Ca is the conclusions.Two quite different activity models distributed equally betweenM(5) and M(7) and the were employed in thesecalculations: the first essen- remainder filled with Mn. The sitesM(l)-M(4) have tially neglectsthe partitioning of cations within the Mg + Fe : x whereMg + Fe :2x in M(6) and : 0 in M(5) and M(7). The activity is then calculatedsite 4 Narita et al. (1977) also obtained unit-cell data on synthetic by site and is taken as the product of the mole fr4c- MnSiO3 rhodonite and pyroxmangite. Calculated unit-cell vol- tion MnSiO, on each site. The activity of MnSiO. in umesare 34.93cc/mole for rhodonite and 34.95for pyroxmangite, rhodonite is: yielding Lft : -9.92 cclmole. This A Z has the wrong sign com- pared to the experiments, and these data will be ignored below. dilo.sio,(ord) : (Xs5. .Xff;. XMI' XMI' XMI)'" (4) WINTER ET AL.: CARBONATES AND PYROXENOIDS. NORTH CAROLINA and in pyroxmangiteis: cPr,frsio,(ord) : (XMl.XME. )ff1. XMI' &y ffiy W)', (5) Acc6rding to this model the activity of MnSiO, ap- proacheszero as CaSiO, reaches l/5 n rhodonite T"C and 2/7 in pyroxmangite,the presumedlimits of Ca in thesephases. Similarly the activity of MnSiO, ap- +I proacheszero as (Mg,Fe)SiO3goes to l/2 in rhodo- I nite and 3/7 n pyroxmangite.While theseestimates cannot be expectedto be exact since activity coeffi- lzT. cientshave beenignored and becausecation distribu- tions are unknown in pyroxenoids at high temper- atures, they may be more realistic than assuming random distribution of Mn on all sitesas in the first model. The orderedmodel reducesto the simpler dis- ordered model for random distribution and ideal be- havior of Mn on eachpyroxenoid site. It attemptsto > Xcoz take into accountthe nonrandom distribution of Mn Fig. 4. Perers'curveat 2 kbar for the reactionMnCO3 + SiO2: among the various octahedralsites but must consider MnSiO3 + CO2 compared wittr the calculated curve using thc ideal mixing on each site becauseof lack of more program EeUILI (Wall and Essene, 1972) fron an equilibrium completemixing data. point at T : 494oC and X(CO) : 0.55. The solid line is Peters' For the Bald Knob pyroxmangite-rhodonitepairs curvb and the dots and circles represent half reversals.The 5 kbar the two thermodynamic models yield similar pres- boundary was calculated. sure estimatesof +8 to -9 kbar. Even small analyti- cal errors causea large shift in estimatedpressures. ism of the deposit allows an estimation of X(HrO) In addition, even small deviations from ideality se- and X(CO,). Peters'(1971) curve for reaction 6 at 2 verely affect the pressuresderived-for instance an kbar is shown in Figure 4 comparedwith the calcu- activity coefficientof l.l instead of I for one phase lated curve.A large discrepancyis noted in the slopes will changethe pressureshift by some30 kbar. Even of the experimental vs. the calculated curves. The the +0.1 cc eror in the AZwill causea 50Vochange EQUILI computer runs were generated using ther- in AP. These are effects expectedfor any reaction modynamic data for rhodonite, since data for py- with a small AZ(
oN
ooll -J
-22 -7 -F -3 +l Log f S,
Fig. 5. Log (/O2)-log (/S) phasediagram calculatedat 575oCand 5 kbar. The stippledarea representsthe rangeofoxygen and sulfur fugacities which prevailed during the metamorphism ofthe Bald Knob deposit" and the shaded area representsconditions close to those required for pyroxmangite/rhodonite * alabandite + quartz and pyroxmangite/rhodonite + tephroite + alabandite. Reactions l-10 couldnotbeconvenientlylabelledonthediagramandare:(l)6CoO+4S2:2Co3Sa+3Oz;Q)6Co2TiOo+4S2=2Co3S4+6CoTiO3 + 3O2;(3) 6CoTiO' + 4S2:2Co3So+ 6TiO2+ 3Oz Q) 6CoSOa* Sr:2gor5o + l2Ozi(5) 2CoTiO3+ 252:2CoS2 + 2TiO2+ Ot (6) 2MnSiO3+52+3O2:2MnSO++2SiOr;(7)2Mn2SiOa+52+3O2=2MnSOc+2MnSiOr;(8)2MnTiO3+2S2:2MnS2+2TiO2+ Or; (9) 2MnSOa* 52 = 2MtrS2+ 4O2;(10) Co2TiOa+ 52 + 2O2 : 2CoSO++ TiO2.
AGi(kcal) : R 7ln[(/O,)" / (f S,)-l The equilibrium constant for the solids in equation I I must be evaluated if there are significant inpu- + av"aP14t 84 + R71nK. (ll) rities in the solid phases.The standardstate is taken to be the pure phase(for the solids)and the ideal gas where Z = 848oK (575oC),R : 0.001987kcal/ (for O, and Sr) at I bar and T. data ob- moloK, V. in cc, AP : 5 kbar, and n and m are the Qualitative tained with a solid-state energy-dispersivemulti- fluid coefficients as in the following generalized reac- channel analyzet indicate that the sulfide and tita- tion: nate phasesin Bald Knob rocks are more than 90Vo m. Sz+X+Y I Z: n. o.2+A+ B + C (12) end-member, and the shift in the oxide-su1fide-sul- WINTER ET AL.: CARBONATES AND PYROXENOIDS. NORTH CAROLINA fate equilibria is negligiblefor solid solutionsin these quire. The conditions under which the sulfates are phasesat Bald Knob. For rhodonite and tephroite stablemay not be reachedin Co- and Mn-rich rocks, the effect of the solid solutionswas again modeled as or if they are obtained may simply causeleaching of in Table 6. For example,holding sulfur fugacity con- Co and Mn from sulfides,silicates, oxides, and tita- stant, the efect of the RZInK term for an average nates,leaving behind materials impoverishedin Co Bald Knob rhodonite composition is to shift the and Mn. Indeed,the solution of CoSOoor MnSOoin rhodonite reactioncurve in the shadedregion of Fig- an aqueousfluid may yield values of a(CoSOo)or ure 5 by not more thanO.2log (/O) units.These er- a(MnSOo) considerably reduced from unity, con- rors are similar in magnitude to those generatedby tracting the stability of the oxides and sulfides still errors in free energydata. The calculatedphase dia- further. The high /S,-/O, part of the diagram may gram (Fig. 5) is consistentwith all observedassem- then representthe conditions for ore transportation blagesat Bald Knob. While it is expresslyfor 848oK as opposed to the two windows for deposition or and 5 kbar, the relations will qualitatively apply to stasisat oxidizing and sulfur-poor conditions on the other crustal metamorphic manganesedeposits. Un- one hand, or reducing sulfur-rich conditions on the fortunately few careful descriptionsof accessorytita- other. The highly oxidized manganesedeposits in- natesand sulfidescoexisting with manganesesilicates volving hausmannite(MnrOo) and other oxidized Mn and oxidesare availablein the literature. One appar- mineralssuch as braunite require quite different f Or/ ent exceptionto the calculatedphase diagram is re- /S, conditions than those at Bald Krob. ported by Peterset al. (1977),who record the assem- Bald Knob f OJf S, conditions are quite diferent blage alabandite-tephroite-pyroxmangite with than those in adjacent rocks and must have been hausmannite. Our calculations indicate that haus- maintained as distinctly different throughout meta- mannite is incompatible with alabandite and py- morphism. Adjacent amphibolites carry accessory. roxmangite, but hausmannitemay possibly have pyrrhotite and magnetite,and require /O, severalor-' been confusedwith jacobsite. ders of magnitude higher and /S, orders of magni- The stability fields of CoSOoand MnSOoappear to tude lower than the manganesedeposit at peak meta- interrupt reactions of the higher oxides of Co and morphic P-7. lf a through-going water-rich fluid Mn from the fully sulfidized CoS, and the mono- interacted with the carbonate deposit as concluded sulfide MnS. The sulfatesare not known in nature. above,the /O, and /S, must have been locally buf- presumablybecause they are too soluble in aqueous fered at quite diflerent levels within the manganese fluids at the relatively high SO, pressurethey will re- deposit.Presumably this condition would have been maintained throughout metamorphism, creating a sulfur-rich and oxygen-poorhalo around the manga- Table 6. Thermodynamic data used in calculating Fig. 5 nese deposit as long as the sulfides persisted.To a
Phase Formula -AG[46(fm) ref. Vigg ref. first approximation it would appear that the Bald kj/mol.K Cclmo1 Knob protolith was a sulfide- and carbonate-rich 'l body poor in Mn oxides, particularly Mn3* oxides. cobalt oxide coO 172.8 (l ) l.64 (1) manganosite of all the phase equilibria supports Mno '1,093.33?2.8 (1) 13.2? (l ) The consistency hausmannite Mn304 (r) 46.95 (t ) the contention that it last equilibrated during meta- partri dgei te Mn203 740.0 (l ) 3t.37 (l ) pyrol usite Mn02 365.4 (r) 16.61 (r) morphism. We have found no evidenceinconsistent quartz sio, (r 757.0 ) ',t8.8222.6s (l ) ruti l e ri02 789.0 (1) (r) with a premetamorphic sedimentary origin for the cobdlt pentl!ndite CogSg 778.4 (4) 147.44 Bald Knob manganesedeposit. linnaeite Co3S4 320.1 (4) 62.55 (2',) cattierite CoS2 1?5.7 (4) 25.52 (?) al abandite Mns 2?2.5 (r ) 21.46 (l ) Appendix haueri te MnS2 222.0 (4) 34.20 (r) cobalt sul fate CoS04 588.3 (8) 39.94 f?\ procedure manganesesulfate MnS04 756.6 (5) 43.62 (r) Microprobe cobalt orthotitanate Co2Ti041,166.2 (6) 45.38 (?) Element analyseswere obtained with an ARL-EMx cobaltmetat'itanate CoTiO3 982.8 (6) 31.05 (Z) pET, pyrophanite MnTiO3 1,148.7 (i) 32.76 (?\ electron microprobe analyzer with rrr, and TAp tephroite Mn2Si041,450.3 (r ) 48.61 (r ) wavelength-dispersivecrystal spectrometers.An ac- rhodonite MnSior ]r]04.0 (f ) 35.16 (l ) celeratingpotential of 15 kV and an emissioncurrent
(l) Robiegt a]. (1978);(2) AsrM;(3) strunz('t970); (4) Mills of 150 ;rA were standard operating conditions. A (1974);(5) Mah(1960); (6) Breznyand Muan (1969); (7) Evans samplecurrent 0.01 pA was used on Mn silicates andMuan (l971)i (8) tle'ller(1965) of and carbonates;the beam current was digitized with 288 WINTER ET AL.: CARBONATES AND PYROXENOIDS, NORTH CAROLINA counting times of 20-30 secondson each of 5-10 ANU) and kaersutite (Irving) were used as stan- areasof each grain analyzed.The presenceof minor dards. Analysesof the rhodonite and kaersutite are elementswas checkedwith an energy-dispersiveLi- given in Table 7. Pyroxenoid intergrowths observed silicon crystalwith multichannel analyzer.Spectrom- on the microscope,as well as discrete grains, were eter data were reduced with the Fortran program EM- analyzed. Zn and Ti were analyzed for some speci- PADR VII written by Rucklidge and Gasparrini mens, and were not found in amounts greater than (le6e). 0.05 weight percent.Zn and Ti were not included in subsequentanalyses. Carbonates Acknowledqments A natural rhodochrositefrom Alma, Colorado, a We are grateful to J. S. Huebner, D. Burt, and J. Papike for dolomite from Delight, Maryland, and siderite from their reviews of the manuscript. We also thank the staffof the elec- Westfalia, Germany were used as standardsfor all tron microprobe laboratory of the University of Michigan, espe- analyses.The compositionsof these standards are cially L. Allard, for their assistancewith the laboratory facilities. given in Table 7. Becausethe specimenswere We are grateful to J. S. Huebner,D. M. Kerrick, S. D. Scott, and providing banded, grains from several areas of each sample A. J. Irving for us with microprobestandards. were analyzed.The data were reducedwith the fixed stoichiometrymode with C : Mn + Ca + Mg * Fe References (Rucklidge and Gasparrini, 1969). Akimoto, S. and Syono,Y. (1972)High pressuretransformations in MnSiO3. American Mineralogist, 57,76-84. Pyroxenoids Burnham, C. W. (1971)The crystalstructur€ of pyroxferroitefrom Proceedings, Lunar Synthetictephroite (provided by Huebner),sphene Mare Tranquillitatis. Second Science Con- fererc,e,47-5'1. (provided by Kerrick), and sphalerite (provided by Brezny, B. and Muan, A. (1969)Phase relations and stabilitiesof Scott), as well as natural rhodonite (Broken Hill, compounds in the system CoG-TiO2. Journal of Inorganic and Nuclear Chemistry,31, 649. Brown, P. E., Essene,E. J., and Peacor,D. R. (1980)Phase rela- Table 7. Compositionsofstandards usedfor microprobe analyses tions inferred from field data for Mn pyroxenes and pyroxe- cited in the t€xt. The syntheticsphene and sphaleriteused for the noids. Contributions to Mineralogy and Pctrology,in press. pyroxenoid analysesare stoichiometric Deer, W. A., Howie, R. A., and Zussman,J. (1978)Rock Forming Minerals, Vol. 24, Single Chain Silicates.Wiley, New York. Al ma Gabbs }iestfal ia Dickson, B. L. (1975)The iron distribution in rhodonite. Ameri- rhodochrosi te dol omite s i deri te can Mineralogist,60, 98-104. Evans,L. G. and Muan, A. (1971)Activity-composition relations MnC0, 97.63 0.10 15.77 of solid solutions and stabilities of manganeseand nickel tita- MqC0- 1.99 46.13 3.78 natesat l250oC as derivedfrom equilibria in the systemsMnO- CoO-TiO, and MnO-NiO-TiO2. Thermochimica Acta, 2, 277- FeC03 0.34 0.1I 80.0l 292. CaC0, 0.M 53.64 0.44 Frondel, C. and Bauer, L. H. (1955) Kutnahorite: a manganese ZnC0, 0.01 0.01 dolomite. American Mineralogist, 40, 7 48-7 60. SrCO, 0.02 0.0] Goldsmith, J. R. and Graf, D. L. (1957)The systemCaO-MnO- CO2: solid solution and decomposition relations. Geochimica et CosmochimicaActa, I l, 310-334. Irvi ng BrokenHill Synthetic kaersutite rhodoni te tephroite Goldsmith, J. R. and Graf, D. L. (1960) Subsolidusrelations in the systcm CaCO3-MgCO3-MnCO3.Journal of Geology, 68, si 02 39.t4 45.74 32+335. Ti 02 4.78 Hodgson,C. J. (1975)The geologyand geologicaldevelopment of Al^0^ I 4.04 0.07 the Broken Hill lode, in the New Broken Hill Consolidated Fer0, 1.12 Mine, Australia. Part II, Mineralogy. Journal of the Geological Societyof Austraha,22, 33-50. Fe0 14.40 I 3.28 Holdaway, M. J. (1971)Stability of andalusiteand the aluminum Mn0 0.17 37.80 70.25 silicate phasediagram. American Journal of Science,271,97- Mgo i0.68 0.06 l3l. Ca0 I 0.88 l.3l Holland, H. D. (1959)Some applications of thermochemicaldata Naro 2.80 0.12 to problems of ore deposits, I. Stability relations among oxides, -l.56 sulfdes, sulfates, and carbonates of ore and gangue minerals. Kzo o.02 Economic Geology, 54, 18+233. 0. Crr0, 0t Holland, H. D. (1965)Some applications of thermochemicaldata to problems of ore deposits, II. Mineral assemblagesand the WINTER ET AL.: CARBONATES AND PYROXENOIDS, NORTH CAROLINA 289
composition of ore-forming ffuids. Economic Geology, 60, Peters,Ij., Schwander,H., and Trommsdorfl, V. (1973) Assem- I 101-1129. blages among tephroite, pyroxmangite, rhodochrosite, quartz: Huebner,J. S. (1967)Stability relationsofminerals in the system experimental data and occurrencesin the Rhetic Alps. Contri- Mn-Si-C-O. Ph.D. Thesis,The JohnsHopkins University, Bal- butions to Mineralogy and Petrology,42,325-332. timore, Maryland. Peters,1j., Schwander,H., Trommsdorff,V., and Sommerauer,J. Ito, J. (1972) Rhodonite-pyroxmangiteperitectic along the join (1978) Manganesepyroxenoids and carbonates:critical phase MnSiO3-MgSiO3in air. American Mineralogist, 57,86+876. relationsin metamorphicassemblages from the Alps. Contribu- Kerrick, D. M. (1974) Review of metamorphic mixed-volatile tions to Mineralogy and Petrology,66, 383-388. (H2O-CO2)equilibria. American Mineralogist, 59, 729-7 62. Peters,!., Valarelli, J. V., Coutinho, J. M. V., Som.merauer,J., Mah, A. D. (1960)Thermodynamic properties of manganeseand and von Raumer, I. (1977) The manganescdcposits of Bur- its compounds.U.S. Bureau of Mines Report of Investigations, itirama (Para, Brazil). SchweizerischeMineralogische und Pet- 5600, 1-34. rographischeMitteilungen, 57, 313-321. Maresch, W. V. and Mottana, A. (1976) The pyroxmangite- Robie, R. A., Hemingway, B. S., and Fisher, J. R. (1978) Ther- rhodonite transformationfor the MnSiO3 composition.Contri- modynamic properties of minerals and related substancesat butions to Mineralogy and Petrology,55,69-79. 298.15Kand I bar (105pascals) pressure and at higher temper- Mason, B. (1973) Manganesesilicates from Broken Hill, New atures.U.S. Geological Survey Bulletin 1452. SourhWales. Journal of the GeologicalSociety of Australia, 20, Ross,C. S. and Kerr, P. F. (1932)The manganeseminerals of a 397404. vein near Bald Knob, North Carolina. American Mineralogist, Mills, K. C. (1974)Thermodynamic Data for Inorganic Sulphides, 17.l-18. Selenidesand Tellurides. Butterworths.London. Rucklidge, J. C. and Gasparrini, E. L. (1969)Specifications of a Momoi, H. (1968) Some manganesepyroxenoids. Journal of the completeprogram for processingelectron microprobe data: EM- Mineralogical Societyof Japan,8, l-5. PADRVII. Departmentof Geology,University of Toronto, un- Narita, H., Koto, K. and Morimoto, N. (1977)The crystal struc- published circular. tures of MnSiO3 polymorphs (rhodonite- and pyroxmangite- Schwerdtfeger,K. and Muan, A. (1966)Activities in olivinc and type). Mineralogical Journal, 8, 329-342. pyroxenoid solid solution of the system Fe-Mn-Si-O at Ohashi,Y. and Finger, L. W. (1975)Pyroxenoids: a comparisonof I150'C. Transactionsof the AIME, 236,206. refined structuresof rhodonite and pyroxmangite.Camegie In- Strunz, H. (1970) MineralogischeTabellen. 5 Auflage. Akade- stitution of WashingtonYear Book, 74, 564-569. mischeVerlagsgesellschaft, Geest und Portig K.-G., Leipdg. Ohashi, Y., Kato, A., and Matsubara, S. (1975) Pyroxenoids:a Stuckey,J. L. (1965)North Carolina: Its Geologyand Mineral Re- variation in chemistryofnatural rhodonitesand pyroxmangites. sourc€s.North Carolina StateUniversity Print Shop,Raleigh. CarnegieInstitution of WashingtonYear Book, 74, 561-564. Sundius, N. (1931) On the triclinic manganiferouspyroxenes. Peacor,D. R. and Essene,E. J. (1980)Caryopilite-a memberof American Mineralogist, 16,41 1429 and 488-51 8. the friedelite rather than ttre serpentinegroup. American Miner- Tracy, R. J., Lincoln, T. N., Robinson,P., and Ashenden,D. D. alogist,65, 335-339. (1980) Rhodonite-pyroxmangite-pyroxene-amphiboleassem- Peacor,D. R. and Niizeki, N. (1963)The determination and re- blages.(abstr.) EOS, 61, 390-391. finement of the crystal structure of rhodonite, (Mn,Ca)SiOr. Wall, V. J. and Essene,E. J. (1972) Subsolidusequilibria in the Zeitschrift fuer Kristallographie, I 19, 98- I 16. system CaO-AI2O3-SiO2-H2O.(abstr.) Geological Society of Peacor,D. R., Essene,E. J., Brown, P. E., and Winter, G. A. America Abstractswith Programs,4, 92. (1978) The crystal chemistry and petrogenesisof a magnesian Weller, W. W. (1965) Low-temperatureheat capacitiesand en- rhodonite. American Mineralogist, 63, I l3'l -l 142. tropies at 298.15'K of anhydrous sulfates of cobalt, copper, Peacor,D. R., Essene,E. J., Simmons,Wm. B., Jr., and Bigelow, nickel, and zinc. U.S. Bureauof Mines Report of Investigations, W. C. (1974)Kellyite, a new Mn-Al member of the serpentine 6669.l-6. group from Bald Knob, North Carolina, and new data on grovesite.American Mineralogist,59, I 153- 11 56. Peters,lj. (1971)Pyroxmangite: stability in H2O-CO2mixtures at a total pressureof2000 bars. Contributions to Mineralogy and Manuscript received, September 4, 1979; Petrology,32, 267-27 3. acceptedfor publication, Iuly 28, 1980.