Manganese-Rich Minerals of the Pumpellyite Group from The

American Mineralogist, Volume 76, pages241-245, 1991

Manganese-richminerals of the pumpellyite group from the Precambrian SausarGroup, India

SoNrN,c,rH D.q.scur.rlo SlN.lrn Crrarnnnonrr, Puu,x SnNcurrl., P. K. Bn,c.rrAcHARyA, H. Bmwnrnn, Surnrva Rov Department of Geological Sciences,Jadavpur University, Calcutta 700 032, India M. Furuoxr, Department of Earth and Planetary Sciences,Klushu University, Fukuoka, Japan

Ansrucr Mn-rich minerals of the pumpellyite group are developed in Mn oxide ores of the Precambrian SausarGroup, India. The ores were intruded by late pegmatite dikes and calcite veins. A hydrothermal origin for thesephases through Ca-Mg-Fe-Al metasomatism is evident from their mode of occurrenceand textures. The phasesstudied show wide- spreadsubstitution between Mn3+ and Al in the l-site and between Mn2+ and Mg in the X-site. An Mg equivalent of okhotskite, reported for the first time, is present.This study establishesthat okhotskite representsthe Mn3+ end-member of the pumpellyite group of minerals and that solid solution between pumpellyite (Al dominant) and okhotskite is nearly complete.

INrnonucrroN dition to pumpellyite, diverse Ca-Mg-Mn-Al-Na silicates piemontite Minerals with the pumpellyite structure having the such as tirodite, Mn-rich pyroxene, and have general formula (Ca,K,Na)r(Mn2+,A13+Mg,Fe,*)o(F€,*,-developed at the contact ofpegmatite dikes and the ores Al3*,Ti4*,Mn3*)8Si,2O56_,(OH),are named on the basis (Roy and Purkait, 1968).Samples of pumpellyite-bearing ofrelative concentration oftrivalent cations in their oc- rocks describedin this work were collected from a small tahedral sites. The phase is called pumpellyite if Al is outcrop of Mn ores at the contact of pegmatite dikes and dominant; ifFe3+ is dominant it is designatedas julgoldite calcite veins. Mineral assemblagesthat contain pumpel- (Moore, 19 7 I ; Passagliaand Gottardi , 1973) . Further sub- lyite are as follows: divisions in the group by adding sufrxes according to the braunite * potassium feldspar + calcite + pum- dominant octahedraldivalentcations have beensuggested pellyite + bixbyite (1) [viz. pumpellyite-(Mn'z+),Kato et al., 1981; pumpellyite- braunite + phlogopite + pumpellyite * calcite (Mg), Passagliaand Gottardi,1973; Lucchetti, 19831.Re- + diopside + bixbyite. (2) cently, a Mn3+-dominant memberofthe pumpellyite group has beenreported (Togari and Akasaka, 1987)and named Thd Mn-Mg-Ca-Al silicatesoccur either within or close okhotskite. to the pegmatite dikes and calcite veins. In Assemblage Data are presentedhere for Mn-rich phasesthat vary l, pumpellyite occurs as narrow rims around braunite at compositionally between pumpellyite and okhotskite from the contact with calcite and/or potassium feldspar. Pum- the Precambrian Sausar Group, India. This is the first pellyite intercleaved with phlogopite occurs between report of such minerals from any Indian Mn deposit as braunite and calcite (Fig. l) in Assemblage2. It also oc- well as from any Precambrian rock. The wide composi- curs as veins in braunite (Fig. l). tional range of these phases from Indian deposits pro- vides additional data on the solid solution relationships ANlr,vrrclr, TEcHNreuEs in the pumpellyite group of minerals. A powder X-ray diffraction pattern (CuKa radiation) was obtained using a Philips X-ray diffractomter and a OccunnrNcn Si internal standard. Becauseof low modal abundance The Mn-rich pumpellyite group of minerals (hence- and the mode of occurrence,it was not possible to sep- forth referred to as pumpellyite in a goup sense) occur arate pumpellyite grains for single crystal analysis. in metamorphosedmanganese oxide ores that have been Chemical analysesof pumpellyite samplesand the as- intruded by pegmatite dikes and calcite veins subsequent sociated phaseswere carried out using a JEOL JXA-733 to amphibolite facies regional metamorphism at Gowari electron probe microanalyzer (EPMA) operatedat 15 kV Wadhona, Madhya Pradesh,India. The ores occur at the accelerating voltage. The following standards were used: contact of the pelitic Mansar Formation and dolomites wollastonite (Ca,Si), magnetite (Fe), rutile (Ti), MnO (Mn), of the Bichhua Formation of the Sausar Group. In ad- periclase (Mg) and corundum (Al). 0003404v9r /0r024241s02.00 24r 242 DASGUPTA ET AL.: Mn-RICH PUMPELLYITE

tural formulae, calculated after the method of Passaglia and Gottardr (1973) are given in Table l. Presenceof braunite in both assemblagesindicates that the ambient .fo2w?sabove the hematite-magnetitebufer (Lattard and Schreyer,1983) where Mn exists in both di- and trivalent statesbut Fe,o,in pumpellyite can be consideredas Fe3+. Following the scheme of Passagliaand Gottardi (1973), Al, Ti, and Fe3+were placed in the l-site and Mn3+ was added until this site was fully occupied. The remaining Mn was placed in the X-site as Mn2+ with Mg (Kato et al., l98l). The most striking compositional characteristic is the wide differencein cation occupancyin the Xand Isites of the pumpellyite samples studied. The Xflr*"[: Mn3+/ (Mn:+ + Al + Fe3+)]vs. Xfln+n[: Mnr*/(Mnr* + Mg)] plots of pumpellyites (Fig. 2) depicts the compositional Fig. l. Backscattered electron image of pumpellyite (P) in- variations in the two sites.Figure 2 showsthat the phases tercleaved with phlogopite (black) between braunite (white) and studied are either pumpellyite or okhotskite, and they calcite (C) in Assemblage2. Note veins of pumpellyite in braun- might be further designatedas pumpellyite-(Mnr+), ok- ite. Irngth of the bar is 0.01 cm. hotskite-(Mn2*), and okhotskite-(Mg) following Passaglia and Gottardi (1973). However, okhotskite-(Mg) and okhotskite-(Mn2*) have nor been approved by the IMA Rnsulrs Commission on New Minerals and New Mineral Names. The pumpellyite samples studied show the following Pumpellyite-(Mnrt) and okhotskite-(Mnr*) associatedwith pleochroic scheme:X : lil€ht yellow, f : Hght pink, Z : phlogopite in Assemblage2 are always more magnesian rose pink, absorption X > Y < Z. The mineral has par- relative to those associatedwith potassium ftldspar in allel extinction with respectto the length of the prismatic AssemblageI (Fig. 2). crystals. Unit-cell valves, calculated from the powder Of the associatedphases in the assemblages,braunite X-ray diffraction pattern on the basis of monoclinic sym- may contain up to 7.43 wto6 CaO (when recalcrrlatedon merry are q : 8.98 + 0.08 A, b :6.00 + 0.06 A, c : the basis of 12 O atoms according to the structural for- 19.19+ 0.06A, p : 98.1 + 0.03.. mula (R,*Rl*Sio,r), neltnerite content is 69 molo/o)and Chemical compositions of pumpellyite and the struc- up to 3.64 wt0/oMgO (Table 2). Such high calcic and high

TffiLE 1. Chemicalcomposition of Mn-rich pumpellyitesdetermined by EpMA Sample number 1 2 3 4 3 6 7 8 9 Assem- blage (1) (1) (1) (1) (1) (1) (1) l2l (21 sio, 33.86 34.14 31.90 33.09 33.39 u.76 32.81 34.10 34.25 Alros 8.40 9.64 1.60 2.15 3.59 3.08 9.23 8.72 Tio, 0.06 0.01 0.01 0.19 o.o2 12.o4 o.17 0.01 FerO. 5.38 0.40 4.48 0.05 0.08 3.93 0.06 0.19 Mn"O" 9.84 12.35 25.93 20.96 24.17 11.82 n.43 't3.77 16.32 MnO 12.19 12.36 11.95 11.92 11.76 12.95 11.81 10.73 7.59 Mgo 0.45 0.36 0.38 o.26 '19.77 0.57 0.19 0.28 1.31 3.31 CaO 10.02 20.17 21.08 21.30 22.O5 2',t.11 20.28 20.91 Naro 0.04 0.05 0.04 0.01 0.01 0.04 0.02 (H.O) 7.08 7.17 6.60 6.61 6.72 6.96 6.60 7.19 7.20 Total 97.32 96.55 98.18 100.75 101.58 100.86 100.22 96.72 98.51 si 12.27 12.36 11.95 12.00 11.92 11.98 11.92 12.30 12.04 {itAl 0.10 0.08 0.02 0.08 rstAl 3.80 4.11 0.71 0.92 1.43 4.87 't.24 3.92 3.61 Ti1* 0.01 0.05 0.05 FeP. 1.47 0.11 1.24 0.01 0.02 1.08 0.02 0.0s Mn3* 2.71 3.78 7.39 5.78 6.55 3.11 5.64 4.06 4.U Mn,* 3.73 3.42 3.79 3.66 3.57 3.76 3.65 2.99 2.26 Mg 0.24 0.20 o.21 0.14 0.30 0.09 0.15 o.71 1.74 Ca 7.77 7.82 7.93 8.19 8.15 8.'14 Ln 7.83 7.88 Na 0.03 0.04 0.03 0.03 0.01 o 40.97 40.32 40.28 40.00 39.97 39.90 40.16 40.28 40.01 OH 15.03 15.68 15.72 16.00 16.03 16.10 15.84 15.72 15.99 A/ofe;Total Fe as FerO3.Total Mn recalculatedto Mn2O3and MnO following 'Dashed Passagliaand Gottardi (1973). portions indicate below detection limit. DASGUPTA ET AL.: Mn-RICH PUMPELLYITE 243

TABLE2. Chemicalcomposition of phlogopite and braunite de- termined by EPMA

Mineral Phlogopite Braunite Sample numDer 15 18 20 16 18 sio, 37.92 36.32 41.58 11.58 10.13 10.62 N =c Tio, 0.03 0.01 0.01 0.01 0.05 >< Al203 16.40 14.41 11.73 0.01 0.55 0.03 Fe2O3 0.82 2.m 1.39 2.O7 3.30 5.64 Mnro3 74.38 76.53 72.32 MnO 2.9 6.34 7.94 4.25 3.47 4.09 Mgo 27.70 26.18 21.55 0.o2 3.64 0.09 CaO o.qt 0.05 0.04 7.43 r.65 6.59 Naro 0.09 0.o2 0.08 K.o 6.90 3.80 9.63 0.01 BaO 0.09 0.04 0.12 HrO 4.19 4.02 4.11 Total 96.48 93.37 98.06 99.75 99.38 99.55 XMnS' Si 2.716 2.708 3.037 1.139 0.992 1.053 Ti 0.002 0.001 0.001 0.001 0.004 Fig. 2. Compositional plots of the pumpellyites studied in AI 1.384 1.266 1.010 0.001 0.063 0.004 xil+"[: Mn3+/(Mn3* + Fe3*+ Al)] - X?i"I: Mnr*/(Mnr* + Mg)l FeP* 0.044 0.123 0.076 0.153 0.243 0.421 5.567 5.706 5.459 pumpellyite-(Mg), pumpellyite-(Mn +), Mn3* spaceshowing the fields of Mn2* 0j42 0.400 0.491 0.354 0.288 0.343 magnesian okhotskite, designated okhotskite-(Mg), and manga- Mg 2.957 2.909 2.U6 0.003 0.531 0.013 noan okhotskite, designated okhotskite-(Mn2*) [following Pas- Ca 0.002 0.004 0.003 0.78!1 0.173 0.700 sagliaand Gottardi (1973) and Kato et al. (1981); note that ok- NA 0.012 0.003 0.011 0.630 0.361 0.897 0.001 hotskite-(Mg) K and okhotskite-(Mn,*), although following from Ba 0.003 0.002 0.005 nomenclature approved by the IMA Commission, have them- selves not been approvedl. Pumpellyite from type I assemblage Note.'Total Fe as Fe"O . 'Dashed portions indicate below detection level : circle, pumpellyite from type 2 assemblage: plus sign.

magnesian braunite in the Indian deposits are recorded semblagesI and 2 on the SiOr-KAlOr-(Mn,Mg)O plane only when these are intruded by pegmatite dikes and cal- projocted from braunite (number l8 in Table 2) and,calcite veins. Phlogopite contains W to 7.94 wto/oMnO (Ta- cite show that the composition of pumpellyite varies with ble 2). the nature of the coexisting phases(Fig. 3). Pumpellyite The compositional plots of the coexisting phases in As- coexisting with phlogopite and diopside in Assemblage 2

TaslE 1-Continuecl

10 11 12 13 14 15 16 17 18 19 20 (2) (21 (2t (2) (2) (21 (2) (2) (2t (2t (2) 34.40 33.24 35.02 31.90 32.97 34.19 33.95 34.38 34.03 u.27 3i1.65 8.83 5.41 10.50 1.60 4.n 8.05 4.78 6.78 7.25 6.10 6.'14 0.03 0.02 0.01 0.01 0.03 0.07 0.05 0.07 0.05 0.03 0.40 2.87 1.85 0.19 0.71 o.23 2.31 1.58 0.61 0.90 16.14 17.03 11.61 25.93 21.12 17.75 22.09 16.92 16.58 19.61 18.32 7.97 10.52 9.90 11.95 10.12 7.61 7.49 10.u 9.36 6.34 9.69 3.12 1.32 2.O5 0.04 1.57 3.O7 2.93 1.49 1.82 3.89 1.64 20.81 19.95 20.94 19.77 20.69 22.06 2',t.73 21.61 21.68 21.47 21.19 0.(M 0.04 0.04 0.01 0.02 o.02 7.20 6.93 7.27 6.60 6.86 6.91 6.79 6.84 6.78 6.&t 6.67 98.94 97.29 99.18 97.U 97.80 100.39 100.08 100.82 99.17 99.17 98.23 12.04 12.18 12.19 11.95 12.08 11.86 11.99 12.O5 12.04 12.M 12.09 0.0s o.14 0.01 3.64 2.U 4.31 0.71 1.85 3.15 1.98 2.80 3.02 2s2 2.60 0.01 0.01 0.01 o.o2 0.01 o.o2 0.o1 0.10 o.79 0.49 0.05 0.18 0.06 0.61 0.42 0.16 0.24 4.25 4.86 3.21 7.39 6.10 4.56 5.94 4.58 4.54 5.30 5.15 2.36 2.81 2.79 3.79 2.94 2.26 2.24 3.04 2.74 1.82 2.81 1.63 1.06 1.06 0.21 0.86 1.59 1.54 0.78 0.96 2.04 0.88 7.81 7.8ri! 7.81 7.93 8.13 8.20 8.22 8.11 8.22 8.08 8.16 0.03 0.03 0.03 0.01 0.01 0.01 39.99 40.11 40.12 4.28 40.20 39.67 40.00 40.10 40.05 40.04 40.10 16.01 15.89 15.88 15.72 15.80 16.33 16.00 15.90 15.95 15.96 15.90 244 DASGUPTA ET AL.: Mn-RICH PUMPELLYITE

. j PRESENTDATA . . ?ifff'9X*'d?i#9i'^ /x' ooK v.i\ \ /:J * :PUMPELLYITE-Mn& "\'4.o A FRoMKAToETAL(]98I) o ;/ oK. oxNor:{'l,E^F.q9[. .J' TOGARIAND AKASAKA(Ig87). . ,.^^-, '_

Fig. 3. Compositionalplots of pumpellyitesin Assemblages Fig. 4. Compositionof Mn-rich pumpellyitesand okhotsk- I and2,diposide, mica, and potassium feldspar on SiOr-KAlOr- itesin Al3+-Fe3+-Mn3+space. (Mg,Mn)O planeprojected from brauniteand calcite. shows sympathetic variations in (Mg,Mn)O content with braunite and phlogopite are the associates,the presence that in phlogopite. of phlogopite does not assurea constantly high Mg content; Mg can be quite variable (Table l, numbers 8-20; DrscussroN Fig. 2). However, there is a positive correlation between Condition of formation the Mg contentsof coexistingpumpellyite and phlogopite Roy and Purkait (1968) describeda number of Ca-Al- in Assemblage2 (Fig. 5). Thesefeatures indicate that the Mg-Mn-Na silicatesthat are developed at the contact of compositional variations in the pumpellyite samples late pegmatite dikes and calcite veins in the manganese studied are related to local bulk composition. oxide ore deposit at Gowari Wadhona. The Mn-rich Experimental data on Mn-free pumpellyite (Schiffman phasesofthe pumpellyite group described herein are new and Liou, 1980, 1983; Schreyeret al., 1986)are clearly additions to this list of silicates. Development of this phase inadequate to explain the stability relations of the Mn- in Precambrian rocks is especially intriguing. The restric- rich pumpellyite studied here. However, Schiffinan and tion of pumpellyite studied here to late pegmatite dikes and calcite veins and textural features such as replace- ment of braunite by pumpellyite indicate a hydrothermal origin for pumpellyite where Ca, Al, Mg, and Fe are metasomaticallyintroduced in preexistingmanganese oxide ores. The processis thus distinct from the metamorphic origin of pumpellyite elsewherein the world (Ka- wachi, 1975; Coombs et al., 1976; Kato et al., l98l; 09 Trzcienski and Birkett, I 982; Lucchetti, I 983; Togari and Fg Akasaka,1987). >< Compositional plots of the pumpellyite in Al3+-Fe3+- Mn3+ space (Fig. a) reveal a wide range of substitution between Al3* and Mn3* in the presenceof aluminous 0.8 phasessuch as potassium feldsparand phlogopite.Within the limits of a single thin section, however, the composition ofthe phasesdoes not show any appreciablevari- ation. The widely varying Al content in pumpellyite from closely spaced samples suggestsdifferential partitioning of cations in the pumpellyite minerals in responseto co- 03 0.4 0.5 existing phasessuch as potassiumfeldspar and phlogopite pu rather than to temperature(cf. Schiftnan and Liou, 1980, XM9 1983). Although the Mg content of pumpellyite is con- Fig. 5. Plot of Xft [: Mg/(Me + Mn,*)] vs. -X!flI: Mg/(Mg siderably higher in Assemblage2 (Fie.2) where high-Mg + Mn'z+)].Data from TablesI and 2 and unpublishedsource. DASGUPTA ET AL.: MN-RICH PUMPELLYITE 245

Liou (1980) noted that the compositional variabilities in filled by only one cation, a situation which may be rare the pumpellyite group of minerals are complex functions in nature. If the alternative suggestion of Passaglia and of temperature,pressure, .for,and local chemical environ- Gottardi (1973) is followed, the pumpellyite group of ment of crystallization. The primary factors that influ- minerals described here might be designated as magne- encedthe composition of the presentpumpellyite formed sian pumpellyite-(Mn'z*), magnesian okhotskite-(Mn'z*), under similar P-Z conditions were high Mn content in and manganoanokhotskite-(Mg) (Fig. 2). the rock, the high, but variable, contents of Ca, Al, Mg, and Fe in the metasomatic fluids, and a high enough/o" AcxNowr-rocunNr for Mn to exist in both di- and trivalent states. Financial support from the University Grants Commission (S.D') is Solid solution in Mn-rich pumpellyites and thankfully acknowledged. The paper has been considerably improved throueft the reviews by W.E. Trzcienski, J. Iaird, and an anonymous nomenclature of the punpellyite group reviewer. We aro thankfiil to them. Figure 4 illustrates the variation in Al, Fe3*,and Mn3+ in the pumpellyite group of minerals studied by us, ok- RrrnnrNcps crrED hotskite (Togari pumpellyite-(Mn'z*) and Akasaka, 1987), Coornbs, D.S., Nakamura, Y., and Vuagnat, M. (1976) Pumpellyite-ac- (Kato et al., l98l), and some of the pumpellyite samples tinolite facies schists ofthe Taveyanne Formation near Lodche, Valais' andjulgoldites described by Passa8[iaand Gottardi (1973). Switzerland. Joumal of Petrology, 17, 440-47 l. Some of the phasesreported here are rich in Mn3+, even Kato, A., Matsubara, S., and Yamamoto, R. (1981) Ihmpelyite-(Mn*) Prefecture, Bulletin of Min- richer than the okhotskite ofTogari and Akasaka (1987), from the Ochia Mine, Yamanashi Japan. eralogy, 104,396-399. and approach the Mn3' end-member composition. Kawachi, Y. (1975) Pumpellyite-actinolite and contiguous facies meta- Therefore, the compositional field of okhotskite is shown morphism in parts of upper Wakatipu district, South Island, New Zea- to be extended farther to the Mn3* end-member of the land. New Zealand Journal ofGeology and Geophysics,18,401-441. garnet pumpellyite group of minerals. Solid solution between Lattard, D., and Schreyer, W. (1983) Synthesis and stability ofthe Fe-Mn-Si-O. Contributions to Mineralogy and pumpellyite (Fig. calderite in tho system and okhotskite is nearly complete a). Petrology, 84, 199-214. Such a relationship has already been recognizedbetween Lucchetti, G. (1983) Mn-rich pumpellyite from Eastern Liguria (ItaI9. pumpellyite and julgoldite (Passagliaand Gottardi, 1973). Neues Jahrbuch fiir Mineralogie Monatshefte, 12,56t-568. As already discussedby Passagliaand Gottardi (1973), Moore, P.B. (1971) Julgoldite, the Fe'?+-Fer+dominant pumpellyite. Lith- the nomenclature of the pumpellyite group of minerals is os, 4, 93-99. Passaglia, E., and Gottardi, G. (1973) Crystal chemistry and nomencla- difficult owing to intersite exchangeof Al and Fe3* (also ture of pumpellyires and julgoldites. Canadian Mineralogist, 12, 219- Mn3+) as well as variable cation occupancyin the X and 223. I sites. We looked for any possible correlation between Roy, S., and Purkait, P.K. (1968) Mineralogy and genesis of metamor- the atomic ratios of the major components in the X and phosed manganese silicate rocks (gondites) of Gowari Wadhona, Ma- Pradesh, India. Contributions to Mineralogy and Petrology' 20, Mn-rich pumpel- dhya I sites from the available data on the 86-114. lyite group of minerals. The Mn'z*/Mg vs. Mn3+/Al plot Schiftnan, P., and Liou, J.G. (1980) Synthesis and stability relalions of did not show any correlation between X- and Y-site oc- Mg-Al pumpellyite Ca.Al'MgSLOr, (OH)'. Journal of Petrology, 2l' cupancies. Such independent variations in the two site 441-474. (1983) Synthesis of Fe-pumpellyite and its occupanciesjustifu the classificationof pumpellyite solid Schiftnan, P., and Liou, J.G. stability relations with epidote. Journal of Metamorphic Geology, l, solution phasesbased on the occupancyof I-site which 9l-101. characteristicallyaccommodates mainly, if not solely, tri- Schreyer,W., Maresch, W.V., Medenbach,O., and Baller, T. (1986) Cal- valent cations. cium-free pumpellyite; a new synthetic hydrous Mg-Al-silicate formed Both the pumpellyite-(Mn,*) and okhotskite-(Mn'*; at high pressures.Nature, 321, 510-511. Togari, K., and Akasaka, M. (1987) Okhotskite, a new mineral, an Mn3*- varieties studied here (Fig. 2) have widely varying Mg dominant member of the pumpellyite group from the Kokuriki mine, content in the X-site, evidently dictated by the nature of Hokkaido, Japan. Mineralogical Magazine, 51, 6l l-614. the associatedphases. Naming a mineral of the pumpelly- Trzcienski, W.E., and Birkett, T.C. (1982) Compositional variations of ite group with only the dominant X-site cation as suffix pumpellyite along the western margin of the Quebec Appalachian. Ca- Mineraloeist, 20, 263-269. (Passagliaand Gottardi, 1973) or prefix (Moore, l97l) nadian thus losesits relevance.Naming with suffix or prefix will M,cNUscFJPrRrcErvED JuNe 29, 1989 be meaningful only where the X-site is almost completely MaNuscnrgr AccEPTEDNover'rnrn 7, 1990