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Manganese-Rich Minerals of the Pumpellyite Group from The

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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 ok- hotskite-(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.
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