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Stability of Braunite and Associated Phases in Parts of the System Mn-Fe-Si-O

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Stability of Braunite and Associated Phases in Parts of the System Mn-Fe-Si-O MINING GEOLOGY, 36(5), 351•`360, 1986 Stability of Braunite and Associated Phases in Parts of the System Mn-Fe-Si-O Somnath DASGUPTA*, Himadri BANERJEE**, Hiroyuki MIURA** and Yu HARIYA** Abstract: Stability of braunite and associated phases in parts of the system Mn-Fe-Si-O is experimentally deter mined using synthetic bixbyite and quartz as starting material by Tuttle-type bomb and piston-cylinder apparatus under Mn2O3-Mn3O4 and Mn3O4-MnO buffers. Some phase boundaries have been bracketed by reverse runs. Mix ture 1 with bixbyite and quartz in 5:1 molecular proportion produces braunite+jacobsite at 675•Ž, 1 kb under Mn2 O3-Mn3O4 buffer and at 600•Ž, 1 kb under Mn3O4-MnO buffer. Mixture 2 bixbyite and quartz (3.5:1) produces braunite+hematite+quartz under Mn2O3-Mn3O4 buffer and braunite+magnetite+quartz under Mn3O4-MnO buffer at 550•Ž, 1 kb. The former assemblage breaks down to jacobsite+pyroxmangite at 675•Ž, 1 kb while the lat ter to jacobsite (2 ph)*1+tephroite+quartz at 650•Ž, 1 kb under the respective buffers. In mixture 3 composition (bixbyite: quartz=2.5:1), braunite+jacobsite+quartz assemblage is stabilized under both buffers at 550•Ž, but breaks down to tephroite+jacobsite (2 ph) (•}quartz) at 750•Ž and 1 kb under Mn3O4-Mnb buffer. All the phase boundaries are rather insensitive to pressure upto 7 kb. The important conclusions of this study are: (a) stability of tephroite is restricted to Mn3O4-MnO buffer even under suitable bulk composition, but this phase can appear in the so far unreported non-carbonatic situations, (b) upper stability of braunite is extended to higher temperatures at higher fO2 and the lower stability is extended to lower fO2 as compared to the Fe free system, (c) stability of bixbyite is reduced in low silica bulk composition with the lowering of fO2, (d) the assemblage jacobsite+pyroxmangite and jacobsite (2 ph)+tephroite are restricted to a nar row range of bulk composition and fO2. Chemical composition of the synthetic phases reveals that the Fe content of braunite varies depending on the bulk composition and temperature of equilibration, but that of silica is restricted to a narrow range. In coexisting braunite and jacobsite, Fe is preferentially partitioned to braunite with increasing temperature, while Mn is partition ed to jacobsite. Jacobsite equilibrating with pyroxmangite has the highest Fe/Mn ratio, while when equilibrating with tephroite+quartz, it has the lowest Fe/Mn ratio. with the exception of the rare braunitell (e.g. 1. Introduction DE VILLIERSand HERBSTEIN,1967), the normal Metamorphism of non-carbonatic Mn-rich braunite contains only a little amount of Mg sediments gives rise to various assemblages of and Ca, but may contain variable amounts of bixbyite, braunite, hematite, hollandite, haus- Fe (e.g. ,ABRAHAMand SCHREYER,1975; MOORE mannite, jacobsite, Mn-pyroxenes and pyroxe- and ARAKI,1976; ABS WURMBACHet al., 1983; noids, spessartine, and quartz (e.g. Roy, 1981). DASGUPTAand MANICKAVASAGAM,1981b; BHA- Braunite is by far the most common mineral in TTACHARYAet al., 1984a, b). Other oxides like these rocks. Several studies have shown that bixbyite and jacobsite contain appreciable amount of Fe. Thus, barring spessartine and Received November 14, 1985, revised June 30, 1986, hollandite, the composition of the different accepted August 15, 1986 manganese oxides and the Mn-Fe end * Department of Geology and Mineralogy , Hokkaido members of the manganese silicates can be University, Sapporo 060, Japan described within the system Mn-Fe-Si-O. Present address: Department of Geological Science, Experimental studies on the stability rela- Jadavpur University, Calcutta-700 032, India tions of braunite and associated phases have ** Department of Geology and Mineralogy , Hokkaido *1 The spinel phase University, Sapporo 060, Japan , the composition of which falls within Keywords: Mn-Fe-Si-O system, Braunite, Bixbyite, the two phase field in the Fe3O4-Mn3O4 system are Tephroite, Jacobsite designated as Jacobsite (2 ph). 351 352 S. DASGUPTA, H. BANERJEE,H. MIURA and Y. HARIYA MINING GEOLOGY: been studied in the system Mn-Si-O (MUAN, were chosen after averaging out hundreds of 1959a, b; HIND et al., 1978; ABS WURMBACH, published and unpublished analyses of natural 1980; MoMOI, 1982; ABS WURMBACH et al., materials. Bixbyite was synthesized by heating 1980, 1983) and in the system Mn-Si-C-O reagent grade Mn2O3 and Fe2O3 (in 2:1 weight (HUEBNER, 1967; PETERS et al., 1974). Phase proportion) to 800•Ž for 10 days in air. Three relationships of Mn-Fe oxides have been deter- different starting materials were prepared in mined in the system Mn203-Mn3O4 (MUAN the following manner. and SOMIYA, 1962) and Mn304-Fe3O4 (MASON, Mixture 1: 1943; VAN HOOK and KEITH, 1958; HALBA et Bixbyite+quartz in 5.0:1 molar ratio al., 1973). DASGUPTA and MANICKAVASAGAM Mixture 2: (1981a) constructed a petrogenetic grid for the Bixbyite+quartz in 3.5:1 molar ratio system Mn-Fe-Si-O which brings out the Mixture 3: phase relations qualitatively. Solubility of Bixbyite+quartz in 2.5:1 molar ratio silica in braunite has been discussed by MUAN The mixtures were severely ground in (1959a) and Abs WURMBACH et al. (1983). alcohol and dried. The compositions of the LATTARD and SCHREYER (1983) studied the sta- starting materials are shown in the projected bility of calderite garnet in a part of the system oxygen-free triangle Mn-Fe-Si, along with the Mn-Fe-Si-O. relevant phases in the system (Fig. 1). For In this background, the present paper deals reverse runs, the products and reactants of the with the experimental study on a part of the relevant runs were mixed in equal proportions system Mn-Fe-Si-O determined under con- and was used as starting materials. All the syn- trolled foz. This study is therefore relevant to thetic materials were checked by X-ray. the metamorphism of manganiferous sedi- The buffer mixtures were prepared by mix- ments devoid of carbonates. Since braunite is ing Mn2O3 and Mn3O4 (obtained by heating the near ubiquitous phase in these rocks, our MnCO3 for 3 days in air at 800•Ž and 1100•Ž experiments were designed to give maximum respectively), and Mn3O4 and MnO (hydrother- information on its stability. Specifically, we mally synthesized by mixing Mn3O4 and aimed to determine the lower and upper stabil- metallic Mn) in 1:1 proportion. After each run ity of braunite and associated phase in P-T- the buffer mixtures were checked by X-ray to foz space using three different starting materials detect any change. containing bixbyite and quartz in different pro- 2.2 Apparatus portions (details given later). Also we intended Hydrothermal synthesis was carried out in to study the partition behaviour of Mn and Fe Tuttle-type vessels. The charge was placed in in the coexisting phases as a function of inten- sealed Ag50Pd50 capsules with water and was sive variables. To obtain maximum relevance enclosed in Au tube containing the buffer mix- to the metamorphism of Mn-rich sediments, ture and water and sealed.Temperature was we restricted the P-T range of our ex- monitored by chromel-alumel thermocouple. periments between 0.5-7 kb and 550•Ž-800•Ž During the long runs pressure and tempera- respectively, thus corresponding to the con- ture varied within the range •}0.1 kb and ditions of crustal metamorphism. The fo e in •}10•Ž respectively. our experiments was buffered at Mn2O3-Mn3 High pressure runs were carried out in O4 and Mn304-MnO buffers that are pertinent piston-cylinder aparatus of 2.5 cm bore and 5 to the metamorphism of such sediments in cm length. The pressure transmitting medium nature (e.g. HUEBNER, 1967; BHATTACHARYA et was molten pyrex glass, and the design of the al., 1984a). pressure cell was similar to that figured by HARIYA and KENNEDY (1968). Sealed Ag50Pd50 2. Experimental Procedure tubes containing the charge (with about 2% of 2.1 Starting material water) was placed inside sealed Au tube con- The compositions of the starting materials taining the buffer and water. Temperature was 36(5), 1986 Stability of braunite and associated phases in parts of the system Mn-Fe-Si-O 353 Fig. 1 Relevant phases in the system Mn-Fe-Si-O projected on to the oxygen-free triangle Mn-Fe-Si (a part of the triangle is shown). Also shown the composition of the starting materials (Mix. 1-Mixture 1, Mix. 2-Mixture 2, Mix. 3-Mix- ture 3). Table I Cell constants of the synthetic phases monitored by chromel-alumel thermocouple and corrections for the pressure value due to friction were made. No correction was however made for the effect of pressure on the electromotive force of the thermocouple. Each capsule was carefully checked after runs for any possible leakage. While there was no significant variation of pressure, temperature varied in the order of •}15•Ž for long dura- tion runs. When certain runs did not show any-notice- able change in the reactants after considerable time, the charge was taken out, reground and put back in the capsule with little water. thetic braunite, jacobsite and tephroite are 2.3 Identification compatible to those in the J.C.P.D.S. data Identification of run products was mainly file. There is a negative correlation between carried out by X-ray powder diffraction tech- chemical composition and cell parameters for nique. Optical confirmations were also made. braunite and jacobsite. In both cases the cell Representative run products were analyzed by constants decrease with increasing Fe content electron microprobe using a JEOL JCMA- in the phases. Hematite and magnetite were 733, at 15 kV operational voltage and 1-2ƒÊm detected only in the X-ray pattern and could beam diameter.
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