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

Home , Bixbyite, Calderite, Hausmannite, Jacobsite, Pyroxmangite

MINING GEOLOGY, 36(5), 351•`360, 1986

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. Synthetic MnO, Fe2O3 and not be anlayzed because of small grain size. In SiO2 were used as standards. In case of reverse many of the run products, minute or trace runs, changes in the X-ray intensity of the reac- amounts of quartz has been detected, which tants or products were used as indicative of re- may be present as a stable excess reactant, equilibrations. Lattice constants of.the syn- paticularly as the starting compositions do not thetic phases are given in Table 1. The fall exactly on the tie line joining the product chemical composition of the corresponding phases (Fig. 1). phases is given in Tables 2-5. The pyroxenoid 3. Phase Relationship phase could best be indexed to pyroxmangite. Moreover, the characteristic rhodonite (240) 3.1 Experiments under Mn2O3-Mn3O4 buffer reflection was not observed in any X-ray pro- With Mixture 1: In this composition (Fig. file. In general, the cell parameters of syn- 2a), the starting material does not show any 354 S. DASCUPTA,H. BANERJEE,H. MIURPAand Y. HARIYA MINING GEOLOGY:

Fig. 2 Phase relations under Mn2O3-Mn3O4buffer. (a) with Mixture 1, (b) with Mixture 2, (c) with Mixture 3. Bx: bixbyite, Qz: quartz, Br: braunite, Hm: hematite, Jb: jacobsite, Pxm: pyroxmangite, Arrow indicates reverse

runs.

Fig. 3 Phase relations under Mn3o4-MnO buffer. (a) with Mixture 1, (b) with Mixture 2, (c) with Mixture 3. Jb (2 ph): jacobsite 2 phase field, See Fig. 2 for abbreviations.

change at 600•Ž and 1 kb. At 675•Ž and 0 .5, 650•Ž, 1 kb and breaks down to jacobsite 1, 1.5 kb, an assemblage braunite+jacobsite +pyroxmangite at 675•K C and I kb. The boun- formed. This assemblage also formed at dary has been bracketed between 650•Ž and 700•Ž and 3-7 kb. Reverse runs at 650•Ž , 1 675•Ž at 0.5 and 1 kb. Several attempts to and 3 kb led to significant decrease of the X- locate the lower stability of braunite did not ray intensity of jacobsite. yield satisfactory results. With Mixture 2: Phase relationship in this With Mixture 3: In this composition (Fig. composition is shown in Fig. 2b. Braunite is 2c) the assemblage braunite+jacobsite+ stable with hematite and quartz from 550•Kto quartz is stable at all pressuret-emperature of 36(5), 1986 Stability of braunite and associated phases in parts of the system Mn-Fe-Si-O 355

Table 2 Composition of synthetic braunite

All analyses recalculated on the basis of Si=Mn2+ and the rest Mn as Mn2O3. Total Fe as Fe2O3. Hm:hematite, Qz:quartz, Jb:jacobsite, Pre:pressure, Tem:temperature

investigation viz. 1-5 kb and 550•K-800•Ž 550•Ž, 1 kb which remains stable upto 700•Ž respectively. at all pressures upto 7 kb. At 750•Ž and 1-7

3.2 Experiments under Mn3O4-MnO buffer kb, this assemblage converts to tephroite+

With Mixture 1: The reactant assemblage is jacobsite (2 ph) (•}quartz) which is stable upto stable at 550•Ž at all pressures of investiga- 800•Ž at 3 and 7 kb. This boundary has been tion even after considerable time and several bracketed between 750•Kand 700•Ž at 1 kb. re-runs. The phase relationship in this The pressure dependence of all the reactions composition (Fig. 3a) shows that braunite+ was found to be small upto 7 kb. jacobsite assemblage formed at 600•Ž and 1- 4. Chemistry 7 kb and remains stable upto 800•Ž and 3.5 kb. Compositions of the coexisting phases are

With Mixture 2: In these experiments (Fig. given in Tables 2-5. For braunite and jacob- 3b) . braunite was stable with magnetite and site, the total Mn obtained in the electron quartz at 550•K and 600•Ž, 1 kb, 650•Ž, 3 kb microprobe results has been recalculated to and 675•Ž, 5 kb. The assemblage breaks Mn2O3 and MnO following the structural for- down to jacobsite (2 ph) (details discussed mulae of each. Following is the synthesis of later)+tephroite+ quartz at 650•Ž, 1 kb and data presented in the Tables 2-5.

700•Ž, 5 kb. The boundary has been (a) Braunite associated with hematite and bracketed between 650•K and 600•Ž at 1 kb. quartz (Table 2) has identical composition With Mixture 3: In this composition (Fig. with XFe3+/Mn3+=0.127 and Si02 percent 3c) the reactants readily produced an around 9 indicating the homogenization ob- assemblage of braunite+jacobsite•}quartz at tained in the experiments. 356 S. DASGUPTA,H. BANERIEE,H. MIURA and Y. HARIYA MINING GEOLOGY:

Table 3 Composition of synthetic jacobsite

Table 4 Composition of synthetic pyroxmangite

Total Fe as FeO and Mn as MnO. Jb:jacobsite, Pre:pressure, Tem:temperature All analyses recalculated on the basis of Mn2+:(Mn3++Fe3+)=1:2. Total Fe as Fe203. varies significantly with temperature at the Br:braunite, Pxm:pyroxmangite, Tph:tephroite, same foz. It increases with increasing tempera- Qz:quartz Pre: pressure, Tem:temperature *:jacobsite in 2 phase field (see text) ture: XFe3+ /Mn 3+ =0.135 at 650•Ž, 0.160 at 700•Ž, 0.169 at 750•Ž and 0.255 at 800•Ž. All

(b) When associated with jacobsite (Table these data excepting the last one are from 1 kb 2) the Si02 content of braunite varies within a experiments. At 3 kb and 700•Ž this ratio is restricted range, but Fe content (as Fe203) 0.163, indicating its insensitivity to pressure. 36(5), 1986 Stability of braunite and associated phases in parts of the system Mn-Fe-Si-O 357

tent (Table 3) and covers a wide range of the Fe3O4-Mn3O4 system as shown in Fig. 4, following MASON (1943); VAN HOOK and KEITH

(1958); HALBA et al. (1973). (i) In the jacobsite coexisting with braunite, XFe3+/Mn3+ varies between 1.678 and 2.119

and there is an excellent negative correlation between the temperature and this ratio e.g. 2.119 at 675•Ž, 1.892 at 700•Ž, 1.824 at 750•Ž and 1.678 at 800•Ž with same buffer.

(ii) The jacobsite associated with pyroxmangite has the highest Fe content. Fig. 4 Composition of the synthetic jacobsite from (iii) When associated with tephroite and different phase assemblage of the present investiga- quartz, jacobsite has the lowest Fe content tion projected in the composition-temperature field and XFe3+/Mn3+ varies between 0.319 and of the Fe3O4-Mn3O4 system prepared by various 0.338. The composition of these jacobsites fall workers. Short dashes from MASON(1943); Solid line within the two phase field in the Fe3O4-Mn3O44 from VANHOOK and KEITH(1958); Long and short system (Fig. 4; cf. MASON, 1943; VAN HOOK dashes from HOLBAet al. (1973). Mt: magnetite, Jb: and KEITH, 1958; HOLBA et al., 1973), where

jacobsite, Haus: hausmannite, Pxm: pyroxmangite, hausmannite should have exsolved out of the Br: braunite, Tph: tephroite, Qz: quartz. spinel solid solution, under the present ex-

Table 5 Composition of synthetic tephroite

Total Fe as FeO and Mn as MnO. Jb(2ph):jacobsite in 2 phase field, Qz:quartz Pre: pressure, Tem:temperature 358 S. DASGUPTA,H. BANERJEE,H. MIURA and Y. HARIYA MINING GEOLOGY:

perimental conditions. However no hausman- 04-MnO buffers, various assemblages were nite could be detected in the optical, X-ray stabilized in conditions covering most of the powder diffraction profiles or in the electron amphibolite facies of metamorphism. The up-

microprobe studies. This can be explained in per stability of braunite is extended at higher the following maner. The exsolved blebs of fO2. In low silica bulk composition, the stabili- hausmannite were so minute that they escaped ty of bixbyite is significantly reduced at lower detection or as pointed out by VANHOOK and fo2. In silica rich bulk composition stability of

KEITH(1958) that unmixing of the cubic and braunite+jacobsite+quartz is enhanced at tetragonal phases is so sluggish that practically higher fo2. Pyroxmangite together with jacobs- it may not be possible to produce it under ite appears only under the Mn2O3-Mn3O4

laboratory investigations. Hence this spinel buffer through reaction between braunite, phase detected in the present investigation has hematite and quartz. Tephroite is restricted to been designated as jacobsite (2 ph). the Mn3O4-MnO buffer even under suitable

(d) Pyroxmangite in the above association bulk composition. Thus contrary to the earlier with around 8% FeO does not change its com- views (HUEBNER, 1967 as cited in Roy, 1981,

position with temperature (Table 4). page 122•`123), tephroite can also appear, (e) Tephroite (with fayalite mole per- even in non-carbonatic situations under cent=22-24.6) associated with jacobsite and suitable conditions and careful search may quartz shows restricted variation in composi- reveal this heretofore unreported phase from tion (Table 5). natural examples of manganese oxide-silicate Analysis of coexisting braunite and jacobs- rocks, at the upper most amphibolite or ite obtained at different P and T eluciated the granulite facies of metamorphism. nature of partitioning of Fe3+and Mn3+ betw- Partitioning of Mn3+ and Fe3+ between een the two. coexisting braunite and jacobsite shows KBDr-Jb (=XFe3+/Mn3+ in braunite/ relative enrichment of Mn3+ in the latter and XFe3+/Mn3+ in jacobsite) of Fe3+ in the former in response to rising =0 .0552 at 675•Ž, 1 kb temperature. Both braunite and jacobsite =0.0846 at 700•Ž , 1 kb show substantial changes in the Fe and Mn =0.0865 at 700•Ž , 3 kb contents in different associations. =0.0927 at 750•Ž , 1 kb Ass WURMBACH et al. (1980, 1983) discussed =0.1520 at 800•Ž , 5 kb the stability of braunite and associated phases This clearly shows that with increasing in the Fe-free system Mn-Si-O and have temperature, jacobsite becomes enriched in predicted qualitatively the changes in the reac- Mn and braunite in Fe at the same bulk com- tion curves as a result of entry of Fe. The pre- position and fo2 level. This also shows the KD sent data more or less confirm their predic- value is rather insensitive to pressure . tions. These authors have shown that in the Fe-free system none of the reactions intersects 5. Discussion the standard buffer curves, though the reac- Phase equilibrium data in parts of the tion braunite = hausmannite + rhodonite + O2 system Mn-Fe-Si-O under controlled fO 2, as lies close to the CuO-Cu20 buffer in fo2 T presented here, have a direct relevance to the space and that braunite is not stable under the metamorphism of carbonate free Mn rich Fe2O3-Fe3O4 buffer. In Fe-bearing system this sediments. This study clearly brings out the reaction leading to the formation of jacobsite

fact that the mineralogy of these is controlled occurs at the Mn2O3-Mn3O4 buffer. Thus, the by the interplay of several factors like bulk assemblage jacobsite+pyroxmangite/rhodo- composition, temperature and prevailing fo2 . nite may form at such high fO 2 within a specific However, the effect of pressure was found to compositional range. Rhodonite did not form be insignificant upto 7 kb. Depending on the in any of our runs though some of these were bulk compositions and Mn2O3-Mn3O4 or Mn3- within the rhodonite stability field, according 36(5), 1986 Stability of braunite and associated phases in parts of the system Mn-Fe-Si-O 359 to the various estimates (MOMOI, 1974; BHATTACHARYA, P. K., DASGUPTA, S., FUKUOKA, M., MARESCH and MOTTANA, 1977). It has been sug- HIROWATARI, F. and Roy, S. (1984a): Mineralogy and mineral chemistry of metamorphosed manganese gested that the pyroxmangite-rhodonite inver- sion curve determined in the system Mn-Si-O oxide ores and manganese silicate oxide rocks-The example from the Precambrian Sausar Group, India. may be altered in the presence of Ca, Mg, and Vol. of Leading Papers 27th. IGC. Moscow. Fe. Our study conveys the same note of cau- BHATTACHARYA, P. K., DASGUPTA, S., FUKUOKA, M. and tion to the application of this curve to the Roy, S. (1984b): Geochemistry of braunite and natural assemblages. In natural samples, associated phases in metamorphosed non-calcareous pyroxmangite contains more Fe than rhodo- manganese ores of India. Contr. Min. Pet., 87, 65 nite (MOMOI, 1974) and it is possible that the •` 71. pyroxmangite structure is stabilized in the pre- DASGUPTA, H. C. and MANICKAVASAGAM, R. M. (1981a): sence of Fe. Also we note that the stability of Regional metamorphism of noncalcareous mangan braunite in the Fe-bearing system is extended iferous sediments from India and the related to fo, lower than the Fe3O4-Fe2O3 buffer. petrogenetic grid for a part of the system Mn-Fe-Si-O . J. Pet., 22, 363•`396. The composition of jacobsite in natural oc- DASGUPTA, H. C. and MANICKAVASAGAM, R. M. (1981b): currences shows fairly wide variation in the Chemical and X-ray investigation of braunite from Fe3O4-Mn3O4 system. The compositional limit the metamorphosed manganiferous sediments of of jacobsite was set by VAN HOOK and KEITH India. N. Jb. Min. Abh., 142, 149•`160.

(1958) between 10 and 54% Mn3O4. As shown DEVILLIERS, JPR. (1975): The crystal structure of braunite in Fig. 4, synthetic jacobsites show wide varia- with reference to its solid solution behavior. Am. tion in its Fe-Mn content but they have com- Min., 60, 1098•`1104.

positional limits with the different associated DEVILLIERS, JPR. and HERBSTEIN, FH. (1967): Distinction between two members of the braunite group. Am. phase assemblage. Composition of jacobsite Min., 52, 20•`30. (2 ph) associated with tephroite +quartz falls in the miscibility gap, where the coexistence of HARIYA, Y. and KENNEDY, G. C. (1968): Equilibrium study of anorthite under high pressure and temperature. jacobsite and hausmannite is reported from Am. J. Sci., 268, 193•`203. natural occurrences. In natural occurrence, HIND, H., MINATO, T. and KUSAKABA, Y. (1978): this phase is named "vredenburgite" which Hydrothermal synthesis of braunite in the system is represented by exsolved hausmannite in manganite (MnOOH)-silica, hausmannite (Mn3O4)-

jacobsite. silica, and pyrolusite (MnO2)-silica (2500 bars).

Acknowledgement: S.D. acknowledges the Mem. Fac. Eng. Kyoto Univ., 40, 16•`29.

financial support obtained through a Japanese HOLBA, P., KHILLA, M. A. and KRUPICKA, S. (1973): On

Government scholarship and thanks Mr. S. the miscibility gap of spinels MnxFe3-xO4+r. J. Phys. TERADA and Y. IWABUCHI for various helps. Chem. Solids, 34, 387•`95. HUEBNER, J. S. (1967): Stability relations of minerals in Reference the system Mn-Si-C-O. Ph. D. Thesis John Hopkins

ABRAHAM, K. and SCHREYER, W. (1975): Minerals of the Univ., 279pp. virdine hornfels from Darmstadt, Germany. Contr. LATTARD, D. and SCHREYER, W. (1983): Synthesis and Min. Pet., 49, 1•`20. stability of the garnet calderite in the system Fe-Mn- ABS-WURMBACH, I. (1980): Miscibility and compatibility Si-O. Contr. Min. Pet., 84, 199-214. of braunite Mn2+Mn6+O8SiO4 in the system Mn-Si MARESCH, W. V. and MOTTANA, A. (1976): The pyrox- -O at 1 atm. in air. Contr. Min. Pet., 71, 393•`399. mangite-rhodonite transformation for the MnSiO3 ABS-WURMBACH, I., LANGER, K. and SCHREYER, W. (1980): composition. Contr. Min. Pet., 55, 69•`79. Studies on braunite stability ralations in the system MASON, B. (1943): Mineralogical aspects of the system Mn-Si-O at controlled oxygen fugacity. XII Gen. FeO-Fe2O3-MnO-Mn2O3. Geol. Forem. Forhandl. Meet. IMA. Orleans, Coll. Abs., 147. 65, 97•`180. ABS-WURMBACH, I., PETERS, Tj., LANGER, K. and MOMOI, H. (1974): Some manganese pyroxenoids. Min. SCHREYER, W. (1983): Phase relations in the system J., 7, 359•`373. Mn-Si-O: an experimental and petrological study. MOMOI, H., HIROWATARI, F. and FUKUOKA, M. (1982): N. Jb. Min. Abh., 146, 258•`279. Natural and synthetic braunites. Ganseki Kobutsu 360 S. DASGUPTA,H. BANERJEE, H. MIURA and Y. HARIYA MINING GEOLOGY:

Koshyo Gakkaishi, Spec. Vol . 3, 281•`289. •` 240. MOORE, P. B. and ARAKI, T. (1976): Braunite, its structure PETERS, Tj., VALARELLI, J. V. and CANDIA, M . A. F. (1974): and relationship to bixbyite and some insights on the Petrogenetic grids from experimental data in the

genealogy of fluorite derivative structure. Am. Min., system Mn-Si-O-H. Revta. Bras. Geol., 4, 15•`27. 61, 1226•`1240. Roy, S. (1981): Manganese deposits. Acad . Press. 458pp. MUAN, A. (1959a): Phase equilibria in the system VAN HOOK, J. J. and KEITH, M. L . (1958): The system manganese oxide-SiO2 in air . Am. J. Sci., 257, 297 Fe3O4-Mn3O4. Am. Min., 43, 69•`83.

•` 315. YUN, I. (1958): Experimental studies on magnetic and

MUAN, A. (1959b): Stabilitry relations among some crystallographic characters of Fe-bearing manganese manganese minerals. Am. Min., 44, 946•`960 . oxides. Mem. Coll. Sci., Univ. Kyoto, Ser. B25, MUAN, A. and SOMIYA, S. H. (1962): The system iron 125•`137. oxide-manganese oxide in air . Am. J. Sci., 260,230

Mn-Fe-Si-O系におけるブラウン鉱の安定領域と共生相

ソムナットダスグプタ・ヒマドリバナジー・三浦裕行・針谷宥

要旨:Mn-Fe-Si-O系の一部におけるブラウン鉱のマンガン石の共存領域および2相領域のヤコブス鉱とテ

安定領域と共生相を合成実験により決定した .実験にはフロ石の共存領域は化学組成とfO2とにより非常に狭い

Tuttle型熱水合成装置とピストンシリンダー型高圧発生範囲に限定される. 装置を用いた.出発物質は合成のビクスビ鉱と石英と合成相の化学成分については,ブラウン鉱中のFe含し,バッファーとしてMn2O3-Mn3O4系とMn3O4- 有量は出発物質と温度により変化するが,SiO2含有量 MnO系とを用いた.本研究から以下のことが明かとなは殆ど一定となる.共存するブラウン鉱とヤコブス鉱にった.(a)Mn3O4-MnOバッファーのもとでのテフP石おいて,温度の上昇と共にFeはブラウン鉱中に , Mn の安定領域は狭い範囲に限られる .(b)ブラウン鉱の安定はヤコブス鉱中に分配される.またパイロクスマンガン領域の上限温度はfO2の上昇に伴って上昇し,下限はfO2 石と共生するヤコブス鉱はFeに富み,テフロ石+石英の低下に伴って低下する .(c)ビクスビ鉱の安定領域はと共生するヤコブス鉱はMnに富む傾向を示す . fO2の減少に伴い狭くなる .(d)ヤコブス鉱とパイロクス