American Mineralogist, Volume 58, pages 979-985, 1973

SubsolidusPhase Relations in Aragonite-typeCarbonates. lll. The SystemsMgCO'-CaCO,-BaCOB, MgOO,-CaGO,-SrCO,, and MgCO,-SrCO,-BaCO,

Wrtr,reu R. BnrcE Department ol Earth and Planeta:rySciences, Uniuersity of Pittsburgh at lohttstown, Iohnstown, Pennsylaania 15901

Lurn L. Y. CnaNc Department ol Geology, Miqmi Uniuersity, Oxford, Ohio 45056

Abstract

Subsolidus phase relations in the three ternary systems were studied at 650"C and at both 5 and 15 kbar using high-pressure, opposed-anvil apparatus. Phases found in the systems, in addition to the end members, are (MgCa(COa),), norsethite (MgBa(COo),), MgSr(CO")a and two forms of CaBa(COs),, a monoclinic and a rhombohedral form with disordered structure. In the system MgCOrCaCOrBaCO", both norsethite and disordered calcite have detectable ranges of solid solution at 650'C and 5 kbar, and six two-phase areas and four three-phase triangles are observed. In the system MgCO-CaCO-SrCOI, a complete series of solid solution along the join MgCa(CO")-MgSr(COg)z and a significantregion of calcite-typesolid solution exist at 650"C and 5 kbar, whereas at 650'C and 15 kbar, aragonite is the stable form of CaCO" and forms a complete series of solid solution with SrCO". Two complete solid solution series, SrCO-BaCO* and MgSr(CO")-MgBa(COo)a were found in the system MgCO- SrCO-BaCOa, and the system is represented by two large immiscibility gaps between the two solid solution series and between the dolomite seriesand MgCOa.

Introduction Experimental Procedures As the third part of a study on the subsolidus Starting materials used were Baker analyzedrea- phase relations in aragonite-type carbonates, the gent grade CaCO3,SrCO3, BaCOs, and basic mag- systems MgCO3-CaCO3-BaCO3,MgCO3-CaCO3- nesium carbonate.The MgCO3 was prepared by SrCOs, and MgCOB-SrCOs-BaCOswere investi- hydrothermallytreating the basic magnesiumcar- gated at 650"C and two pressures,5 and 15 kbar. bonate with H2O and CO2 in a Morey-type vessel phasespresent in the system,in addition to at 200'C. the well known end members, include dolomite, All mixtures were calculatedon a molar percent- norsethite,barytocalcite, , and benstonite. agebasis. The accuracyof the weighingsis estimated The temperatureof 650'C was chosenbecause to be 10.05 mg, and the total weightof eachmixture previous studies of carbonatesindicated that mod- preparedvaried from about 0.75 to 1.5 g. Mixing erate temperaturesare required for mixtures involv- was made under acetonein a Spex Mixer/Mill for ing MgCO3 to react in a reasonablelength of time 15 minutes, followed by hand grinding for 45 (Goldsmithand Graf, 1960;Goldsmith and Newton, minutes. 1969). Becausetwo of the systemsinclude CaCO3, Experimentalwork was performedin the high- which exhibits both calcite- and aragonite-type pressure,oppose.d-anvil apparatus. The apparatus structures,it was decidedto selectpressure-tempera- and the proceduresto operateit were describedin ture relationshipswhich would offer two situations: detail by Griggsand Kennedy(1956) and by Mac- (1) conditionsunder which the calcite form is stable, Donald (1956), while Goldsmith and Newton and (2) conditions under which the aragonite form (1,969\ discussedthe suitability of this apparatus is stable. The two pressuresof 5 and 15 kbar for equilibrium studiesin carbonates. satisfiedthese requirements. Periods of 28 hours to I32 hours, with a few 979 980 W. R, BRICE AND L. L. Y. CHANG runs up to 221 hours,were used at 650"C depend- the reversibility of reaction in the systems are ing on the amount of MgCO3 in the the sample; listed in Table 1. the greater the amount, the longer the required run time. This apparently stemsfrom use of crystalline The SystemMgCo'3-CaCO:r-BaCO'r MgCO3 instead of a fine-grained precipitated ma- Subsolidusphase relations in the systemsMgCO3- terial. A small amount of Li2CO3 (approximately CaCOg (Goldsmith and Heard, 1961; Goldsmith 2percent of the sample)was addedto runs to pro- and Newton, 1969) and CaCO3-BaCOa(Chang, mote the reaction. Uncertaintiesare estimatedto 1965, l97I) have been well established,whereas be :L5oCfor temperaturesand within i0.5 kbar for the only reported work on the system MgCO3- pressures. BaCOa has centeredaround the synthesisof norse- X-ray powderdiffraction was used in phaseidenti- thite, the 1: 1 compound MgBa(CO3)2 (Chang, fication. Since all phasesinvolved are well crystal- 1964; Lippman, 1.967\. No studies on the ternary lized, any phase of more than 2 mole percent in phase relations in the system are known. the quenched assemblagecan be easily detected. In the present study, subsolidusphase relations Measurementof lattice parameters,construction of in the systemMgCOs-BaCOs were found to be very tie lines, and checking of attainment of equilibrium simple. Three phases,witherite, norsethite,and mag- were made in the manner described previously nesite, are stable, and there is no solid solubility (Chang, t971,). Results from runs made to show among them. According to Mrose et al (1.961),

Tenre 1. Pertinent Data on Runs Related to the Reversibility of Reactions in the Systems MgCOrCaCOrBaCos and MgCO"-CaCO-SrCOg

composition Eirst Period of Second Period of Products, mole% Equilibriun Equilibriuh phases* Pres. Temp, fime, Pres. Temp. Iime, oC oC kbar Hrs " kbar Hrs.

MqCO- CaCO- BaCO-

40.0 50"0 I0.0 t5 650 84 - Do + N ss + C ss 40.0 50.0 I0.0 15 650 A4 5 650 160 Do + N ss + C ss 40.0 50.0 10"0 5 650 A4 - Do + N ss + C ss 40,0 50.0 10"0 5 650 A4 15 650 160 Do + N ss + C ss

20.0 50.0 30"0 15 650 64 - A + N ss + B ss 20,0 50.0 30.0 15 650 64 5 650 96 c ss + N ss + cd ss 20.0 50.0 30.O 5 650 64 - c ss + N ss + cd ss 20"0 50.0 30.0 5 650 64 15 650 96 A + N ss + B ss

10"0 80"0 I0"0 15 650 48 - A + N ss I0"0 80"0 10.0 15 550 48 5 650 64 c ss + N ss + cd ss I0.0 80"0 10.0 5 650 4A - C ss + N ss f Cd ss I0"0 80,0 10.0 5 650 4a 15 650 64 A + N ss

Io"o 4o"o 50.o t5 650 4a - i - N ss l B ss 10.0 40.0 5O"0 15 650 4A 5 650 64 N ss + Cd ss I0"0 40"0 50"0 5 650 48 - N ss t Cd ss I0.0 40.0 50.0 5 650 4A 15 650 64 N ss I B ss

Mqco- caco- srco- 5"O 92"O 3"0 15 650 32 - A ss + C ss 5"O 92"O 3.O 15 650 32 5 650 68 C ss 5"0 92"0 3.0 5 650 32 - c ss 5"O 92.O 3.0 5 650 32 15 650 6A A ss + c ss

5.0 85"0 10"0 15 650 32 - A ss + C ss + Do ss 5.0 85.0 I0,0 15 650 32 5 650 68 c ss 5.0 85.0 10.0 5 650 32 - C ss 5.0 85.0 10"0 5 650 32 15 650 6a A ss + C ss + Do ss

5.0 80.0 15.0 15 650 32 - A ss + C ss + Do ss 5.O 80.0 15.0 15 650 32 5 650 68 C ss 5.0 8O.0 15.0 5 650 32 - c ss 5.0 80.0 15.0 5 650 32 15 650 68 A ss + c ss + Do ss

20.0 50.0 30.0 15 650 96 - A ss + Do ss 20.0 50,0 30.0 15 650 96 5 650 L24 A ss + C ss + Do ss 20.0 50.0 30.0 5 650 96 - A ss + c ss + Do ss 20.0 5O"0 30.0 5 650 96 15 650 124 A ss + Do ss

20.0 20.0 60.0 15 650 96 - A ss + Do ss 20.0 20.0 60.0 t5 650 96 5 650 L24 A ss + Do ss 20.0 20.0 60.0 5 650 96 - A ss + Do ss 20.0 20.0 60.0 5 650 96 15 650 L24 A ss + Do ss

* Abbreviations: A = aragonite? B =barytocalclte; C = calclte; Cd = disordcrcd cafciter Do = dolomiter N = norseLhiter ss = solld solution. PHASE RELATIONS IN ARAGONITE.TYPE CARBONATES, III. 981 norsethiteclosely rememblescalcite and dolomite in crystal structurewith a possiblespace group of.R32. X-ray powder diffraction data of norsethite pre duced in this study correspond well with those reported in literature (Mrose et aI, 196l; Chang, 1964;Sundius and Blix, 1965). Phase relations in the ternary system at 650oC and 5 kbar are shown in Figure 1. Norsethite has an extensive range of solid solution and forms equilibrium assemblagewith every phase presented in the ternary system.A maximum amount of 22 mole percentCaCOs entersthe norsethitesolid solu- tion along the join CaCOe-MgBa(CO3)2,a caseof substitutionof Ba * Mg by Ca. On the other hand, no solid solutionwas detected between MgCa(COe)z and MgBa(COs)2. Barytocalcite,the 1: 1.compound CaBa(COs)2, is CoBo(CO3)2 BoC0g not a stable phase in the systemCaCOs-BaCOs at Fro. 1. Subsolidus phase relations in the system MgCO'- 650'C and 5 kbar. Instead.the disorderedcalcite- CaCO-BaCOs at 650'C and 5 kbar. A - aragonite; C - type (Ca,Ba)COssolid solutionforms betweencom- calcite; Cd = disordered calcite; Do : dolomite; M - positionsof CaEznBa*.sand Ca62.5Ba375along the magnesite;N - norsethite;W = witherite. binary join CaCO3-BaCOa and extends into the ternary system,taking a maximum of 9 mole percent calcite and dolomite at 660oC and in the pressure MgCO, in solid solution. This disorderedphase can range 1-10 kbar, whereasthe latter synthesizedthe be differentiatedfrom the calcite-typephase by the 1:1 compound,MgSr(COs)z and presenteda phase absenceof a (113) peak and by the differencesin diagram of the ternary systemat 660'C and 3.9 position of other peaks in the X-ray diffraction pat- kbar. The diagramshows there are no solid solutions terns.The absenceof this (113) peak, as described betweenMgCO3, SrCOa, and MgSr(COa)2,and an previously (Chang, 1965), is due to the random arrangementof COs groupsin the structure,and the differencein position of other peaksis due to differ- MgCOI ent chemical composition. Subsolidusphase relations at 650'C and 15 kbar are shown in Figure 2, basedon the few runs illus- trated in the diagam. Th.y are similar to those shown in Figure 1 except that both barytocalcite and aragonite become stable phases representing CaBa(COg)zand CaCOB, respectively, in the system. The presenceof the calcite-typephase in the binary system MgCOg-CaCOs (Goldsmith and Heard, 1961; Goldsmithand Newton, 1969) was observed in the present study. It requires the existenceof a three-phaseregion of aragonite,a calcite-type solid solution, and a dolomite-type solid solution in the system.

B+w The SystemMgCO3-CaCOs-SrCOa coco. c'B coBo(coa), Boco3 Phase relations in the system have been studied FIc. 2. Subsolidusphase relations in the system MgCO- part by Froeseand Winkler (1,966) and Froese in CaCO"-BaCOa at 650'C and 15 kbar. A = aragonite; - (1967). The former determinedthe compositionof B - barytocalcite; C - calcite solid solution; Do aragonite-strontianitesolid solutionscoexisting with dolomite; M = magnesite;N - norsethite;W - witherite. 982 W. R, BRICE AND L. L, Y. CHANG

MgCO3 Subsolidusphase relations in the systemMgCOs- CaCOa-SrCO3determined in the present study at 650"C and 5 kbar are shown in Figure 3. A com- plete solid solution seriesforms along the dolornite join with little deviationfrom the 1:1 cation ratio, and interplanar spacingdro+ shows a linear relation- ship with composition(Fig. a). As a result of this complete series only one three-phaseregion exists in the system,and it is definedby an aragonite-type (/ / \) solid solution with a composition of Mg5C8a6Srs7, a dolomite-type solid solution near MgroCzl36Srla, 0\ / and a calcite-type solid solution of MgsCa66Sr12. /{ o.o. .t ^\tA The calcite-type solid solution has a range from A.C.D. a maximum of 22 mole percent SrCOain the binary ,^.l A ^' system CaCOs-SrCO3to a maximum of 10 mole percent MgCOr in the binary system MgCO3- SrCOg CoCOs CaCOa. The aragonite-type solid solution in the Fro. 3. Subsolidusphase relations in the systemMgCO- binary system CaCOg-SrCO3was found to extend 5 kbar. The tie lines in this and CaCOTSTCOgat 650"C and with a maximum of 3 mole following diagrams are representedby dashed lines. A = into the ternary system aragonite-type solid solution; C = calcite solid solution; percent MgCO3 near the composition of CamSroo' Do - dolomite-type solid solution; M = magnesite. Subsolidusphase relations in the systemat 650oC and 15 kbar are shown in Figure 5. Aragonite is the stable form of CaCOs under these conditions, immiscibility gap exists betweenMgSr(CO3)2 and and there is a completeseries of solid solution with MgCa(COe)2. Froese refers to the possible diffi- SrCOs. No effect of changing pressurewas found culty of quenchingCa-rich Mg(Ca,Sr)(COs)z solid on the solid solution along the join MgCa(COg)z- solutions.However, an apparentsolubility gap could MgSr(COe)2,but the three-phaseregion is greatly also be produced by incomplete reaction. reduced in size, as is the region of the calcite-type solid solution which is now confinednear Mgr6Caoo. At 650"C and 15 kbar, the three-phaseregion is defined by an aragonite-typephase with a composi- 3.02 MgBo(CO3)2 tion of Cas3Sr17,a calcite-typesolid solution with a composition of MgeCaeaSrl,and a dolomite-type solid solution with a composition of Mg56Ca+rSre. 3.OO In the Mg-poor half of the systern,a large two- phase region of dolomite-type and aragonite-type 2.98 solid solution exists. The dolomite-typephases tend o< to be Sr-poor as compared with their co-existing 2.96 MgSr(CO3)2aragonite-typephases. \t o The SystemMgCOg-SrCO3-BaCO3 E 2.94 Subsolidusphase relations in the systemat 65OoC 2.92 and 5 kbar are shown in Figure 6. Two complete seriesof solid solutionsexist along the joins SrCO3- 2.90 BaCOg and MgSr(COs)z-Mgtsa(COr)r, and the systemis representedby two large immiscibility gaps between 2.88 MgCo(Coj2 between the two solid solution series and the dolomite series and MgCO3. In the Mg-poor o20406080too half of the system,tie lines in the two-phaseregion phases, Frc. 4. Variation of do",t along the joins, MgCa(CO")- show that the dolomite-type Mg(Sr,Ba) MgSr(CO"), and MgSr(CO")-MgBa(CO,),. (COr)r, tend to be Sr-pooras comparedwith their PHASE RELATIONS IN ARAGONITE.TYPE CARBONATES, IN. 983 coexisting aragonite-typephase, (Sr,Ba)CO3. This MgCOg combined with the observation that the dolomite-type phasesof compositionMg(Ca,Sr) (COr), tend to be Sr-poor as comparedwith their coexistingaragonite- type phasesof composition(Ca,Sr)CO3 clearly illus- trates that the strontium atom has an affinity for the aragonite-structureover the dolomite structure. The phase relations at 65OoC and 5 kbar are expectedto be similar at higher pressures,because no phase transition is anticipated in any of the phasesinvolved. A. D. ,/ I The Dolomite Plane l / Subsolidusphase relations in this plane at 650'C f and 5 kbar (Fig. 7) reflect the difference in ionic I size between calcium, strontium, and barium. The immiscibility gap along the join MgBa(COr)r SrCO3 CoCO3 MgCa(COg)2 extendsinto the ternary plane and Fro. 5. Subsolidus phase relations in the system MgCOr representsa large two-phaseregion of two dolomite- CaCO"-SrCOa at 650"C and 15 kbar. A - aragonite-type - type solid solutions; one Ba-rich, the other Ca-rich. solid solution; C calcite solid solution; Do : dolomite- M - magnesite. With increasing Sr-content, the immiscibility gap type solid solution; decreasesin range and a complete series of solid solution forms when the Sr-content is greater than Norsethite at the Rosh Pinah deposit (Steyn and 70 mole percent. The tie lines in this two-phase Watson, 1967) occursin dolomite gangueas irregu- region are essentiallyparallel to the base line of larly shapedveins and as massesseveral feet across. the join MgCa(COs)z-MgBa(COs)2. For both It is also associatedthroughout with calcite in more joins,MgSr(COs ) rMgBa(COa ) z andMgSr(COa )r- or less equal quantities. A similar assemblagewas MgCa(COg)p,dr,oc varies linear$ with composition found at Langban, Sweden (Sundius and Blix, (Fie.a). 1965), and chemicalanalyses of the three co'exist- Similar phaserelations are observedin a few runs made at 650"C and 15 kbar. A run of composition 15:15:70 mole percent of MgCa(CO3)2:MgBa (COa)z:MgSr(COs)z producesa single phase in- stead of the twodolomite assemblageobserved at 5 kbar. This suggeststhat the eftect of Sr-content on the formation of solid solution in the plane is greater at higher pressures. Geological Consideration This investigationhas revealed the possibility of extensivesolid solubility of strontium in both norse- thite and dolomite. At pressuresof 5 and 15 kbar , Do+A and a temperature of 650'C, the solid solution is complete from norsethite through MgSr(COs)2 to u, dolomite. Experimental results also indicate that strontium is more likely to be found in an aragonite structure than in a dolomite structure. This may &@g SrCOg explain, at least in part, why MgSr(COs)z has not Frc, 6. Subsolidus phase relations in the system MgCOa- yet been found in nature. STCOTBaCOa at 650'C and 5 kbar. A - aragonite-type A number of occurrencesof norsethitehave been solid solution; Do = dolomite-type solid solution; M = reportedsince the first report by Mrose et al (1961). magnesite. 984 W, R. BRICE AND L. L. Y. CHANG

MgCo(CO3)2 cent MgCOa plane of the four-component system MgCOs-CaCOs-SrCO3-BaCOs.At 650'C and 5 kbar (Fig. 1) compositionsclose to that of bens- tonite produce phasesof the disorderedcalcite-type structure.A comparisonof X-ray powder diffraction data of benstonite with those of phasesprduced from compositionswhich are close to that of bens- tonite is shown in Table 2. The match of spacings is close enough to suggestthat benstonite may be a low-temperature ordered eguivalent of the dis- ordered phases at 650oC and 5 kbar. Benstonite has been reported in depositswith barytocalciteand norsethite (Steyn and Watson, 1967) and with dolomite and norsethite (Sundius, 1.963).

Dolomit!-lyp. lolid sotution Acknowledgments MgBo(CO3)2 M9S(C03)2 The auhors are indebted to Professor Julian R. Goldsmith, Fra. 7. Subsolidus phase relations in the dolomite plane University of Chicago, and Dr. Edgar Froese, Geological MgCa(COa)-MgSr(CO,)-MgBa(COr), at 650'C and 5 Survey of Canada, for reviewing the manuscript. This work kbar. was supported in part by the National Science Foundation Grant GA-1366.

ing are given. Except for the high percent- References age of MnCOs (in mole percent, 4.7 in norsethite, CueNc, L. L. Y. (1954) Synthesis of MBa(CO'), com- 7.03 in dolomite, and 6.5 in calcite), these min- pounds. Amer. Mineral. 49, 1142-1143. erals fit the three-phase assemblagepredicted by (t965) Subsolidus phase relations in the system the phaserelations shown in Figure 1. Temperature BaCOrSrCOa, STCOTCaCO', and BaCOTCaCO"' /. and pressureconditions for this deposit were esti- GeoI. 73, 346-368. (1971) phase relations in aragonite-type mared (1965) Subsolidus by sundius to be 500o-600oc and carbonates: I. The system CaCO-SrCO"-BaCO". Amer, about 10 kbar. Mineral. 56, 1660-1673. Norsethite in association with and FnoEse, E. (1967) A note on strontium magnesium car- other minerals was reported by Kapustin (1965) bonate. Can. Mineral. 9, 65-:70, -, in carbonatite veins in pyroxenite. Analysis shows lNo H. G. F. WrNxrpn (1966) The systemCaCO- pressures 500'C to 700"C. Can. norsethite SrCOs at high and that contains 1.4 mole percent SrCOs, Mineral. 8. 551-556. indicating solid solution in nature. Gorosrvrrrn,J. R., eNo D. L. Gur (1960) Subsolidusphase Benstonite,according to Lippman (1962), has relations in the system CaCO-MgCO"-MnCO', I. Geol, a derived frorn that of calcite by 6E. 324-335. the ordering of cations within the cation plane, and lNo H. C. Heeno (1961) Subsolidus phase rela- tions in the system I. GeoI. 69,45-74. a compositionclose CaCO-MgCOs. to Mg6Caa7Sr6Ba4.This comtrxF -, lNo R. C. NewroN (1969) P-T-X relations in the sition should occur as a phase on the 5 mole per- system CaCO-MgCO, at high temperatures and pres- sures.Amer. ,1. Sci. 2674, 160-190. Gnrccs, D. T., eNp G. C. KeNNeov (1956) A simple ap- Tt'rl,B 2. X-ray Powder Diffraction Data for Benstonite paratus for high pressures and temperatures, Amer. I. and Disordered Calcite with Compositions to That of Sci. 254,722-735. Benstonite Klpusrnl, Yu L. (1965) First find of norsethite in the U.S.S.R. Dokl. Akad. Nazk. S,SSR,161,922-924. (Min- Benstoniter Benstonite2 Disordered caLcite3 eral. Abst. 19, 312\. Mg6Sr6ca47Ba4I M9lgh3c"33PbZB"44 1496c^+78^47 c"a2.5st5B"47.5 LrrrlreNN, F. (1962) Benstonite, Ca?Bao(CO3)8,a new d, A I/Io d, A a/Io d, A t/to d, A a/to mineral from the barite deposit in Hot Springs County, 3a 3.89 60 40 50 3.04 95 3.O7 IO0 Arkansas.Amer. Mineral. 47, 585-598. '70 3.r3 100 r00 2A 50 (1967) Synthesisof norsethiteBaMg(COs), at 2O"C 2.I27 2.It 60 2.145 40 2.149 40 and 1 atm,: a model of dolomitization. Neues lahrb. Benstonite from Arkansas, Lippmnn (196?) Mineral. Monatsch. l, 23-29, BensConite from hngban, Sundius (1953) Disordered calcite obtained at 650oC and 5 kbars MecDoNero, G. J. T. (1956) Experimental determination PHASE RELATIONS IN ARAGONITE-TYPE CARBONATES. lil. 985

of calcite-aragonite equilibrium relations at elevated tem- SuNorus,N, (1963) Benstoniteand tephroite from Langban' peratures and pressures,Amer. Mineral. 41, 744-756, Ark. Mineral. Geol. 3,407-411. Mnose, M. E., E. T. Cneo, J. J. Fenrv, AND C. MtLtoN (1965) Carbonates in the manganese ore at Lang- (1961) Norsethite, BaMg(COa)p, a new mineral from ban. Ark. Mineral. Geol. 4,279J85. the Green River formation, Wyoming. Amer. Mineral. -, AND R. Br.rx (1965) Norsethite from Langban. 46, 420-429. Ark. It4ineral.Geol. 4,277-278' Srevx, J. G. D., eNp M. D. Wersox (1967) Notes on a new occurrenceof norsethite,BaMg(CO")r, Amer. Min- Manuscript receirsed,April 18, 1973; accepted eral, 52. 1770-1775. lor Publication,lune 19, 1973'