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 dolomite (MgCa(COa),), norsethite (MgBa(COo),), MgSr(CO")a and two forms of CaBa(COs),, a monoclinic barytocalcite and a rhombohedral form with disordered calcite 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 Mineral 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, alstonite, 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.