Subsolidus and melting relationships for the system CaCO 3 –MgCO 3 have been reexamined using a Kawai-type multi-anvil apparatus at 3 and 6 GPa in graphite capsules. Phase boundaries were delineated according to the chemical composition of phases measured by electron microprobe in energy dispersive mode and identification of crystal phases by Raman spectroscopy. At 3 GPa, the dolomite-magnesite solvus intersects the melting loop at about 1250 °C, and the isothermal three-phase line so produced represents the peritectic reaction: dolomite (Ca# 43) = magnesite (Ca# 13) + liquid (Ca# 48), where Ca# = 100·Ca/(Ca+Mg). The melting loop for the CaCO 3 –MgCO 3 join extends from 1515 °C (CaCO 3 ) to 1515 °C (MgCO 3 ) through a liquidus minimum at 1230 °C (near 53 mol% CaCO 3 ). Starting from 1425 °C at ≤ 30 mol% CaCO 3 in the system, the liquid quenches to dendritic carbonate and periclase and contains rounded voids, indicating an incongruent melting reaction: MgCO 3 (magnesite) = MgO (in liquid) + CO 2 (fluid and/or liquid). At 6 GPa, aragonite + magnesite assemblage is stable up to 1000 °C. The reaction aragonite + magnesite = dolomite locates between 1000 and 1050 °C. The presence of dolomite splits the system into two partial binaries: aragonite + dolomite and dolomite + magnesite. The dolomite-magnesite solvus intersects the melting loop between 1400 and 1450 °C, and the isothermal three-phase line so produced represents the peritectic reaction: dolomite (Ca# 31) = magnesite (Ca# 21) + liquid (Ca# 57). The melting loop for the CaCO 3 –MgCO 3 join extends from 1660 °C (CaCO 3 ) to 1780 °C (MgCO 3 ) through a liquidus minimum at 1400 °C and 62 mol% CaCO 3 . The compositions of carbonate crystals and melts from the experiments in the carbonated eclogite (Yaxley and Brey 2004) and peridotite (Dalton and Presnall 1998) systems are consistent with the geometry of the CaCO 3 –MgCO 3 melting loop at 3 and 6 GPa: Ca-dolomite melt coexists with Mg-calcite in eclogite and peridotite at 3 GPa and dolomite melt coexists with magnesite in peridotite at 6 GPa.

The reaction between clinopyroxene and Mg-carbonate is supposed to define the solidus of carbonated lherzolite at pressures exceeding 5 GPa. To investigate the effect of alkalis on this reaction, subsolidus and melting phase relations in the following systems have been examined at 6 GPa: CaMgSi₂O₆+2MgCO₃ (Di+2Mgs); CaMgSi₂O₆+NaAlSi₂O₆+2MgCO₃ (Di+Jd+2Mgs); CaMgSi₂O₆+Na₂Mg(CO₃)₂ (Di+Na₂Mg); and CaMgSi₂O₆+K₂Mg(CO₃)₂ (Di+K₂Mg). The Di+2Mgs system begins to melt at 1400 °C via the approximate reaction CaMgSi₂O₆ (clinopyroxene) + 2MgCO₃ (magnesite) = CaMg(CO₃)₂ (liquid) + Mg₂Si₂O₆ (orthopyroxene), which leads to an essentially carbonate liquid (L) with composition Ca_0.56Mg_0.44CO₃ + 3.5 mol% SiO₂. The initial melting of the Di+Jd+2Mgs system occurs at 1350 °C via the reaction 2CaMgSi₂O₆ (clinopyroxene) + 2NaAlSi₂O₆ (clinopyroxene) + 8MgCO₃ (magnesite) = Mg₃Al₂Si₃O₁₂ (garnet) + 5MgSiO₃ (clinopyroxene) + 2CaMg(CO₃)₂ (liquid) + Na₂CO₃ (liquid) + 3CO₂ (liquid and/or fluid), which yields the carbonate liquid with approximate composition of 10Na₂CO₃·90Ca_0.5Mg_0.5CO₃ + 2 mol% SiO₂. The systems Di+Na₂Mg and Di+K₂Mg start to melt at 1100 and 1050 °C, respectively, via the reaction CaMgSi₂O₆ (clinopyroxene) + 2(Na or K)₂Mg(CO₃)₂ (solid) = Mg₂Si₂O₆ (orthopyroxene) + (Na or K)₄CaMg(CO₃)₄ (liquid). The resulting melts have the alkali-rich carbonate compositions Na₂Ca_0.4Mg_0.6(CO₃)₂ + 0.4 mol% SiO₂ and 43 K₂CO₃·57Ca_0.4Mg_0.6CO₃ + 0.6 mol% SiO₂. These melts do not undergo significant changes as temperature rises to 1400 °C, retaining their calcium number and a high Na₂O, K₂O, and low SiO₂. We suggest that the clinopyroxene–Mg-carbonate reaction controlling the solidus of carbonated lherzolite is very sensitive to the carbonate composition and shifts from 1400 to 1050 °C at 6 GPa, which yields K-rich carbonate melt if the subsolidus assemblage contains the K₂Mg(CO₃)₂ compound. Such a decrease in solidus temperature has been previously observed in the K-rich carbonated lherzolite system. Although a presence of eitelite, Na₂Mg(CO₃)₂, has a similar effect, this mineral cannot be considered as a potential host of Na in carbonated lherzolite, because the whole Na added into the system dissolves as jadeite component in clinopyroxene if bulk Al/Na ≥ 1. The presence of jadeite component in clinopyroxene has little impact on the temperature of the solidus reaction decreasing it to 1350 °C at 6 GPa.

—This paper reports the synthesis of double K–Ca carbonates at atmospheric pressure in closed graphite capsules. The mixtures of K₂CO₃ and CaCO₃ corresponding to the stoichiometry K₂Ca(CO₃)₂ and K₂Ca₂(CO₃)₃ were used as starting materials. The low-temperature modification of K₂Ca(CO₃)₂ was synthesized by a solid-state reaction at 500°C during 96 h. The high-temperature modification of K₂Ca(CO₃)₂ as well as K₂Ca₂(CO₃)₃ were synthesized by a solid-state reaction at 600°C for 72 h and by cooling of the melt from 830 to 650°C for 30 min. The obtained carbonates were studied by Raman spectroscopy. The Raman spectrum of bütschliite is characterized by the intense band at 1093 cm^–1 and several bands at 1402, 883, 826, 640, 694, 225, 167 and 68 сm^–1. The Raman spectrum of fairchildite has characteristic intense bands at 1077 and 1063 cm^–1, and several bands at 1760, 1739, 719, 704, 167, and 100 сm^–1. In the Raman spectrum of K₂Ca₂(СO₃)₃, the intense lines were found at 1078 and 1076 cm^–1 and several lines at 1765, 1763, 1487, 1470, 1455, 1435, 1402, 711, 705, 234, 221, 167, 125 and 101 сm^–1. The collected Raman spectra can be used to identify carbonate phases entrapped as microinclusions in phenocrysts and xenoliths from kimberlites and other alkaline rocks.