Synthesis and Crystal Structure of New Carbonate Ca₃Na₂(CO₃)₄ Homeotypic with Orthoborates M₃Ln₂(BO₃)₄ (M = Ca, Sr, and Ba)

Abstract

Single crystals of the new double alkali carbonate Ca 3 Na 2 (CO 3 ) 4 were synthesized using a high-pressure large volume apparatus at 6 GPa by slow cooling of the stoichiometric carbonate mixture from 1400 to 1150 °C for 2.5 h. The structure was solved and refined to R = 0.044 using 7489 independent reflections: P1n1, Z = 8, a = 31.4421(8) Å, b = 8.1960(2)Å, c = 7.4360(2) Å, and β = 89.923(2)°. The structure is characterized by the maximal number of nonparallel sets of CO 3 groups among carbonates. The compound is homeotypic with the orthoborates M 3 Ln 2 (BO 3 ) 4 (where M = Ca, Sr, Ba and Ln = Er, Sm, Nd, Pr, La, Y, Gd, Eu, Dy, Ho, and Bi). No homeotypic analogs have been found among carbonates. The structure is described as a packing of two-capped trigonal prisms formed by Na + and Ca 2+ cations and centered by CO 3 triangles.

Phase relationships in the system K2CO3-CaCO3 at 6 GPa and 900-1450 C

“…The binary carbonate with similar stoichiometry, Na 2 Ca 3 (CO 3 ) 4 , was also established in the Na 2 CO 3 -CaCO 3 system at 900-1300 °C and 6 GPa (Shatskiy et al 2013d;Gavryushkin et al 2014).…”

“…Since carbonates could participate in various mantle processes (kimberlite magma generation, mantle metasomatism, and diamond formation) that have occurred at the base of subcratonic mantle (150-230 km depths), pressure of about 6 GPa is the most interesting for the study of those systems. Although phase relations in the CaCO 3 -MgCO 3 (Buob et al 2006), CaCO 3 -FeCO 3 ), K 2 CO 3 -MgCO 3 (Shatskiy et al 2013c), Na 2 CO 3 -MgCO 3 (Shatskiy et al 2013a), and Na 2 CO 3 -CaCO 3 (Shatskiy et al 2013d) systems were already studied at 6 GPa, the data on phase relations in the K 2 CO 3 -CaCO 3 system are limited by pressures ≤0.1 GPa (Niggli 1916;Eitel and Skaliks 1929;Kröger et al 1943;Ragone et al 1966;Chattaraj et al 1973;Cooper et al 1975;Malik et al 1985;McKie 1990;Gavryushkin et al 2014;Jago and Gittins 1991;Arceo and Glasser 1995) (Fig. 1).…”

High‐Pressure Na‐Ca Carbonates in the Deep Carbon Cycle

“…Considering the phase relations in the Na 2 CO 3 -CaCO 3 system (Cooper et al, 1975;Podborodnikov et al, 2018;Shatskiy et al, 2013Shatskiy et al, , 2015, a range of Na-Ca double carbonates changes in the following sequence upon pressure and temperature increase: Na 2 Ca(CO 3 ) 2 , Na 2 Ca 2 (CO 3 ) 3 , (0.1 GPa) → Na 2 Ca(CO 3 ) 2 , Na 2 Ca 3 (CO 3 ) 4 (3 GPa) → Na 4 Ca(CO 3 ) 3 , Na 2 Ca 3 (CO 3 ) 4 , Na 2 Ca 4 (CO 3 ) 5 (6 GPa). The Na-Ca carbonates recovered from experiments at 0.1, 3, and 6 GPa were characterized by Raman spectroscopy and their crystal structures were solved: Na 4 Ca(CO 3 ) 3 (Ia−3d) , Na 2 Ca(CO 3 ) 2 (P2 1 ca) nyerereite (Gavryushkin et al, 2016), Na 2 Ca 3 (CO 3 ) 4 (P1n1) (Gavryushkin et al, 2014;Shatskiy et al, 2015), Na 2 Ca 4 (CO 3 ) 5 (P6 3 mc) burbankite (Rashchenko, Bakakin, et al, 2017). Although in situ high-pressure studies of Na-Ca carbonates have just begun (Borodina et al, 2018;Vennari et al, 2018), they already suggest an existence of unquenchable high-pressure polymorphs of Na 2 Ca 2 (CO 3 ) 3 and probably Na 2 Ca(CO 3 ) 2 at pressures exceeding 15 GPa.…”

“…Na 2 Ca 3 (CO 3 ) 4 . The crystal structure of Na 2 Ca 3 (CO 3 ) 4 , stable at least between 3 and 6 GPa, was reported by Gavryushkin et al (2014). The low symmetry (monoclinic) and long unit cell (a = 31.4421 Å, Table 13.2) of the compound are compensated by its strong pseudosymmetry, which allows reliable description of the structure using an orthorhombic cell with halved a parameter.…”

High-Pressure Phase Diagrams of Na2CO3 and K2CO3

“…Natural occurrence of alkaline carbonates Na 2 CO 3 and K 2 CO 3 is in the form of rare minerals natrite Na 2 CO 3 [1] and gregoryite (Na 2 ,K 2 ,Ca)CO 3 [2,3] and as a constituent part of double carbonates: Nyerereite and zemkorite (Na,K) 2 Ca(CO 3 ) 2 [4][5][6][7][8], shortite Na 2 Ca 2 (CO 3 ) 3 [9], eitelite Na 2 Mg(CO 3 ) 2 [10] and polymorphs of K 2 Ca(CO 3 ) 2 butchelite and fairchildite [11]. Other than these minerals, a number of double Na-Ca, Na-Fe and K-Mg carbonates were synthesised experimentally at pressures up to 6 GPa: Na 2 Ca 3 (CO 3 ) 4 , Na 2 Ca 4 (CO 3 ) 5 , Na 4 Ca(CO 3 ) 3 , Na 6 Ca 5 (CO 3 ) 8 , Na 2 FeCO 3 , K 2 Mg(CO 3 ) 2 [12][13][14][15][16][17][18][19][20].…”