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Although alkali-alkali earth carbonates have not been reported from mantle-derived xenoliths, these carbonates may have a substantial role in mantle metasomatic processes through lowering melting temperatures. On the Na 2 Mg(CO 3 ) 2 -K 2 Mg(CO 3 ) 2  join only the Na-end-member eitelite (R3̅ space group), was reported in nature. The K-end-member (R3̅m) readily hydrates even at low temperatures, therefore, only baylissite, K 2 Mg(CO 3 ) 2 ·4H 2 O, has been observed. Because of the role of (K,Na)Mgdouble carbonates in mantle metasomatism, we performed high P-T experiments on K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 . Structure refinements were done upon compression of single crystals from 0 to 9 GPa at ambient temperature employing synchrotron radiation. Fitting the compression data to the second-order Birch-Murnaghan EoS resulted in V 0  = 396.2(4), 381.2(5), and 347.1(3) Å 3  and K 0  = 57.0(10), 54.9(13), and 68.6(13) GPa for K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 , respectively. These compressibilities are lower than those of magnesite and dolomite. The KMg-double carbonate transforms into a monoclinic polymorph at 8.05 GPa; the high-P phase is 1% denser than the low-P polymorph. The NaMg-double carbonate has a phase transition at ~14 GPa, but poor recrystallization has prevented structure refinement. The parameters for a V-T EoS were collected at 25-600 °C and ambient pressure and are ɑ 0  = 14.31(5) × 10 -5  K -1  and 16.73(11) × 10 -5  K -1  for K 2 Mg(CO 3 ) 2  and Na 2 Mg(CO 3 ) 2 , respectively. Moreover, fitting revealed an anisotropy of thermal expansion along the a- and c-axis: ɑ 0 (a) = 2.84(6) × 10 -5  and 4.78(5) × 10 -5  K -1  and ɑ 0 (c) = 10.47(11) × 10 -5  and 8.72(5) × 10 -5  K -1  for K 2 Mg(CO 3 ) 2  and Na 2 Mg(CO 3 ) 2 , respectively.

Crystal structure, high-pressure, and high-temperature behavior of carbonates in the K 2 Mg(CO 3 ) 2 –Na 2 Mg(CO 3 ) 2 join