Novel dense crystal structure of high-pressure synthetic K₈Ca₃(CO₃)₇ as described through cation–carbon nets

K 8 Ca 3 (CO 3 ) 7 was synthesized at 1200°C and 6 GPa. Its layered triclinic crystal structure presents a new type characteristic of high-pressure K–Ca carbonates. It is also the second example of K/Ca disorder coupled with orientational disorder of carbonate anions. Descriptions based on the most symmetric (tri/hexagonal) and the densest ( krd -type) cation–carbon nets reveal its peculiarities and common features with the crystal structures of chemically related carbonates. The four-layered ABA ′ B packing in K 8 Ca 3 (CO 3 ) 7 is based on hexagonal cation nets, wherein carbon atoms occupy two types of positions. Most of the carbon atoms are localized close to net B , within the centers of triangular loops. This forms a bütschliite layer, which is found in other alkali-bearing carbonates and is composed of alkali cations. Its presence in the K 8 Ca 3 (CO 3 ) 7 crystal structure implies that the K/Ca disorder is incomplete. In nets A and A ′, carbon atoms replace the cations at some nodes, resulting in the aggregation and increased concentration of carbon atoms and their stronger mixing with cations in the densest ( krd -type) nets. Thus, a new type of densest mixed net is formed in the K 8 Ca 3 (CO 3 ) 7 crystal structure. A quantitative measure of the packing density of high-pressure K–Ca carbonates is the reticular density of the symmetric atomic nets, as well as the distance between the densest nets.