Correlation of habitus forms, twins and aggregates of diamond crystals with the composition of its indicator minerals from kimberlite pipes of Angola

The method of cluster (R and G methods) analysis of the allocation of the cluster (CG) and chemical-generic (CGG) groups of average values of the compositions of diamond indicator minerals (DIM) (Ivanov, 2017) was used, supplemented with data on the frequency of occurrence (FO) of habitus forms, twins and clusters of diamond crystals of three kimberlite pipes of Angola – Catoca, Luele and Txiuzo (Ganga et al., 2021). Clustering of MSDS by their chemical composition was carried out on the basis of chemical-genetic classifications of Dowson J. and Soboleva N.V. for garnets (Dowson et al., 1975; Sobolev, 1973) and Garanin V. K. for Cr-diopsides, of ilmenite and chromite (Garanin et al., 1991).

It is found that FO CG/СGG of МSD are indirect and inverse significant correlation with FO habitus forms, twins and adhesion of diamond crystals of these kimberlite pipes. This is demonstrated by histograms of the joint distribution of CG DIM and habitus forms, twins and splices of diamond crystals from geological samples of kimberlites at their deposits (Fig. 1).

The fractions of octahedra (O) and transition habit crystals (OD) decrease in parallel with a decrease in the proportions of CG G9, G10 pyropes and an increase in G1 and G2a, an increase in the proportions of CGG 2b and 4b picroilmenites. The shares of rhombododecahedron, including dodecahedrons (RD), grow with the growth of the shares of CG pyropes G1a, G2a, as well as CGG picroilmenites 2b and CGG Cr-diopsides S2 and S5. The shares of twins (Tw), splices (Agr) and polycrystalline bead (PC) decrease in the studied tubes with a decrease in the shares of CG pyropes G10, G10a and an increase in the shares of CGG picroilmenites 2b and 4b and CGG Cr-diopsides S2 and S5 (Fig. 1). The presence of Ti and Fe compounds, which are part of DIM in elevated concentrations, in the process/medium of diamond crystal formation contributes to the formation of habitus forms OD and RD (D - dodecahedrons) during dissolution associated with low-chromium pyropes CG G1 and G2. Medium-high chromium pyropes CG G10 and G10a are associated with octahedral habitus (O) diamond crystals and their spinel counterparts (TwSp), whose shares they control.

Petrogenetic affiliation of CG/CGG MSD to various associations of deep mantle rocks allows us to identify the most favourable conditions and environments for the origin and growth of diamonds (high FO of O+TwSp+Tw) and environments (conditions) of their dissolution (high FO of OD+RD+Th+C). Interesting that the diamond grade calculated diamond deposits (Ct /T) is positively correlated with FO (Ar g+Tw), SGG S6 picroilmenites and SG G10 garnets, but the FO (RD+OD) has a negative effect on diamond grade, which allows determining the degree of the fertility of the mantle sources by DIM diamond.