Crystal chemistry and isomorphism of high-calcium eudialyte-group minerals from the Tamazeght peralkaline complex, Morocco

Crystal-chemical features of high-calcium and hypercalcium eudialyte-group minerals (EGMs) from a carbonatite-related rock of the Tamazeght peralkaline complex, High Atlas Mountains, Morocco were studied using electron probe microanalysis, single-crystal X-ray structure analysis, IR and Raman spectroscopy.The major components of the host rock are calcite, fluorite and EGMs; aegirine-augite is present in subordinate amounts.The specific features of the studied EGMs are chemical heterogeneity, a complex zoning, reaction zones around calcite and apatite inclusions, Na-and Cl-deficiency, high contents of Ca, Mn, REE, Nb, carbonate and H-bearing groups, positive correlation between Nb and Mn and negative correlations between the pairs Fe-Mn and Zr-Mn.These features confirm previous assumptions about the role of infiltration of carbonatite fluid rich in Ca, Mn, REE, Nb, CO 2 and H 2 O and a depletion of Cl in the remaining fluid after the crystallization of sodalite at the expense of nepheline in the formation of carbonatite-related rocks of the Tamazeght complex.The crystal structure of a single-crystal fragment extracted from the Nb-rich zone refined to R 1 = 0.0335 has shown a high degree of ordering of Na, REE and H 3 O + and the dominance of Fe 3+ at the M2 site with five-fold coordination.The composition of EGMs from the reaction zones around calcite and apatite inclusions corresponds to Mn-dominant (with Mn > Fe at the M2 site) analogue of feklichevite with the simplified formula Ca 3 (Na,K) 9 (H 3 O) 3 Ca 6 Zr 3 (Mn 2+ ,Fe 3+ ,Zr) 3 NbSi(Si 24 O 72 )(OH,H 2 O) 5 (F,Cl) 2/3 (CO 3 ) 1/3 .