Crystal-chemical aspect of the sequential change of phases in a part of system BeO-MgO-Al2O3-SiO2 rich in magnesium
Demina T. V., Belozerova O. Yu., Bogdanova L. A.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 4. P. 94-111
The regular change of phases has been found in a part of the system Cordierite—Beryllian Indialite — Beryl, rich in magnesium. It results from the gradual realization of six schemes of isomorphism: 2Si → Be + Al (1), Si → Al (2), Si → Be (3), Al → Be + R (4), 2A1 → Be + Si (5) and MgVI + AlIV → AlVI + BeIV (6), caused by value of Mg/Be ratio in the media. At Mg/Be = 27, the scheme (1) was mainly appeared providing the priority of indialite development. At Mg/Be = 7—13, all six schemes (1—6) took place with comparable shares of all revealed phases; and for Mg/Be = 3—4 schemes (5) and (6) become predominant determining the dominance of beryllian indialite, solid solutions and emerald. It was found that growth of the BeO content in the mixture from 1.5 to 15 % is accompanied by change of the structural state of phases: hexagonal → orthorhombic → hexagonal. There has been revealed the simultaneous crystallization of beryllian indialite and the orthorhombic beryllium-free cordierite in one of studied compositions (medium with CBeO = 10 %).
Key words: phase transitions, circuit isomorphism, crystal chemical formula, one-vertex individuals.