Si, Ca, Al and Mg rich magnetites from iron ore deposits of the angara province

摘要

Magnetites from the ore forsteritites, fosteritemagnetitolites and apatite-magnetite ores (nelsonites) have beenstudied. the amounts of SiO_2 and CaO (mass. %) achive 3.42 and1.89, accordingly, or as minals Mg_2SiO_4 and Ca_2SiO_4 (mol.%), 8.5 and 3.6. the magnetites show constant ratios ofSiO_2/CaO (molecular quantity) varying within 1.46—1.80(average 1.61) and low titanium content, n. d. –0.89mass.%TiO_2 (average 0.46). Si-Ca-Al-Mg-rich magnetites crystallizedfrom residual portions of Fe-Mg silicate (phoscorite) meltstransformed due to accumulation of H_2O, P, S, F, Cl into fluid—melts. The change of aggregate state of the mineral-formingmedium hedral(?), etc and their chemical heterogeneity (simple,rhytmical and sectorial-rhytmical zonality), the favorable factorsfor the isomorpous inclusion of Si and Ca into structure ofmagnetites are the increase of oxidation potential and presence ofS~2-. Distribution of Si and Ca in crystals was controlled by (a)differences in the energetic characteristics of faces in accordancewith the first law of crystal anatomy (S. P. Grigoriev, 1984), morepreferably for capture of admixtures of the faces (111) and (311?)and (b) local effects of the change of mineral-forming environment composition.