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The hydrograndite and magnesioferrite in the Budureasa area, Romania: genetical implications

Domenii publicaţii > Ştiinţele pământului şi planetare + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: Ghergari L., Ionescu C.

Editorial: E. Schweizerbart'sche Verlagsbuchhandlung, Neues Jahrbuch für Mineralogie - Monatshefte, 11(2000), p.481-495, 2000.

Rezumat:

The Budureasa hydrogarnet appears in the shape of idiomorphic crystals of yellow – brownish color, usually having 2 – 3 mm diameter, sometimes more (5 – 6 mm). The chemical formula of the Budureasa hydrogarnet is: (Ca2,88Mg0,05Fe0,03Mn0,03Na0,03)(Fe1,59Al0,41)[(SiO4)2,89(OH)0,43] and the calculated cell unit lattice parameter is 12.07(1) Å. The infra-red absorption spectrum presents the „hydrogarnet – type substitution” bands. The hydrogarnet is constituted by 76% andradite and almost 20% grossular and hydrogrossular. Based on the high amount of andradite, the chemical homogeneity, the grossular – hydrogrossular content, and the impossibility to place the [(HO)4]4- only around the Al ion, this hydrogarnet may be called hydrograndite (the andradite amount is < 80%). In the zones with hedenbergite inclusion, the hydrograndite contains isometric microcrystals of magnesioferrite, with the formula (Mg0,274Fe0,726)(Mg0,726Fe1,274)O4 and a unit - cell parameter ao = 8.39(8) Å. The magnesioferrite has an inversion parameter x = 0.735, corresponding with the Néel-temperature (temperature of the magnesioferrite genesis) to about 300°C. It was formed during the garnet hydroxylation process (appears in hydrogarnet only) as a result of the hydro - metasomatic processes, when calcium was consumed in the hydrogarnet while Mg and Fe from pyroxene generated the spinel.

Cuvinte cheie: Laramian magmatism, garnet-skarn, hydrograndite, magnesioferrite