Crystal chemistry and petrology of coexisting galaxite and jacobsite and other spinel solutions and solvi '
Galaxite (Gx:MnAlzO+) and jacobsite ("ID:MnFezO+)-rich spinels occur together in the metamorphosed manganese deposit near Bald Knob, North Carolina. They show limited mutual solubility with lz-lgVa Jb in Gx and 19-22% Gx in Jb. The implied solvus is consistent with syntheses oflshida et al. (1977) along the Gx-"ID pseudobinary. Refinement of the structure of one galaxite (Gxg5JbrcSpaGhr) reveals that Mn is located on tetrahedral sites and Al on octahedral sites and that galaxite is therefore a normal spinel. All available data suggest that jacobsite is also a normal spinel at low temperatures, and the solvus between galaxite and jacobsite does not seem to be caused by differences in normality/inverseness. Chemical and experimental data available for (Mg,Zn,Mg,Fe2*XFe3*,Al)zOa spinels suggest hat the ferrites (acobsite, franklinite, magnesioferrite and magnetite) show extensive solid solution and are separated from aluminate solid solutions (galaxite, gahnite, spinel (s.s.) and hercynite) by wide solvi at metamorphic temperatures.
Galaxite, MnAl2O4, a spectroscopic standard for tetrahedrally coordinated Mn2+ in oxygen-based mineral structures
Abstract Chemical analyses, crystal structure refinement, cation order determination, and single-crystal optical absorption spectrum of synthetic galaxite are presented. New optical absorption spectra of natural Mn-bearing willemite, rhodochrosite, Mn-rich forsterite, and tephroite are reported for comparative purposes. The structure of a synthetic galaxite end-member is characterized by a relatively large unit-cell edge, a0 = 8.2104(3) Å, a u-parameter equal to 0.26588(7), a T-O distance of 2.0034(6) Å, and an M-O distance of 1.9310(5) Å. Mn2+ is strongly ordered at the tetrahedral T-site as demonstrated by the refined structural formula T(Mn2+0.90Al0.10)M(Mn2+0.10Al1.90)O4. The optical absorption spectrum of galaxite in the range 300.800 nm shows a set of five relatively sharp bands at 20 300, 22 250, 23 390, 25 970, and 27 780 cm-1 marking spin-forbidden transitions in Mn2+ at the tetrahedral site. The molar absorption coeficient of the field-independent 6A1(S) → 4Eg4A1g(G) absorption band at 23 390 cm-1 equals 1.90 L/(mol·cm), which is approximately an order of magnitude higher than for corresponding bands in spectra of compounds that contain isolated Mn2+-centered octahedra. The calculated crystal field splitting, 10Dq, for Mn2+ at the T-site in galaxite equals 5290 cm-1. This compares well with derived 10Dq-values of 5860 and 5510 cm-1 for Mn2+ at the tetrahedral T1- and T2-site in Mn-bearing willemite. In agreement with theory, the 10Dq for Mn2+ in MnO4 tetrahedra is ca. 30% smaller than corresponding values in MnO6 octahedra. The lower Racah B-parameters of the spectroscopic data indicate that the degree of covalency of Mn2+-O bonds is higher in tetrahedra than in octahedra.
The distribution of ions and their valencies in manganese ferrites I. MnFe2O4+γ ferrites
A model for the distribution of cations in the spinel lattice of manganese ferrites MnFe2O4+γwas elaborated on the basis of the experimental studies of the basic magnetic quantities, electrical conductivity and magnetic relaxation of the given ferrites, taking into consideration their crystallographic properties. The conclusions following from this model are in good agreement with the experimental results obtained by us and by other authors both for stoichiometric manganese ferrite and for a ferrite where γ≠0.AbstractНа основе эксперимен талъного иэучения ос новных магнитных вел ичин, электропроводн ости и магнитных релаксац ий у марганцевых ферр итов типа MnFe2O4+γи с учет омих кристаллографич еских особенностей бы ла разработана модел ъ для размещения катионов в решетке ти па шпинели у упомянут ых ферритов. Эаключен ия, сделанные на основе зтой модели, на ходятся в хорошем сог ласии с зксперимента лъными резулътатами нашими и других автор ов, и для стехиометрич еского марганцевого феррита, и для феррита, у которого γ≠0.