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Galaxite, MnAl₂O₄, a spectroscopic standard for tetrahedrally coordinated Mn²⁺ in oxygen-based mineral structures

Department of Mineralogy, Swedish Museum of Natural History, P.O. Box 50007, SE-104 05 Stockholm, Sweden

Correspondence: ^* E-mail: ulf.halenius{at}nrm.se

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⁺ _0.90Al_0.10)^M(Mn²⁺ _0.10Al_1.90)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 coefficient of the field-independent ⁶A₁(S) ⁴E_g ⁴A_1g(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.

Key Words: Crystal structure • galaxite • optical spectroscopy • willemite • tephroite • forsterite