Galaxite, MnAl2O4, a spectroscopic standard for tetrahedrally coordinated Mn2+ in oxygen-based mineral structures
Galaxite, MnAl2O4, a spectroscopic standard for tetrahedrally coordinated Mn2+ in oxygen-based mineral structures
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DOI：
10.2138/am.2007.2481
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发表时间：
2007-07
影响因子：
3.1
通讯作者：
U. Hålenius;F. Bosi;H. Skogby
中科院分区：
文献类型：
--
作者：
U. Hålenius;F. Bosi;H. Skogby
摘要 介绍了合成方钠石的化学分析、晶体结构精修、阳离子顺序测定和单晶光学吸收光谱。为了比较目的，报道了天然含锰硅锌矿、菱锰矿、富锰镁橄榄石和软镁石的新光学吸收光谱。合成方钠石端元的结构特点是具有相对较大的晶胞边缘，a0 = 8.2104(3) Å，u 参数等于 0.26588(7)，T-O 距离为 2.0034(6) Å，M-O 距离为 1.9310(5) Å。 Mn2+ 在四面体 T 位点上具有强有序性，如精炼结构式 T(Mn2+0.90Al0.10)M(Mn2+0.10Al1.90)O4 所示。方钠石在 300.800 nm 范围内的光学吸收光谱显示，在 20 300、22 250、23 390、25 970 和 27 780 cm-1 处有一组五个相对尖锐的谱带，标志着四面体位置处 Mn2+ 的自旋禁止跃迁。 23 390 cm-1处与场无关的6A1(S)→4Eg4A1g(G)吸收带的摩尔吸收系数等于1.90 L/(mol·cm)，比含有孤立Mn2+中心八面体的化合物光谱中的相应吸收带大约高一个数量级。计算得出的方解石 T 位处 Mn2+ 的晶体场分裂 10Dq 等于 5290 cm-1。这与含锰硅锌矿四面体 T1 和 T2 位点处 Mn2+ 的 10Dq 值 5860 和 5510 cm-1 相比较。与理论一致，MnO4 四面体中 Mn2+ 的 10Dq 约为 10Dq。比 MnO6 八面体中的相应值小 30%。光谱数据的较低 Racah B 参数表明四面体中 Mn2+-O 键的共价程度高于八面体。
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.