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Notes: Abstract A new single beam microtechnique has been developed for measuring the polarized absorption spectra in the region 44,000-4,000 cm−1. Spectra of a natural garnet (Spess70Alm30), measured by the microtechnique and by conventional macrotechniques, are consistent and thus prove the applicability of the microtechnique described. It is possible to obtain well resolved spectra down to about 13,000 cm−1 with crystals as small as about 10 μm. Thus spectra of crystals obtained in routine high-pressure high-temperature silicate syntheses can be measured. The polarized spectra of Mn3+, Fe3+, Fe2+, and Cr3+ in the following synthetic silicate minerals are presented: piemontite (I), acmite (II), orthoferrosilite (III), and kyanite (IV) or uvarovite (V), respectively. O-Cr3+, O-Mn3+, and O-Fe2+ charge transfer band maxima in the UV region are identified at 38,700 cm−1, in V; at 33,200, 35,300, and 39,000 cm−1, in I; and at 32,800, 35,200, and 37,300 cm−1, in III, respectively. Bands in the region ≦25,000 cm−1 are assigned to spin-allowed and spin-forbidden dd transitions as predicted from crystal field theoretical considerations for the foregoing ions in the respective structures.

Notes: Abstract Mössbauer and polarized optical absorption spectra of the kyanite-related mineral yoderite were recorded. Mössbauer spectra of the purple (PY) and green yoderite (GY) from Mautia Hill, Tanzania, show that the bulk of the iron is Fe3+ in both varieties, with Fe2+/(Fe2++Fe3+) ratios near 0.05. Combining this result with new microprobe data for PY and with literature data for GY gives the crystallochemical formulae: $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.95}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{0}}{\text{.01}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.34}}}^{{\text{3 + }}} {\text{Mn}}_{{\text{0}}{\text{.07}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.57}}} )_{5.97}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.98}}} {\text{P}}_{{\text{0}}{\text{.03}}} ){\text{O}}_{{\text{18}}{\text{.02}}} ({\text{OH)}}_{{\text{1}}{\text{.98}}} ] \hfill \\ \end{gathered}$$ and PY and $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.98}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{〈 0}}{\text{.001}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.45}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.56}}} )_{6.02}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.91}}} {\text{O}}_{{\text{17}}{\text{.73}}} {\text{(OH)}}_{{\text{2}}{\text{.27}}} ] \hfill \\ \end{gathered}$$ for GY. The Mössbauer spectra at room temperature contain one main doublet with isomer shifts and quadrupole splittings of 0.36 (PY), 0.38 (GY) and 1.00 (PY), 0.92 (GY) mm s−1, respectively. These values correspond to Fe3+ in six or five-fold coordination. The doublet components have anomalously large half widths indicating either accomodation of Fe3+ in more than one position (e.g., octahedraA1 and five coordinatedA2) or the yet unresolved superstructure. Besides strong absorption in the ultraviolet (UV) starting from about 25,000 cm−1, the polarized optical absorption spectra are dominated by strong bands around 16,500 and 21,000 cm−1 (PY) and a medium strong band at around 13,800 cm−1 (GY). Position and polarization of these bands, in combination with the UV absorption, explain the colour and pleochroism of the two varieties. The bands in question are assigned to homonuclear metal-to-metal charge transfer transitions: Mn2+(A1) Mn3+(A1′) ↔ Mn3+(A1) Mn2+(A1′) and Mn2+(A1) Mn3+(A2 ↔ Mn3+(A1) Mn2+(A2) in PY and Fe2+(A1) Fe3+(A1′) ↔ Fe3+(A1) Fe2+(A1′) in GY. The evidence for homonuclear Mn2+ Mn3+ charge transfer (CTF) is not quite clear and needs further study. Heteronuclear FeTi CTF does not contribute to the spectra. In PY, additional weak bands were resolved at energies around 17,700, 18,700, 21,000, and 21,900 cm−1 and assigned to Mn3+ in two positions. Weak bands around 10,000 cm−1 in both varieties are assigned to Fe2+ spin-alloweddd-transitions. Very weak and sharp bands, around 15,400, 16,400, 21,300, 22,100, 23,800, and 25,000 cm−1 are identified in GY and assigned to Fe3+ spin-forbiddendd-transitions.