The stress-birefringence which is caused by imperfections in single crystals of synthetic rareearth garnets has been observed and analysed by means of polarizing microscopy using a BraceKöhler compensator and the Sénarmont's method using a He-Ne laser. The stress-birefringence associated with the inhomogeneities in the garnet crystal can be divided into three types according to the origins. One is associated with growth striations, another with facets and the other with dislocations and inclusions. In this study, the features of stress-birefringence of the three types are briefly described. It is demonstrated that measuring the value of the birefringence (Δn) and the vibration directions of polarized rays (X', Z') is important to characterize such defects. Besides the validity as the characterization method, such a photoelastic study is helpful to understand the environment during crystal growth.

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To measure the optical spectra of microcrystals up to -20μm in diameter, microcomputer processed, polarizing microscope-spectrophototometer method has been introduced. The 2nd optical source, Hg-lamp (Epi-illuminater), is mounted in the polarizing microscope to measure the fluorescence spectrum. Transmittance of reference (I₀) and that of a sample (I) are measured independently in the same single beam condition, and each optical data are stored in the floppy disk of microcomputer. Absorbance, log(I₀/I), can be calculated using the data stored in the disk. The power of detection and resolution of the spectrophotometer is tested by using ruby and Hg spectra. The weak spin-forbidden transitions, R and B-lines of Cr⁺³ in ruby are clearly detected. Precision of the spectral position and resolution (full width at half maximum Γ), are obtained : ruby fluorescence R1 694.32±0.07nm, Γ0.69, R2 692.89±0.06nm, Γ0.62: Hg Γ0.05 nm at 546.074 nm. Some examples of visible absorption spectra are given.

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Modern electronics industry requires very many crystals. And the crystals must be characterized not only before using device process but also before their research and development. The first approach to crystal characterization starts with optical observation. The observation is originated from natural history including mineralogy and crystallography. The first step of optical observation is seeing outlook, and the second is sampling. If the sampling is not suitable, the following observation will be a waste of time.

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Manganese spinel minerals reported till now include hausmannite, iwakiite, Jacobsite, galaxite, hetaerolite, manganochromite and vuorelainenite. The first four minerals occur commonly in the bedded manganese deposits formed in Paleozoic to Mesozoic sediments of Japan. Their mutual phase relations are usually considered in the ternary system Mn₃O₄-MnFe₂ O₄-MnAl₂O₄. The immiscibility range between jacobsite and galaxite in the system MnFe₂O₄-MnAl₂O₄ disappears with increasing Mn₃O₄ content in their solid solutions at metamor. phic temperature and an extensive jacobsite∼galaxite complete solution becomes to exist with another restricted hausmannite solution in the ternary system. Jacobsite also has extensive solution with Mn₂TiO₄ and the Mn₂TiO₄ content of the solution attains to about 70 mole % in the metamorphosed deposits. Consequently jacobsite solution needs to be treated in the quarternary system Mn₃O₄-MnFe₂O₄-MnAl₂O₄-Mn₂TiO₄. Iwakiite, Mn (Fe, Mn)₂O₄, has the only extensive solution with Mn₂TiO₄ in the quarternary system but its stability relation is unknown because it never coexist with any other spinels and has not been synthesized. This mineral must hold the keys to decipher the complex phase relations in this system.

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The Cr-bearing silicate systems have a wide liquidus surface for chrome spinel, indicating that a small amount of Cr causes the presence of chrome spinel which gives a significant effect on phase relations in the silicate system. The behavior of chrome spinel is examined by the addition of a small amount of Cr (0.16-0.40 wt% Cr₂O₃) to the joins CaMgSi₂O₆-CaAl₂Si₂O₈-Mg₂SiO₄ and CaMgSi₂O₆-CaAl₂SiO₆ in the form of MgCr₂O₄ and CaCrAlSiO₆, respectively. The liquidus surface for chrome spinel expands considerably with the small increase in Cr content of the bulk composition, and liquidus phase relations in the both joins are significantly changed. The spinel phase with very high Cr content (40-50 wt% Cr₂O₃) is crystallized from melt with very low Cr content. The Cr content of the spinel phase increases with increasing that of bulk composition at 1 atm. The Cr content of spinel phase increases considerably (up to 70 wt% Cr₂O₃) with increasing pressure when excess water is present, and decreases with decreasing water content of host melt.

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Since RM₂X₄ compounds having a spine! structure have been payed attentions not only for earth science but also for material science, crystal chemical features of spinel structures are reviewed. Atomic movement due to thermal vibration of atoms at high temperature and/or under high pressure comes to be one of the important subjects for the comprehension of structure dynamics with respect to transition, decomposition and melt. Analyses of anharmonic thermal vibrations of atoms of MgAl₂O₄ (a=8.0806Å, z=8, S. G.=Fd 3m) have been conducted by means of X-ray single-crystal diffraction study at high temperatures up to 1700°C besides the correction of thermal diffuse scattering. Structure refinements of the spinel in the series of the corrections of thermal effects, (1) elipsoidal harmonic vibrations of atoms, (2) thermal diffuse scattering, (3) anharmonic thermal vibrations of atoms, are presented as a function of temperature. Several theoretical approaches to anharmonic thermal vibrations are also reviewed and compared with each other.

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