The Anomalous Optical Properties of Some New Series of Isomorphous Double Tartrates

Harold E. Buckley
Assistant Lecturer in Crystallography.

II. In the series which contain all three substituents NH4, Na, and K the following effects are to be observed: Initial crossed dispersion of the optic axial planes, with subsequent changes to a normal orthorhombic dispersion or to a crossed axial-plane dispersion of the brookite type. A change in optic axial angles for different colours of the spectrum as the crystals settle down to the final state. In crystals which contain only two of the substituent bases NH4, Na, and K only the second (b) of these effects is invariably present.

III. In basal sections of the freshly prepared crystals, the greatest optical anomalies are exhibited by the terminal sections, and as the distance from the surface increases the anomalous effects fall off in magnitude. All sections, whether terminal or otherwise, ultimately possess the same optical properties when the crystals have reached the final state. This may be considered as evidence in favour of the homogeneity of these crystals.

IV. A rise in temperature hastens considerably in these crystals the process of settling down to the final state. Application of pressure, normal to the section, has a contrary effect. In neither case is there any effect on the final state when this is ultimately reached.

V. The strains involved degrade the symmetry of the freshly prepared crystals to that of a monoclinic class. Externally, however, judging from a large number of angular measurements, the crystals appear to be still orthorhombic holoaxial, and there is no corresponding readjustment of angular values as the settling down process in the interior takes place.

Mineralogical Magazine; September 1926 v. 21; no. 114; p. 55-72; DOI: 10.1180/minmag.1926.021.114.01
© 1926, The Mineralogical Society
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