Minerals In Thin Section Research Articles

The Determination of Minerals under the Microscope: with Special Reference to the Interpretation of Interference Phenomena

IN very clear and simple style this book describes the outline to be followed for the complete determination of the optical characters of minerals in thin sections, with some remarks on the application of the same methods to minerals in small grains. The title rightly emphasises the importance attached to the interpretation of interference phenomena, as Chapter vi. on the ‘directions image’ gives detailed instructions for the determination of optical characters from the interference figures by methods which are neglected altogether in some schools of petrology. The frontispiece gives a good reproduction of the polarisation colour scale seen when a quartz-wedge is viewed between Nicol prisms, both crossed and parallel, as recently published, by Drs. W. R. Jones and A. Brammall. Chapters vii. and viii., on dispersion and ‘other determinations,’ are less carefully written than the earlier chapters, and the diagrams illustrating the effects of dispersion on the interference figures are liable to be misleading, as it is not clear which of the two kinds of shading used is meant to represent blue colour and which red.

A Key for the Determination of Rock-forming Minerals in Thin Sections

A Key for the Determination of Rock-forming Minerals in Thin Sections

I. On the determination of the minerals in thin sections of rocks by means of their indices of refraction

In my address at the Annual Meeting of the Mineralogical Society last year, in Plymouth, I gave an account of a new method of studying the optical properties of minerals, and of determining their indices of refraction. I then showed that an approximate knowledge of these characters would, in many cases, enable us to identify minerals with great confidence, and also would sometimes furnish us with valuable hints as to their general chemical composition. My remarks had reference exclusively to detached specimens of more or less considerable thickness, and I scarcely thought that it would ever be possible to apply this method successfully with comparatively high powers in the case of thin sections of rocks. I feared that certain errors, which appeared to be unavoidable, would then become so great in proportion to the difference in the indices of many of the commoner minerals, that it would be impossible to distinguish them with confidence.

I. On a definite method of qualitative analysis of animal and vegetable colouring-matters by means of the spectrum microscope

My attention was first directed to this subject by reading a report of Professor Stokes’s very excellent lecture at the Royal Institution, Friday, March 4th, 1864. It immediately occurred to me that a spectroscope might be combined with a microscope, and employed to distinguish coloured minerals in thin sections of rocks and meteorites. I was soon led to examine many other coloured substances, and found that the instrument is ore useful in connexion with qualitative analysis, when only very small quantities of material can be obtained. At first I employed the imperfect apparatus described in my Paper in the ‘Quarterly Journal of Science’, not afterwards, along with Mr. Browning, I constructed that described in my Paper in the ‘Popular Science Review’. For general purposes I do at think this could be much improved; but for chemical testing it is much less fatiguing to use a binocular instrument. There were many difficulties to contend with, but at length I constructed one which appears to answer all the requirements of the case.