Leonard James Spencer

“ It is, however, possible to determine whether a crystal is isotropic or anisotropic, and in the latter case to determine the directions of the principal axes of refringence and of absorption.
One result of this study of opaque minerals is to draw attention to the extremely intimate association and intergrowth of many of the ore-minerals; this is well shown in the numerous photomicrographs published by American workers in economic geology. What to all appearances by ordinary methods is a homogeneous mineral may be found by the new method to be really heterogeneous ”

“ Many minerals and allied compounds have been prepared artificially in silicate and salt fusions. The conditions necessary for their formation and their ranges of stability—either when alone or when in the presence of other compounds—have been studied in detail. One important result obtained by experimenting over wide ranges of temperature has been to show that practically all compounds known as minerals exist in several polymorphous forms. ”

“ It is therefore concluded that the excess sulphur is absorbed by the ferrous sulphide, or in other words present in “solid solution,” the formula being written FeS (S) x.. The absence of any excess of sulphur in the troilite of meteorites is accounted for by the coexistence of free iron. Radioactivity.—The strong radioactivity of uranium minerals affords a ready means of recognizing these valuable ores in the search for them by prospectors. ”

“ During the war period of 1914-8 much attention was given in all countries to the development of home resources of various minerals of economic value, and to meet new circumstances new sources of supply were developed. Further, there was an increased demand for certain kinds of minerals, for example those which yield the rarer metals used in the hardening of steel. ”

“ But in many cases ordinary simple tests made on fragments detached from the polished surface are more reliable. Electrical tests can be made with quite simple apparatus: for example, the electrical conductivity can be determined with a dry cell and voltmeter using needles as terminals on the polished surface. Certain optical determinations can also be made in reflected polarized light; but whilst the use of polarized light is of prime importance for the examination of transparent minerals (e.g. in thin slices of rocks) , it has only a limited application in the case of opaque minerals. ”

“ Fortunately, however, pure science has not been altogether neglected. Many new facts have been recorded, and new methods of investigation have been devised. A review of the recent scientific literature is given in the Mineralogical Society's series of “Mineralogical Abstracts.”
X Rays and Crystal-Structure.—The new X-ray method of investigating the internal structure of crystals has been applied with much success to the study of minerals (see Crystallography) . ”

“ It has, however, not hitherto been possible, by optical means alone, to distinguish with certainty a colloidal form from a microcrystalline mineral which is opaque or which is cubic in crystallization. ”

“ As an example, the recent discussion on the composition of nephelite (see 19.383) may be cited. Analyses of this mineral invariably show an excess of silica over that required by the orthosilicate formula NaAlSiO4, and sodium is always partly replaced by an equivalent amount of potassium in the varying ratio (Na:K) of 5.5:1 to 3:1, whilst a small amount of calcium is also present. Here the excess of silica (the ratio SiO2:Al2O3 ranging from 2.1:1 to 2.2:1) has been assumed to be present in “solid solution” in an isomorphous mixture of NaAlSiO4 and KAlSiO4. ”

“ Patronite, vanadium sulphide, VS4, forming dark, greenish-black, compact masses. It occurs abundantly at Minasragra, Cerro de Pasco, Peru, where it is a valuable ore of vanadium. It weathers very readily with the production of various highly coloured vanadium compounds; even on material kept in collections there is a slow growth of blue and green efflorescences. ”