Microchemical Research

Of all the avenues of approach followed by investigators of ores, none has been more consistently followed than the avenue of microchemical research. From the time of Becke during 1886, when such etch reagents as nitric and hydrochloric acids and potassium chlorate, can be seen under a microscope, were used to differentiate minerals of the magnetite group, until the present day, notable contributions to our knowledge of this phase of the subject have been made by scientific workers.

These contributions have taken the form either of a multiplication of etches reagents, or the development of new methods of application. In the former, two schools of thought soon emerged, the one represented by such eminent foreign scientists as Van der Veen, Ramdohr and Schneiderhohn, using many reagents, each one adapted to the differentiation of one or a few minerals that can be examined using a microscope and the other followed by such writers as Murdoch, Davy and Farnham, Farnham and Short, using a limited number of reagents with more general application to a whole host of ore minerals. In the development of new methods of application there must be mentioned the staining methods of Granigg, Head and Crawford and Gaudin, the electrochemical and photochemical methods of McKinstry, the investigations of Osborne into the relative merits of the immersion and drop methods of applying reagents, the research of Fraser and Dreyer into the role played by interfering ions and the important contribution by Galopin, Geysin, Gutzeit, Hillier and Wenger of the a la touché method, whereby the minerals are attacked by a solvent absorbed in a gelatin paper, which is later treated by reagents. These should be considered as important milestones in the path of progress. The text-books of Chamot and Behrens-Kley on the other hand, and that portion of short text-books dealing with this phase of the subject should also get honorable mention in this connection.

Still another attractive angle of approach lay in the study of the electrical conductivity of ore minerals, which can be examined under a microscope. The earlier investigations in this direction, such as those of Beijerinck, Koenigsberger and Reichenheim, Borgström and Dannholm, Wartman, Braun, Dufet, and many others tested this property either on macroscopic specimens or on mineral powders, while later researches by such workers as Davy and Farnham, Kerr and Cabeen, and Harvey were carried out on polished sections under the microscope. Their observations, although covering a wide range of mineral specimens, were made chiefly on sulphide, arsenide, and oxide compounds and seemed to indicate very clearly not only that the conductivity of a natural opaque mineral is not constant, but that it may show variations of from hundreds to even millions of per cent. The principal factors causing these wide variations were considered to be the existence of impurities, non-uniform temperature, change in orientation, variation in size of mineral crystals, destruction of the crystal lattice by heating, grinding, fracturing, or oxidation, and exposure to light.

While most investigators in the field of ore mineral study have relegated to a place of secondary importance observation of such physical properties as hardness, color, and streak, a few workers have studied these properties intensively. Although most workers have made use of hardness as a means of identifying the ore minerals, the determination of this property has been rather imperfect in most cases. Some form of steel needle or other simple hardness tester, in the opinion of many workers, has been amply sufficient for the purpose. Nevertheless, attempts have been made by some workers to determine this property more accurately. In 1923 Bierbaum made a study of the hardness of bearing metals, using an instrument, called a microcharacter, for measuring the width of the cut. This instrument, which was later used by Hodge and McKay for determining the microhardness of minerals, consists essentially of a tiny diamond under which the mineral to be tested is moved, a constant load being maintained by a small weight mounted on a lever-spring system. The width of the cut, called the microcut, produced by this device, is measured by means of a filar micrometer eyepiece. Hodge and McKay determined the microhardness of nine members of scale of hardness to be as the following talc 1, gypsum 11, calcite 129, fluorite 143, apatite 517, orthoclase 975, quartz 2700, topaz 3420, and corundum 5300. Talmage, another worker in this field, used a scratch-sclerometer with a diamond blade substituted for the original steel needle and established a seven-way classification of hardness. This method thus has now become the standard practice in many scientific laboratories.