Contributions to Mineralogy and Petrography from the Laboratories of the Sheffield Scientific School of Yale University (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1901 Excerpt: ...very little hydroxyl, so that the material may be regarded as practically the pure fluorine compound, AlF2Si0v In addition to the complete analyses, isolated determinations have been made on material from the other localities mentioned on pages 234 and 235, and these results will be given beyond in tabular form. Physical properties and tlieir relations to the chemical composition.--The specific gravities were very carefully determined on a chemical balance, pains being taken to boil the crystals for some time in water to expel any air bubbles. The results vary within the limits 3.574 and 3.533, a difference of only 0.041, and as a rule they decrease as the molecularly lighter hydroxyl replaces fluorine. Also basal plates were prepared and the divergence of the optical axes 2E measured on a large axial angle apparatus. The values for 2E have been found to vary in topaz from different localities and according to the observations of Des Cloizeaux they extend from 129 30' on crystals from Durango, Mexico to 71 32' on those from Mugla, in Natolien, Asia Minor, f both measurements being for red. These variations have generally been supposed to be connected with some change in chemical composition, but a satisfactory explanation has never been given. In the following table the measurements that we have made are arranged according to decreasing values of 2E for yellow, and with these the determinations of the specific gravity, fluorine and water are given: On the last mentioned crystal the value of 2E was not measured owing to the strong optical anomalies which the section presented; it was observed, however, that the angle was small. It is evident from the results given in the table that the value of 2E decreases as the percentage of water increases or as fluorine ...