Proceedings of the Section of Sciences Volume 7, No. 1

Koninklijke Akademie Natuurkunde

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. 1904 Excerpt: ... by a very fine diagonal on two of the. broadest planes. We also may obtain needle shaped or very elongated thick pillar shaped crystals. The planes are generally angular and give plural reflexes. The crystals are also frequently bordered by curved planes and by vicinal forms in (he prism-zone. These circumstances render an accurate investigation very difficult occasionally, however, I obtained better formed crystals, which gave very sharp reflexes and served for the following accurate measurements. They are monoclino-prismatic with the axial relation: a: b: c= 0,5217: 1: 0,7803 0= 8512. Forms observed are: m = 11OJ andp = 120, broad and lustrous a = 100 and n=130, very narrow a is generally hazy 6=010, a little broader, but is generally absent c=J001, large and very lustrous r = 101, well developed and lustrous t. = narrower and is often absent t= 112, generally small and dull, occasionally a little broader and better reflecting = 132, large and lustrous, but generally there are only two parallel planes present. Combinations of all the forms rarely occur. Generally such with m, p, c, r m, p, b, c, r, t, s and o m, p, a, b and c, etc. The more typical crystals are shown in figures 1-3. In the properly formed crystals, the angular values are very constant the reflexes are as sharp as possible. The substance has a decided tendency to twin-formation in this, one form 102 is a twin-plane with a twin-axis normally standing on it. On the plane of / may be often observed a delicate line parallel p: c in this vertical zone the most important geometrical anomalies are found. The following are the calculated and observed angular values. A distinct cleavability was not observed. On c, r, and a the position of the optical elasticity-axis is orientated perpe