178 T. L. Walker — Examination of Triolinic Minerals. 



Etching Figures on Tourmaline. — I selected a transparent 

 greenish yellow crystal of tourmaline about a quarter of an 

 inch in diameter. The prismatic zone was composed of the 



trigonal prism (1010) and the prism of the second order 



(1120). One end was terminated by a single plane, the basis 

 0001, while the other end was not crystallographically termi- 

 nated, a part of the crystal being broken off. The crystal was 

 sent to R. Fuess of Berlin with the request to prepare two 

 thin polished plates parallel to the base. After several fruit- 

 less attempts, I finally succeeded in obtaining good etching 

 figures, by bringing the already slightly warmed tourmaline 

 plate into a red-hot mixture of potassium bisulphate and 

 coarsely powdered fluor spar. Below red heat no corrosion 

 took place. After ten minutes the crystal was withdrawn from 

 the red-hot fusion and allowed to cool slowly so as to prevent 

 cracking. By treating with hot dilute hydrochloric acid it was 

 easily cleansed from the coating of the corroding mixture. On 

 examination with the microscope the face corresponding to the 

 natural basis, 0001, was seen to be well covered with small 

 figures which may be described as sharply angular equilateral 

 triangles, with their sides parallel to the edges of the trigonal 

 prism and basal plane and the angles pointing toward the above 

 mentioned edge (fig. 4). The figures on the parallel face — the 

 artificial basis 0001 — are also equilateral triangles, but the sides 

 are curved and the angles blunt. A closer examination 

 showed that the little figures are probably nine-sided and 

 closely resemble sections through many tourmaline _crystals 

 whose prismatic zone is composed of the forms (1010) and 

 (1120), the former predominating so as to give a trigonal habit 

 to the crystal, with the latter forming narrow faces bevelling 

 the edges of the trigonal prism. The chief sides of the cor- 

 rosion figures are parallel to the edges of the trigonal prism 

 and the base, but the angles of the triangle are turned away 

 from the edge of the trigonal prism. This is shown in a more 

 or less idealized form in fig. 5. Thinking this difference be- 

 tween sharp and blunt triangles might be due to the position 

 or temperature of the crystal during the reaction, I etched the 

 second polished plate, taking the precaution to hold it in such 

 a position, by means of platinum pointed forceps, that both 

 should be subject to exactly the same conditions. The result- 

 ing figures were quite like those on the first plate. The high 

 temperature caused the greenish yellow crystal plates to be- 

 come water-clear. 



These figures are different, but as the basal planes of tour- 

 maline contain three lines of symmetry the figures on both 

 planes must also contain three lines of symmetry and these 



