MINERALOGY AND CRYSTALLOGRAPHY 377 



of parallel submicroscopic crystals ; diametrically opposite 

 sections have usually similar optical orientation. 



(d) Elliptical Figures. — ^These figures are composed of an 

 aggregate of minute particles arranged in a heterogeneous 

 fashion, and generally distinguished from the other types by 

 their deep-seated development. Examination under high magni- 

 fication shows the figure to have an undulating form which 

 in the limiting case approximates to an ellipse. 



Of these types, the first is the most common, and is shown 

 by some of the monoclinic sulphates ; the second is instanced 

 by zinc sulphate ; the third by gypsum, and the ferrocyanides, 

 and the last by zinc sulphate. 



The sectored figures obtained have already undergone 

 several investigations, notably by Sohncke {Zeit. Kryst. Min., 

 30, I, 1899), and Weiss {Zeit. Deutsch. Geol. Ges., 29, 211, 1877), 

 and are known as the " Weiss 'sche Brennfigur." 



The former investigated the effect of temperature on the 

 form of the figures, and found that the higher the temperature 

 of formation, the more tendency do they show to orientate 

 themselves parallel to the vertical axis of the gypsum crystal ; 

 the average deviation from parallelism amounted to approxi- 

 mately 3°. This has been verified by R. Grengg {Zeit. anorg. 

 chein., 90, 327, 191 4 ; Tscher. Min. Mitt. =33, 201, 191 5; 

 Zeit. Kryst., 55, i, 191 5), who has made an elaborate investiga- 

 tion of the subject. He endeavours to show that, in some 

 cases, a very concentrated solution of calcium sulphate forms 

 and this afterwards consolidates to form an o/ientated group 

 of crystals of the hemihydrate CaS04,|H20. When the 

 gypsum is dehydrated on the oil bath, this aggregate consists 

 of fine acicular crystals whose optical properties indicate the 

 above-named salt. When the gypsum plate is in contact with 

 concentrated sulphuric acid, the composition of the needles is 

 not so certain, but it is probable that in this instance it is 

 anhydrite which forms. When the dehydration occurs at 

 temperatures just under 100° C, rounded or elliptical figures 

 are obtained. These have the greater axis inclined to the 

 c-axis of the crystal at a somewhat larger angle than in the case 

 described above, while their composition is variable, anhydrite 

 being common, though the hemihydrate may occasionally occur. 



The etch and solution figures of alum crystals have been 

 examined by H. Bauhans {Beitr. Kryst. Min., 1, 11, 1918) ; and 

 the same volume contains papers on similar phenomena in 

 fluorspar by H. Bauhans and V. Goldschmidt {ibid., 1, 219, 191 8) 

 and in quartz, by the action of sodium borate solutions under 

 pressure, by G. Lincio {ibid., 1, 87, 191 8). The analogies between 

 such solution and erosion have been discussed by V. Goldschmidt 

 {ibid., 1, 183, 191 8; Bull. Imp. Acad. Petersburg, 339, 1914). 



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