190 



Fig. 146. Leucite. 



8H 2 0, (a : b: c = 0,34.63 : / : 0,5525; /3 = pojo'); of natrolithe: 

 Na 2 Al 2 Si 3 O w + 2H 2 0; of prehnite 1 ); CaH 2 Al 2 Si 3 12 ; of pennine*): 

 2 or 3 (Mg 3 H^Si 2 9 ) -f Mg 2 H^Al 2 Si0 9 \ and of some other sub- 

 stances, which even by 

 Brauns, who in gene- 

 ral does not agree with 

 Mallard's views, are 

 considered to be true 

 mimetic aggregates of 

 lower symmetrical la- 

 mellae. 3 ) 



The cases of the 

 pseudo-cubic minerals 



boracite; Mg 7 Cl 2 B w 30 , and leucite; KAIS1 2 & , as optically anoma- 

 lous crystal-species, are wellknown, and numerous investigations 

 have already been made in connection 

 with these remarkable substances. In 

 particular, the work of Mallard and 

 C. Klein has contributed much to the 

 explanation of their abnormal beha- 

 viour 4 ). Without going into details, 

 we may mention here, that in both these 

 cases the lamellar structure is beyond 

 all doubt. 



In fig. 146 a crystal of leucite with 

 its typical striation of the apparent icosahedron-faces is reproduced, 

 and the aspect of a plate parallel to the cube-face, when observed 

 between crossed nicols. The lamellae disappear abruptly at about 

 500 C., an reappear on cooling. 



Of boracite in fig. 147 a plate parallel to 111} in its condition 



Fig. 147. Boracite. 

 Plate parallel to {\\\}. 



!) C. Stadtlander, Neues Jahrb. f. Miner., (1885); //, p. 113; A. Von Koenen, 

 Sitzber. d. Ges. zu Beford. d. Naturwiss. Marburg, (1874); R. Brauns, loco cit. 



2 ) A. Des Cloizeaux, Bull, de la Soc. Miner., 5, 58, 125, (1882); E. Mallard, 

 ibid. p. 70, 195, (1882); R. Brauns, loco cit., p. 69. 



3 ) R. Brauns, Die Optischen Anomalien, (1891), p. 74. 



4 ) E. Mallard, Ann. des Mines, (7), 10, 79, 93, (1876); Bull, de la Soc. Miner., 

 2, 147, (1879); 5, 144, 216, (1885); E. Mallard and H. Le Chatelier, ibid., 6, 

 122, (1883); 6, 129, (1883); 9, 69, (1886); C. Klein, Neues Jahrb. f. Miner., (1880), 

 //, p. 209; (1888) /, p. 239; Gott. Nachrichten, (1881); (1884); p. 189, 421 ; Neues 

 Jahrb. f. Miner., (1884) /, p. 235; //, p. 49, (1887). /, p. 224, 234, (1885) Beil. 

 End., ///, p. 522; R. Brauns, loco cit., p. 88, 106, (1891); H. Baumhauer, 

 Zeits. f. Kryst, 1, 257, (1877); etc. 



