

185 



in JXIJK ndii iil.n tc each other; all four sectors \ :inul- 



tanroiisly dark between crossed nicols, if the sides >l th quadratic 

 pl.it. iih hide an angle of about 34 or 56 respectively, with tin- 

 ]>l,iii( s of vibration of polarizer and analyser. 



Tlu whole behaviour of these crystals is in full agnvmmt with 

 -apposition that they are composed by monn<lmi< laim-llar, 

 crossing at 90, and intercalated in such a way that an apparently 

 tetragonal crystal is produced. There cannot be the least doubt 

 to the correctness of Mallard's view in this case. 



The same is valid for the case of autunite 1 ): Ca(UO z ) z (PO 4 ) t + 

 SH.,0, (a : b : c = 0,3463 : i : 0,3323; /3 = pojo'); of natrolithe: 



a z Al.,Si 3 10 -f 2H Z 0; of prehnite' 1 ): CaH z Al z Si 3 O lz ; of Pennine 3 ): 

 2 or 3 (Mg 3 H 4 Si z 9 ) + Mg 2 H l Al z Si0 9> ; and of some other sub- 

 stances which even by Brauns, who in general does not agree with 

 Mallard's views, are considered to be true mimetic aggregates 

 of lower symmetrical lamellae. 



The cases of the pseudo-cubic minerals boracite: Mg^l^B^O^, 

 and leucite : KAlSi z O n , as optically anomalous crystal-species, are 

 \\vll-known, and a great number of investigations has already been 

 made with respect to these remarkable substances. In especial 

 the work of Mallard and C. Klein has much contributed to the 

 explanation of their abnormal behaviour 5 ). Without going into 

 details, we may mention here that in both these cases the lamellar 

 structure is beyond all doubt. 



In fig. 144. 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 when observed between crossed nicols. The 



1) A. Brczina, Zeits. f. Kryst. 8. 273. (1897) ; A. Madelung, ibid. 8. 75. (1884) ; 

 K. Brauns, loco cit. p. 63. 



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

 [oenen, Sitzber. d. Ges. zu Beford. d. Naturwiss. Marburg, (1874); R. Brauns, 

 cit. 



) 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. 



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



*) 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. i. 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 

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

 1. Kryst. 1. 257. (1877); etc. 



