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Crystallography. — "About the uf/iiqiw e.rthiction of rhombic crystals." 

 By J. ScHMUTZEK. (Communicated by Prof. A. Wichmann). 



(Communicated in the meeting of November 30, 1907j. 



As late as the year 1901 Ai,fked Hakker in a brief communi- 

 cation ^) pointed out the fact that with rhondjic crystals the oblique 

 extinction on planes making a small angle with the c-axis, is to be 

 neglected only when the angle of the optic axes has no great value. 

 That this ' was not superfluous is perhaps partly owing to the fact 

 that, iji the application of the theoretically deduced results concerning 

 tlie extinction of crystal-sections mineralogists have con lined themselves 

 to monoclinic and triclinic minei'als, preferably to the feldspars. ■) 

 It seems, therefore, that the circumstance that rhombic minerals as 

 a rule show an oblique extinction and oidy exceptionally a straight 

 one, is not laid sufficient stress on, though the fact is of course 

 well-known. ') That is why, even in the younger petrographical 

 literature, it is often alleged, in verification of the rhombic nature 

 of a minei'al, that all its sections show a straight extinction, whilst 

 at partly straight, partly oblique extijiction of the crystal-sections the 

 monoclinic nature of the mineral is considered to have been proved. ■*) 

 A separation of rhombic and monoclinic pyroxenes, oli\ine and 

 diopside, zoisite and klinozoisite liowever on the ground of the 

 character of the extinction is not to be insisted upon ; only in case 

 of small axis-angles this characteristic has some value as a criterion. 

 What gave rise to the calculation of the angles of extinction for 

 oli\ine was that considerable extinctions were fouiid with respect 

 to a {)articularly well developed pinacoidal cleavage of this mineral, 

 whilst, to compare them with the results obtained from this, I have 

 also made the same calculations for talc. 



1) Mineralogical Magazine, XllI, 1903, p. 00—68. 



2) Michel Levy, Ann. d. Mines, (7), XII, 1877, p. 392—471, Abstract Zeitschr. f. 

 Krysl. Ill, 1879, 217—231; Minéraux des Roches, 1888. p. 9 seq.; Fouque et 

 Michel LÉvy, Mineralogie Micrographique, Paris 1879; A. Harker, Min. Mag. X, 

 1898, p. 239—210; G. Gésaro, Mém. cour. Acad. Roy. Belg. LIV, 1895; ÖALy, 

 Proc. Americ. Acad. Arts a. Sc. XXXIV, 1899, p. 311—328; A. A. Ferro, Riv. 

 di Min., Padua XX, 1898; Atli Soc. Lig. di Sc. nat. Genova, IX, 1898, Abstract 

 Zeitschr. f. Krysl. XXXll, 1900, 532; Vicente de Sousa Brandao, Communicacoes 

 da direcQaO d. servi(^. geol. de Portug. IV, 1901, 13—126. 



3) Gf. FouguÉ et Michel LÉvy, Mineralogie Micrographique, p. 55 — 57. 



■*j Gf. Lacroix, about Fouqueït in Contributions a l'étude des gneiss a pyroxene 

 et des roches a wcrnénte, Bull. Soc. frang. de Mineralogie XII, Paris 1889, 

 p. 328. 



