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XIII. On a New Species of Double Refraction, accompanying a 

 remarkable Structure in the Mineral called Analcime. By 

 DAVID BREWSTER. LL.D. F.R.S. Lond. & Sec. R. S. Ed. 



(Read Jan. 7- 1822J 



HE Mineral called Analcime or Cubizite has been ranked by 

 HAUY among those crystals which have the Cube for their primi- 

 tive form ; an opinion which has been adopted by all succeeding 

 mineralogists. No distinct cleavage-planes, however, so far as 

 I can learn, have been observed in it*. Crystallographers 

 presumed that such planes must exist, and, allowing conjec- 

 ture to supply the place of observation, they considered Anal- 

 cime as differing in no respect from other crystals of the same 

 series. This opinion was first rendered doubtful by the obser- 

 vation, that at thicknesses of -/^th of an inch, it displayed 

 a considerable action upon polarised light f ; but though, 

 from the tessular form of the mineral, this fact indicated some- 

 thing singular in its organization, yet, owing to the great diffi- 

 culty of obtaining proper specimens, I have been baffled in re- 

 peated attempts to investigate its structure. 



* Mr PHILLIPS observes, " that there have been occasional appearances of 

 cleavage-planes parallel to the faces of the cube." Mineralogy, 1823, p. 129. M. 

 MOHS describes the cleavage as hexaJtedral, but imperfect. HAUY does not seem to 

 have observed any cleavage. In the first edition of his Traits, published in 1801, he 

 says, Les cristaux diaphanes offrent seuls quelques indices de lames paraUeles aux 

 faces du aibe, torn. iii. p. 181 ; but in the second edition, published in 1822, he has 

 struck out this observation. See torn. iii. p. 170. In the transparent and perfectly 

 crystallised specimens, I cannot find any cleavages. I consider Analcime, therefore, 

 as a mineral without cleavage ; and if cleavage-planes are discovered, they will no 

 doubt be found in the direction of the planes of no polarisation. 



f Philosophical Transactions, 1818, p. 255. 



A a 2 



