[ 26 ] 



III. Crystallographic Notes. By L. Fletcher, M.A.* 



VIII. A Twin of Zircon. 



ALTHOUGH zircon, rutile, and cassiterite have been long- 

 looked upon as isomorphous, zircon has been, till within 

 the last three years, remarkable for the absence of the twin 

 growth which is so usual a feature in the other two species. 

 In 1878 Herr Otto Meyerf, in a paper relative to the rocks 

 met with in the St. Gotthard tunnel, called attention to the 

 fact that zircon was present in considerable quantity as a mi- 

 croscopic constituent of some of the crystalline schists, and 

 further observed that the minute crystals were occasionally 

 twinned according to the same twin plane (101) as the crys- 

 tals of r utile and cassiterite. In the same year Herr HussakJ 

 mentioned his discovery of simple microscopic twins of zircon 

 according to the same law in the eklogite from Styria, and 

 of both simple and polysynthetic twins in the mica schist of 

 the St. Gotthard tunnel. One of the crystals was measured 

 by him, and found to be 0*16 millim. long and 0*04 millim. 

 broad. 



Very lately large crystals of zircon have been found at Ren- 

 frew in Canada. The largest specimen which has yet reached 

 this country weighs 408 grams: it is a simple crystal presenting 

 the forms { 1 1 }, {ill}, and {2 2 1 } . A second specimen, 

 weighing 52*785 grams, is of great in- 

 terest as showing the twinning of zircon 

 on a far from microscopic scale. The 

 habit of the individual crystals and the 

 general aspect of the twin will be evi- 

 dent from the accompanying figure. 

 The forms present are the prism { 1 1 } 

 and the pyramids {ill}, {2 2l}, 

 {331}: while on one quoin there is a 

 well-developed face, which proved upon 

 measurement to belong to the ditetra- 

 gonal pyramid (311). The angle be- 

 tween two corresponding prism-faces of the different indivi- 

 was found by measurement to be 44° 47', a result sufficiently 

 near to the calculated value 44° 50' to prove that the growth is 

 really due to twinning about the above-mentioned plane (101). 



* Read before the Crystallological Society, May 30, 1881. 



t Zeitschrift der Deutschen geologischen Gesellschaft, 1878, Band xxx. 

 p. 10. 



X Tscherniak, Mineralogische und PetrograpMsche Mittheilungen, 1878, 

 p. 277. 7 



A twin of zircon. 



