TWIN, OR COMPOUND CRYSTALS. 



57 



Fig. 1 represents a threeling, and 2, 3, and 4, fcwolings. In 3 

 and 4 the combined crystals are simply in contact along the 

 plnne of junction ; in 2 they cross one another; the former are 

 called co/ttar.t-twins and the latter 2^netration-twins. 



B< sides the above, there are also geniculated crystals, as in 

 hs annexed figure of a crystal of rutile. The bending has litre 

 ttktvi place at equal distances from the centre 

 £>f the crystal, and it must therefore have been 7. 



subsequent in time to the commencement of 

 the crystal, xhe prism began from a simple 

 molecule ; but after attaining a certain length 

 an abrupt change of direction took place. The 

 angle of geniculation is constant in the same 

 mineral species, for the same reason that the 

 interfacial angles of planes are fixed ; and it is such that a cross 

 section directly through the geniculation is parallel to the posi- 

 tion of a common secondary plane, in the figure given the 

 plane of geniculation is parallel to one of the terminal edges. 

 In rutile the geniculated crystals sometimes repeat the bendings 

 at each end until the extremities meet to form a wheel-like 

 twin. 



In some species, as albite, the reversion of position on which 

 this kind of twin depends, takes place at so short intervals that 

 the crystal consists of parallel plates, 

 8. 9. each plate often less than a twen- 



tieth of an inch in thickness. A sec- 

 tion of such a crystal, made trans- 

 verse to the plate, is given in fig. 8 ; 

 without the twinning the section 

 would have been as in fig. 9. The 

 plates, as the figure shows, make with 

 one another at their edges a re-enter- 

 ing angle (in albite an angle of 172° 

 48'), and hence a plane of the albite 

 crystal at right angles to the twin- 

 ning direction, is covered with a series of ridges and depressions 

 vrhich are so minute as to be only fine striatums, sometimes 

 requiring a magnifying power to distinguish. Such striationa 

 m alike are therefore an indication of the compound struc- 

 ture. 



This kind of twinning is owing to successive changes of 

 polarity in the molecules. a3 the enlargement of the crystal 

 went forward. It occurs in all the triclinic feldspars, and is a 

 means of distinguishing them from orthoclase. 



