106 Wright — Optical Study of 



elongated crystals and interprets tliem as indicative of the 

 monoclinic system. Doelter also mentions in passing the rare 

 occurrence of twinning lamellae in his preparations, but seems 

 to have attached no significance to the fact. The present 

 writer also observed in several of his sections sharp and 

 occasionally polysynthetic, twinning lamellae which were not 

 unlike oligoclase feldspar twins in appearance. On a plate 

 perpendicular to the optic normal the twinning lamellae were 

 normal to the plate and parallel to the basal pinacoid, their 

 trace running parallel to the basal cleavage lines. The 

 lamellae showed an extinction angle a : a = 2°. Since the 

 cleavage cracks are not perfect, the small extinction angle of 

 2° might easily be overlooked, under ordinary circumstances, 

 and the extinction be considered parallel. In the hexagonal 

 and orthorhombic crystal systems the basal pinacoid is a plane 

 of symmetry and cannot act as a plane of twinning nor show 

 an extinction angle, however small. The fact, then, that twin- 

 ning after the basal pinacoid does occur in the pseudo-wollas- 

 tonite crystals and does show an extinction angle, the double of 

 which when taken between adjacent lamellae is 4°, precludes the 

 uniaxial and orthorhombic crystal systems. The writer con- 

 siders the mineral with Bourgeois as probably monoclinic. 

 The twinning law is analogous to that of Tschermak in the 

 micas, where the basal pinacoid is also the plane of composition. 

 .Since its crystals frequently similate hexagonal forms, pseudo- 

 wollastonite may well be treated as pseudo-hexagonal and 

 probably monoclinic in form. It is not a modification of the 

 natural monoclinic wollastonite. and differs from the latter 

 profoundly in optical as well as crystallographical features. 



Par amorphic changes. — The phenomenon of paramorphism, 

 the change of crystal structure of a chemical substance in the 

 solid state with consequent preservation of the crystal habit of 

 the original form, is well illustrated in the inversion of wollas- 

 tonite to pseudo-wollastonite. Since in certain paramorphic 

 minerals it has been noted that a plane of symmetry or other 

 direction may be common to both simulated and simulating 

 mineral, several experiments were made to ascertain whether 

 any crystallographic or other relations exist between the origi- 

 nal wollastonite crystals and the pseudo-wollastonite which 

 replaces them. Cleavage fragments of natural wollastonite 

 from Diana, X. Y., were heated in an electric arc and then 

 cooled rapidly by plunging them into mercury. Sections from 

 this preparation showed that the wollastonite had thereby 

 passed into the pseudo-form without any apparent regularity. 

 The fibers of the original wollastonite were unaltered up to 

 that portion which had touched the electric arc, from which 

 point outwards irregular grains of pseudo-wollastonite occurred 



