SYMMETRICALLY DISTORTED CRYSTALS FROM WEST AUSTRALIA. 335 



Pilbarra occurs intercrystallised with monazite it is quite possible 

 that it may contain a certain proportion of some of the rare earths. 

 This may account for the distortion of the crystals. 



A cross section of one of the rather imperfect pseudo-monoclinic 

 crystals is dark reddish-brown, almost opaque, and shows under 

 the microscope the fine lamellar twinning characteristic of cassi- 

 terite, parallel to the faces of the second order pyramid [101]. 

 The specific gravity at 16° C. determined by means of the pycno- 

 meter is 6*876. Before the blowpipe with Na 2 C0 3 , abundant 

 globules of tin are obtained with ease. Fused with Na 2 C0 3 and 

 sulphur, and dissolved in hot water, an abundant yellow precipitate 

 of Sn 2 fS 3 is obtained on addition of HOI. 



Explanation of Plate and Figures. 



(a) — Photograph of a pseudo-monoclinic crystal of tinstone. 

 (6) (c) — Photograph of a pseudo-rhombic crystal of tinstone. 

 Fig. 1 — Figure of pseudo-monoclinic crystal in the conventional position 



for reading as a monoclinic crystal. 

 Fig. 2 — Same, in conventional position as tetragonal crystal. 

 Fig. 3 — Faces of tetragonal pyramid with its faces in equipoise. 

 Fig. 4 — Figure of pseudo-rhombic crystal in the conventional position 



for reading as a rhombic crystal. 

 Fig. 5 — Stereographic projection on the basal plane, indicating the 



tetragonal symmetry of the crystal. 

 Fig. 6 — Stereographic projection as a monoclinic crystal on the apparent 



orthopinacoid. 



A 



FIG 5 



FIG 6 



