1918] 



The Ottawa Naturalist 



CERUSSITE FROM SALMO, B.C. 



By a. Ledoux and T. L. Walker, Royal 

 Ontario Museum of Mineralogy, Toronto. 



At the H. B. Mine, Salmo, B.C., where the 

 principal ores are oxidised zinc minerals (silicate, 

 carbonate and phosphate) cerussite is found in con- 

 siderable quantity. The cerussite is not well crys- 

 tallized as a rule but occasionally exceedingly beau- 

 tiful crystallized specimens are encountered. The 

 crystals are water clear with very brilliant faces and 

 well suited for exact goniometric measurements. 

 This probably is the finest crystallized cerussite 

 found in Canada. 



The crystals are almost invariably twinned form- 

 ing six rayed structures such as have been frequently 

 observed for this mineral. In these complex growths 

 the twining ordinarily observed occurs on the face 

 of the prism (110) but in the case of the Salmo 

 mineral the structures are often more complex in 

 that several of the individuals are twinned on (110) 

 while one of these is twinned on another individual 

 with (130) as the twinning plane. In many min- 

 erals complex twins involving more than one 

 twinning law are common but in the past the stellate 

 interpenetrating twins of cerussite had been regarded 

 as resulting from twinning according to one law 

 only until Hubrecht observed the participation of 

 both twinning laws in the same complex group.* 

 On groups of cerussite from Salmo the same com- 

 plexity has been observed. 



On crystals measured the following forms have 

 been obseved: 



(a) Pinacoids 



Basal Pinacoid (001) usually rough and 

 when present large. 



Brachypinacoid (010) always the largest 

 face so that the crystals are tabular. 

 Macropinacoid (100) narrow and well 

 defined ; 



(b) Prisms (110) and (130); 



(c) Brachydomes (012), (Oil), (021), 

 (052), (031), (041), (092). (051), 

 (061), (071), (081), (091), (0.10.1), 

 and (0.12.1). 



The domes (012) and (021) are the most 

 prominent. The others are present in cer- 

 tain crystals and give with the goniometer 

 a long series of reflections; 



(d) Macrodome (102); 



(e) Pyramids (111) and rarely (112). 



The ordinary form of single crystals is represented 

 in fig. I ; it will be noticed that the faces are gen- 

 erally not very numerous. The more complex type 

 of crystal is represented in fig. 2, where the de- 

 velopment of numerous brachydomes is especially 

 characteristic. 



Fig. 2 The brachydomes lying 

 between the faces (001) and 

 (010) are as follows: (012), 

 (Oil), (021), (052), (031). (041), 

 (092), (051). (061), (071). (081). 

 (091), (010.1). 



-^ 



^ 



:? 



Zeitschrift f. Kryst. XL p. 169. 



U 

 Fig. 3. Fig. 4. 



Repeated twinning on the unit prism (110) is 

 very common, producing forms such as are repre- 

 sented by fig. 3 or 4. When this twinning law is 

 the only one involved the angle between the a axes 

 of successive individuals is 62 46'. In the crystal 

 represented schematically on fig. 5, the four in- 

 dividuals 1 , 2, 3 and 4 are twinned according to 

 this law, but a fifth individual (x) is twinned on 

 No. 2 with (130) as twinning plane. The angle 

 between the a axes of those two individuals is 

 57 18'. The interpenetration of several individuals 



