walker: crystal form of spencerite 



457 



Under the circumstances the survival of crystals of the phosphate 

 was hardly to be expected. 



Fig. 1. The crystal is twinned on 

 (100) and as a result of the twinning 

 the forms (241) and (221) are each repre- 

 sented by four faces, thus giving the 

 crystals a rhombic appearance. 



no- 



oio 



Fig. 2. Crystals of this type exhibit 

 twinning in the duplication of the termi- 

 nal faces, (346) being represented by four 

 faces while the two faces of (T02) occur 

 as striations on a large plane nearly 

 coinciding in direction with a rhombic 

 basal pinacoid. 



Some specimens of this mineral recently obtained from the 

 original locality contain small druses lined with sharp crystals of 

 spencerite, the preservation of the crystals being apparently due 

 to the cavities having been completely sealed up by calamine. 



