156 



At Nutfield, Surrey (Eng.), a yellow form of barytes is found 

 in the EuUer's earth deposits. A large crystallised specimen is 

 shown in case 119. 



10331, Leadville ; transparent purple variety. 



Although of common occurrence in JS^ew South Wales, none 

 • of commercial value has yet been found. 



GROUP VI. 

 Case 101. Carbonates. 



Carbonates. Carbonate of lime : one of the most common gangue minerals 



Calcite. found in lodes. It presents great variety of form and colour. 



The most useful practical test for calcite is the effervescence that 



results w^hen a drop of weak acid (vinegar for instance) is placed 



upon it. Calcite can be easily scratched with a knife. 



A very common form of calcite is the rhoniboliedron ; moreover, all 

 calcite, irrespective of its crystalline form, breaks up into little 

 rhomboheclral when tapped with a hammer, by reason of its good, 

 rhombohedral cleavage (fig. A). The purest form of calcite is 

 Iceland spar, which breaks up into perfect rhombodedra as clear 

 as glass. Iceland spar is in great demand by optical instrument 

 makers ; it is by no means of common occurrence, and is not known 

 from New South Wales. 



Rhombohedral forms will be noticed from Tarrangobilly, 

 Bombala, and the Consols Mine, Broken Hill. Another common 

 form is a six-sided (Jiexagonal) prism, sometimes terminated by 

 a rhoniboliedron .^ 9279 is a good illustration ; also 257, Lious- 

 ville ; 4170, Consols Mine, Broken Hill ; 253, Golden Crown 

 Mine, Solferino. 



A third common form is the seaJenoliedron, constituting dog- 

 tooth spar. This is shown by 270, Derbyshire, and others. 

 Like the prism, the scalenohedron is often terminated by the 

 rhombohedron. 



Twin Crystals. 7445, " Butterfly calcite," Furness (Eng.). 



In 3806, Burragorang, the intimate twin structure has been 

 revealed by weathering, a sort of natural etching. 



* Good crystals of calcite will be found in case 33. 



