PROPERTIES OF MINERALS 1 3 



in the form of globules and hair-like rods, may be detected with 

 the microscope. 



Secondary Forms of Crystals. — A great variety of crystalline 

 forms is produced by the occurrence of secondary planes or faces 

 on the angles or edges of the primary forms. All the similar parts 

 of the crystal may be modified in the same way, or alternating 

 similar parts may be so modified. 



Certain faces may be obliterated by the enlargement of others ; 

 but however great the variation, the angle at which corresponding 

 faces meet almost invariably remains constant for each mineral. 



Massive and imperfectly crystallized minerals may consist of 

 grains, fibres, or thin layers {lamina) . 



Hardness. — The hardness of minerals is a useful means of iden- 

 tifying them. For this purpose they are referred to a scale of 

 hardness, ranging from such soft substances as may be readily 

 scratched with the finger-nail, to the hardest known substance, 

 diamond. The degree of hardness is expressed by the numerical 

 place of the mineral in the scale, and intermediate grades are 

 indicated by fractions. Thus a mineral which is scratched by 

 quartz and scratches orthoclase with equal ease, has a hardness of 

 6.5. The scale is as follows : — 



1. Talc. 6. Orthoclase. 



Selenite. Td+Zti^b 7- Quartz. 



Calcite. 8. Topaz. 



Fluor-spar. 9. Sapphire. 



Apatite. 10. Diamond. 



Cleavage. — Many minerals split readily along certain planes, 

 still retaining a crystalline form, while in other directions they 

 break irregularly. This property is called cleavage. Cleavage 

 is uniform in the different varieties of the same mineral, and 

 takes place either in planes parallel to one or more faces of the 

 fundamental form of the crystal, or along the diagonals of that 

 form. 



Pseudomorphs occur when one mineral assumes the crystalline 

 form proper to another. This may take place either by the addi- 



