Properties of Minerals. 89 
Cleavage is that property possessed by most min- 
erals to part or break in certain directions. The direc- 
tion of cleavage is always parallel to the faces of the 
geometrical figure to which the mineral tends to as- 
sume, and generally will indicate its crystalline system. 
In the I, or isometric system cleavage is generally 
cubic, octahedral and dodechedral. In the VI, or hex- 
agonal system the cleavage is said to be basal when the 
cleavage is parallel to the base of the prism, and pris- 
matic when parallel to the sides. If the cleavage is 
equal in three directions, but not at right angles to one 
another, the mineral belongs to the R, or rhombic form 
of crystalization, and is designated as rhombohedral. 
In the remaining systems the term basal expresses cleay- 
age as taking place at right angles to the vertical axis 
of the crystal. Cleavage is called pinacoidal when it is 
in one direction parallel to the vertical pinacoids of the 
ITI, IV, or V systems. In the II system, if the cleav- 
age is in two directions parallel to the sides of a prism 
it is called prismatic, or in general, the term prismatic 
means that the ¢leavage is easiest parallel to the long 
sides of the crystal of which the mineral partakes. (See 
Determinative Mineralogy and Blowpipe Analysis by 
Brush and Penfield, Pages 155 to 225.) 
SEVENTH PROPERTY. 
_ TENACITY. 
The following terms are used in denoting those 
characteristics possessed by some minerals, such as ap- 
pear in the columns of the tables under the headings of 
Tenacity : 
(a) Brittle—When a mineral breaks easily, or 
when parts of the mineral separates in powder on at- 
tempting to cut it. 
