192 



Cleavage. 



Many minerals, when tapped with a hammer, show a tendency to 

 separate into fragments having a certain definite form or into plates with 

 parallel surfaces. This property is known as "cleavage," and is sometimes 

 useful. It is generally best determined by placing the edge of a knife - 

 blade upon the mineral in the direction in which it is expected to cleave 

 and tapping it with a hammer. Frequentlj^ naturally-produced cleavages 

 ■ean be detected. 



The direction of cleavage is dependent upon the form in which the 

 mineral crystallises. For example : fluor crystallises in the cubic system, 

 and its cleavages are parallel to the faces of the octahedron, this being the 

 fundamental form of that system. 



" Cleavage " may be described as : — 



1. Highly perfect : mica, selenite. 



2. Perfect : fluor. 



3. Imperfect : garnet. 



4. Yerj imperfect : quartz. 



When the cleavage is imperfect the planes of separation will be more or 

 less irregular. 



The following are the principal instances of characteristic cleavages : — 

 Cubical : galena. 

 Octahedral : 



Octahedron : fluor. 



Tetrahedron : fluor. 

 Prismatic : hornblende. 



Basal : 7nica, topaz, sappJdre, heryl, cJiIorife. 

 Pinacoidal : selenite, orthoclase. 

 Domatic : harytes. 

 Rhombohedral : calcite. 

 Two cleavages : orthoclase, harytes. 



Note. — The terms applied to the cleavages are derived from the crystallo- 

 graphic faces to which they ai'e parallel. It is not necessary to understand 

 these terms in order to make use of this method of mineral determination ; 

 it is quite sufiicient to become familiar with the appearance of the forms. 



The forms assumed by minerals on crystallising are frequently of use in 

 their determination. A number of common minera-ls Avhich thus assume 

 characteristic forms are arranged in the cases. The descriptions have been 

 printed witli inks of different colours, with a view to making a distinction 

 between the information suited to those visitors who have already acquired 

 some knowledge of crystallography and those who have not. The 



