nnsH \i. PROPERTIES 



such a cleavage is tin- cubic cleavage of galena, in which it is 

 alnio>t impossible, even on grinding, to obtain p.-irt ides not cubical 

 in form. Cleavage faces may be highly pofi>hed and smooth, 

 even though difficult to obtain, as the rhornbohedral cleavage of 

 quarts. 



Cleavage surfaces parallel to the faces of the same crystal form 

 are all of the same character ; they possess the same luster and are 

 obtained with the same ease, as the three directions of the rhombo- 

 hedral cleavage in calcite, or the four directions of the octahedral 

 cleavage of fluorite. In anhydrite, where there are also three cleav- 

 age directions at right angles, they are obtained with unequal ease 

 and each differs from the other in luster. In gypsum there is 

 one very perfect cleavage, parallel to the clinopinacoid, cleavage b, 

 and another parallel to the orthopinacoid, cleavage a, which is 

 obtained with more difficulty; the resulting flat cleavage plates 

 are parallel to the easy clinopinacoidal cleavage, and are bounded by 

 two straight edges parallel to the less easy orthopinacoidal cleav- 

 age. This is also the usual shape of cleavage fragments in ortho- 

 clase. The comparative ease of coexisting cleavages materially 

 influence the general shape of the fragments into which a mineral 

 breaks. The descriptive terms as applied to cleavage are: per- 

 fect, as the rhombohedral cleavage of calcite ; distinct, as the pris- 

 matic cleavage of rutile ; imperfect, as the basal cleavage of beryl 

 or apatite ; traces or indistinct, as the cubic and octahedral cleavage 

 of pyrite ; difficult, as the cleavage of quartz or tourmaline. 



When the cleavage pieces are flat and it is possible to cleave very 

 thin laminae, the cleavage is said to be micaceous, as in muscovite 

 or biotite. The laminae are described as flexible when they can 

 be bent, even though they may crack, but without parting, as in 

 chlorite ; elastic when they bend with an even curve without 

 cracking and on being released spring back to their original flat 

 condition, as in muscovite. The laminae are brittle when they 

 break easily on bending, as in margarite ; tough in one direction 

 and brittle in another, as in gypsum. 



Parting is not a true cleavage, but is the result of pressure or 

 strain to which minerals have been subjected and is therefore not 

 characteristic of all specimens of the same species, but is peculiar 

 to localities and formations and not necessarily confined to crys- 

 tals. Some garnets have a parting parallel to the rhombic dodec- 

 ahedral faces, others have no indication of it. Magnetite and 

 franklinite have an octahedral parting. 



