GUIDK TO THE MIXEKALOGIC COLLECTIONS 



41 



mamillary, like the botryoidal but composed of larger and 

 flatter protuberances, example malachite, pi. 6 2 



globular, imperfect spheres of radiating fibers, examples 

 wavellite, pectolite, pi. 7 X 



oolitic, composed of small rounded grains, like the roe of 

 fish, example calcite, pi. 7 2 



coralloidal, consisting of branching forms like coral, exam- 

 ple aragonite, pi. 8 X 



dendritic, in branching treelike forms, example pyrolusite, 



pi. So 



stalactitic, in pendant columns from the roofs of caves, 

 formed by percolation of water carrying dissolved mate- 

 rial, example calcite, limonite, pi. 9 X 



acicular, slender, needlelike forms, example stibnite, pi. 9 2 



capillary, hairlike, example millerite, pi. lO^ 



reticulated, interlaced fibers like a net, example stibnite, 



pi. 10 2 . 



Cleavage 



Closely related to the crystalline structure of a mineral is 

 the tendency, common in a varying degree to most mineral 

 species, to break or split parallel to cer- 

 tain crystallographic planes. This ten- 

 dency, which is called cleavage, takes place 

 along the lines of minimum cohesion. 

 Thus in a cube, the molecular arrangement 

 of which is shown in fig. 2, it would be 

 reasonable to expect cleavage to take place 

 along planes parallel to the cube faces, 

 that is,along the planes of molecular crowd- 

 ing. In fig. 156, assuming the dots to represent the position of 

 molecules, the lines of least resistance to cohesion and con- 

 sequently the lines of cleavage would be the vertical and hori- 

 zontal rather than either of the inclined lines because the par- 

 allel lines of molecules on either side of the vertical and hori- 

 zontal directions are further apart and consequently the attract- 

 ive force betweeE adjacent molecules would lie least along flics'- 

 lines. 



In isometric minerals cleavage takes place parallel l<» the 



Fig. 156 



