IMPERFECT CRYSTALLIZATIONS. 53 



Radiated, divergent ; when the crystals radiate from a 

 center, without producing stellar forms. Ex. quartz, gray 

 antimony, 



2. Lamellar Structure. In the lamellar structure, the 

 laminae or leaves may be thick, or very thin ; they some- 

 times separate easily, and sometimes with great difficulty. 



When the laminae are thin and separate easily, the struc- 

 ture is said to be foliaceous. Mica is a striking example 

 and the term micaceous is often used to describe thi 

 structure. 



When the laminae are thick, the term tabular is often ap 

 plied ; quartz and heavy spar afford examples. 



The laminae may be elastic, as in mica, flexible, as in talc 

 or graphite, or brittle, as in diallage. 



Small laminae are sometimes arranged in stellar shapes 

 this occurs in mica. 



3. Granular Structure. When the grains in the 

 texture of a mineral are coarse, it is said to be coarsely gran- 

 ular, as in granular marble ; when fine; finely granular, as 

 in granular quartz ; and if no grains can be detected with 

 the eye, the structure is described as impalpable, as in 

 chalcedony. 



Granular minerals, when easily crumbled by the fingers, 

 are said to he friable. 



Imitative Shapes. — Massive minerals also take certain 

 imitative shapes, not peculiar to cither of these varieties of 

 structure. The following terms are used in describing imi- 

 tative forms: 



Globular; when the shape is spherical or nearly so: the 

 structure may be columnar and radiating, or it may be con- 

 centric, consisting of coats like an onion. When they are 

 attached, they are called implanted- globules. 



Reniform ; kidney-shaped. In structure, they are like 

 globular shapes. 



Botryoidal*; when a surface consists ot a group of rounded 

 prominences. The prominences or globules usually consist 

 of fibers radiating from the center. 



Mammillary ; resembling the botryoidal, but consisting of 

 larger prominences. 



Filiform ; like a thread. 



Acicular ; slender like a needle. 



Explain the varieties of lamellar structure ; of granular structure ; the 

 •everal imitative shapes, globular ; reniform, &c. 



