268 



MINERALOGY 



and represent always the more general form of the type. Vicinal 

 faces are common on fluorite, where the face of the cube seems to 



be replaced by a very flat 

 tetrahexahedron, Fig. 381. 

 When a zone of vicinal 

 faces occurs, the crystal 

 appears rounded or 

 bounded by curved faces. 

 Drusy. A surface 

 formed of numerous small 

 individual crystals and 

 therefore dull is said to 

 be drusy. Drusy surfaces 

 may be caused by a sec- 

 ondary mineral produced 

 by surface decomposition, 

 or it may be the result of 

 very fine crystals usually 

 in parallel position, repre- 

 senting a second generation of the same species; or it may be 

 caused by a parallel or radiated aggregate of fine crystals, each 

 terminated at the surface, as in crusts of millerite or smithsonite. 

 Internal structures. Granular is where the individual crys- 



FIG. 381. A Cube of Fluorite, showing Four 

 Vicinal Faces replacing the Cube Face. 



FIG. 382. Calamine with Radiated Structure. Franklin, New Jersey. 



tals are equidimensional and irregularly packed together, the indi- 

 viduals being distinguishable by the unaided eye. 



Massive or compact. A granular structure in which the indi- 

 viduals are not to be distinguished by the unaided eye. 



