GUIDE TO THE MIXERALOGIC COLLECTIONS loo 



Divergent. Composed of elongated crystalline individuals which diverge 



or radiate from a center. 

 Dodecahedron. An isometric form bounded by 12 (dodeca) rhombic faces. 

 Domes. Horizontal prisms which in the orthorhombic, monoclinic aud 



triclinic systems are parallel to one basal axis. The term is derived 



from the Latin domus. a house, to describe their resemblance to a hip 



roof. 

 Drusy. Covered with extremely minute crystals producing a roughened 



surface. 

 Faces. The bounding surfaces of a crystal. 

 Fibrous. Composed of slender filaments or fibers. 

 First order. A term applied in the tetragonal and hexagonal systems to 



pyramids and prisms the faces of which intersect two basal axes with 



equal intercepts; any plane of the hexagonal forms is parallel to the 



third basal axis. 

 Foliated. Composed of layers of imperfectly formed crystals which may 



be separated from one another with ease; derived from the Latin folio, 



a leaf. 

 Geode. A hollow rounded fragment lined with crystals. 

 Granular structure. Composed of irregular particles or grains. 

 Habit of crystals. The general preponderance of certain forms in 



crystals of a given species and from a given locality. 

 Hemimorphic. Having a dissimilar development of crystal planes on the 



two extremities. 

 Hexagonal. Sixfold. 

 Hexakistetrahedrons. Tetrahedral forms of the isometric system 



bounded by 24 triangular faces arranged in four groups of six each. 

 Hexoctaherons. Forms of the normal group of the isometric system 



bounded by 48 triangular faces. The name derived from the Greek 



refers to the grouping of the faces in eight groups of six each. 

 Inclusions. Foreign matter of a solid, liquid or gaseous nature inclosed 



within the crystal. 

 Isometric. Presenting the highest degree of symmetry in which the three 



crystallographic axes are interchangeable. The term is derived from 



two Greek words meaning equal measure and refers to the ideal 



development in which the three axes are of equal length. 

 Isomorphic. Presenting, the close chemical and crystallographic relations 



stated on p. 45. 

 Macroaxis. The longer of the two basal axes in the orthorhombic aud 



triclinic systems. The term macro is derived from a Greek word 



meaning long. 

 Macrodome. A dome or horizontal prism parallel to the macroaxis. 

 Macropinacoid. A pinacoidal piano parallel to the vertical and the macro 



axis. 

 Macroprisme, macropyramlds. Crystal forms, the planes of which are 

 more nearly parallel to the macroaxis than those of the form which 



determines the axial ratio. [See p. •*!2. liix- 124] 

 Mammillary structure. Consisting of rounded prominences; the term is 



derived from the Latin mamma, meaning a female breast. 

 Micaceous structure. In thin leaves which may be separated from one 

 another as typified in the mica group "i" minerals. 



