L34 



NEW YORK STATE MUSEUM 



Octahedron. A crystal form bounded by eight equilateral, triangular 



faces; derived from a Greek word meaning eight. 



Orthoaxis. The axis which, in the monoclinic system, is perpendicular to 

 the plane of the other two. 



Orthodomes. Domes or horizontal prisms parallel to the orthoaxis. 



Orthopinacoid. A pinacoid parallel to the vertical and ortho axes. 



Orthoprisms. orthopyramids. Crystal forms the faces of which are more 

 parallel to the orthoaxis than the form which determines the axial ratio. 



Ortho-rhombic. Presenting the symmetry of the orthorhombic system, ref- 

 erable to three uninterchangeable axes which are at right angles to 

 one another. Orthorhombic crystals are characterized by binary sym- 

 metry in three directions. 



Pinacoids. Crystal forms composed of planes parallel to two axes and 

 corresponding in position to the faces of a cube; the term is derived 

 from a Greek word meaning a board. 



Plane. One of the bounding plane surfaces of a crystal; the term is 

 extended to include the imaginary extension of this bounding surface to 

 meet the axes. 



Prisms. Crystal forms the planes of which intersect two basal axes and 

 are parallel to a third. Domes as described above may be regarded as 

 horizontal prisms. 



Pseudohexagonal. Apparently hexagonal; many crystals seem, by reason 

 of twinning, to be hexagonal though belonging to a system of lower 

 symmetry. 



Pseudoisometric. Apparently isometric; note above. 



Pseudomorph. A substance having the form of one mineral and the com- 

 position of another; the term is derived from two Greek words meaning 

 a false form. 



Pyramids. Crystal forms the planes of which intersect all three axes. In 



the isometric system forms of this type are designated by special terms 



such as octahedron, trioctahedron, etc. 

 Pyritohedrons. Isometric forms so named from their common occurrence 



in the species pyrite. 

 Radiated structure. Consisting of crystalline individuals which radiate 



from a center. 



Reniform. Kidney-shaped; from the Latin renes, a kidney. 

 Reticulated. Interlaced like a net; from the Latin reticulum, a net. 

 Rhombohedrons. Hexagonal forms of trigonal symmetry bounded by six 

 rhombic faces. 



Scalenohedrons. Crystal forms of the tetragonal and hexagonal systems 

 bounded by scalene triangles and presenting in general a somewhat 

 wedgelike shape. 



Second order. A term applied, in the tetragonal and hexagonal systems, 

 to pyramids and prisms the faces of which are related to those of the 

 corresponding first order forms as shown on p.26, fig.98. 



Sphenoids. Crystal forms of the tetragonal and orthorhombic systems 

 bounded by four triangular faces and closely related by analogy to the 

 isometric tetrahedron. 



Stalactitic structure. Consisting of pendant columns or forms resembling 

 icicles. 



