THE PROPERTIES OF MINERALS. 675 



No. 49698, U.S.N.M.); Pyrite, Sherwood, Jasper County, Missouri (Cat. No. 51341, 

 U.S.N.M.). 



Inenidarily of planes, due to uneciual rapidity of gntwth: Beryl, Oxford County, 

 Maine. (Cat. No. 46125, U.S.N.M.) 



Iiiqxrfcction ofplaiiea, due to striatiou : Rutile, Parkesburg, Pennsylvania (Cat. No. 

 49649, U.S.N.M.); Beryl, Aduii-Cbalon, Nertschiusk, Siberia (Cat. No. 16246, 

 U.S.N.M.). 



IrrefiHlaritu of pUincK, due to polysynthetic twinuing: Kntile, Parkesburg, Penn- 

 sylvania. (Cat. No. 49649, U.S.N.M.) 



In-iiiuhiriiji of plan ex, curvature due to irregularity of growth: Dolomite, Oswego 

 land, Jopliu.Missouri. (Cat. No. 82120,U.S.N.M.) 



Irn'nularity of planes, curvature due to oscillatory combination : Calcite, Jopliii, 

 Missouri (Cat. No. 82160, U.S.N.M.); Smoky Quartz, St. Gothard, Switzerland 

 (Cat. No. 82358, U.S.N.M.). 



fnet/iilarUy of planes, the result of corrosion: Galena, Joplin, Missouri (Cat. No. 

 51225, U.S.N.M.); Franklinite, Franklin, Sussex County, New Jersey (Cat. No. 

 83941, U.S.N.M.). 



Internal impurity: Andalusite inclosing carbonaceous matter, Lancaster, Massa- 

 chusetts (Cat. No. 82667, U.S.N.M.); touniialiue inclosing orthoclase, Crown Point, 

 New York (Cat. No. 18380, U.S.N.M.); quartz inclosing sand, Edwards, St. Lawrence 

 County, New York (Cat. No. 48286, U.S.N.M.); quartz inclosing mud, Burke County, 

 North Carolina (Cat. No. 82916, U.S.N.M.); calcite inclosing sand (Fontainel)leau 

 limestone), Fontainebleau, France (Cat. No. 49286, U.S.N.M.). 



rLEOMORI'HISM. 



Pleoniorpliism is that tendency of some chemical compounds to 

 crystallize in two or more distinct forms. Specimens of the following- 

 compounds illustrate this tendency: 



Calcium carbonate, CaCOs, crystallizes in thc^ rhombobedral system as calcite, in 

 the orthorhombic system as aragonite. 



Titaniimi dioxide, TiOj, cryst-allizes in the tetragonal system as rutile and octahe- 

 drite, in the orthorhombic system as brookite. 



Iron bisulphide, FeS-, crystallizes in the isometric system as pyrite, in the ortho- 

 rhombic system as marcasite. 



ISOMf)RPHISM. 



Isomorphism has been defined by Mitscherlich, its discoverer, as 

 "substances which are analogous chemical compounds have the same, 

 or nearly the same crystalline form." That is, two more distinct chemi- 

 cal compounds may crystallize in like forms. Examples of this ten- 

 dency are shown in — 



Spinel, MgALO^, Magnetite, FeFccO^, franklinite, (FeZnMn) (FeMn),04. Crystals 

 isometric, commonly octahedral. 



Rutile, TiO.2, Cassiterite, SnOj. Crystals tetragonal, commonly prismatic. 



Apatite, (CaF)Ca4(PO^)3, pyromorphite, (PbCl)Pb^(PO,):, minietite, (PbCl)Pb4 

 (P04):!, vanadinite, (PbC)Pbj(V04):i. Crystals hexagonal with pyramidal hemi- 

 hedrism. 



Barite, BaS04. Celestite SrS04, anglesite, PbSO,. Crystals orthorhombic, com- 

 monly tabular. 



PSEUDOMORPHS. 



Pseudomorph is that term applied to certain crystals or forms which 

 have the angles and general habits of a certain mineral with the com- 

 position and, in some instances, the structure of another; that is, the 



