62 LITHOLOGICAL GEOLOGY. 



Pyrophyllite. — A mineral resembling talc in its appearance and soapy feel, 

 but consisting of silica, 66, alumina, 29, water, 5. There is a massive waxy 

 variety, which resembles the pagodite and some soapstone. The massive pyro- 

 phyllite differs from pagodite in containing no alkali and more silica. The 

 preceding hydrous silicates of alumina have the soapy feel of talc, a hydrous 

 silicate of magnesia, and by most persons Avould on first examination be pro- 

 nounced magnesian. They are the basis of a slaty rock much like talcose slate, 

 but containing no magnesia, — a point of interest to the geological observer. 



68. (21.) Zeolites and related minerals. — The zeolites are hydrous minerals, 

 consisting of silica and alumina, with lime or an alkali. They resemble the 

 feldspars closely in composition, but contain water, are less hard and more 

 fusible. They occur in cavities and veins in igneous rocks, or disseminated 

 through the mass of the rocks ; also in cavities in granite and some other feld- 

 spathic rocks. The following are the more prominent : — 



a. Analcime : occurs in trapezohedral crystals, glassy and clear, or white. 

 Hardness a little less than that of feldspar. 



b. Chabazite : in rhombohedral crystals, nearly cubic, white and reddish. 



c. Natrolite : in fibrous masses and acicular crystals, usually white, without 

 a pearly cleavage. Scolecite closely resembles natrolite. 



d. Stilbite: in flattened prisms with dihedral summits, and having a broad, 

 pearly cleavageTSurface parallel to one face of the prism ; prisms often grouped 

 into sheath-like forms, and usually white. 



e. Heulandite : in rhomboidal prisms, with pearly basal cleavage, often trans- 

 parent, and usually white. 



Water, 8.1. 



" 21.0. 



" 9.5. 



" 13.7. 



" 16.3. 



" 14. 7. 



They resemble the feldspars in having the same ratios between the protoxyds, 

 sesquioxyds, and silica, this ratio being — in Heulandite 1:1:4; in Stilbite 

 1:1:3; in Natrolite 1:1:2; in Chabazite 1 : 1 : 2§ ; in Analcime 1 : 1 : 2§. 



/. Prehnite is a species related to the zeolites, and similar in its modes of 

 occurrence. It is harder, the hardness being that of feldspar. The surface is 

 usually clustered, convex, and crystalline; the color, pale green to white; 

 translucent. G. = 2.8-3.0. Before the blowpipe, intumesces and melts. Com- 

 position: Silica, 43.8, alumina, 24.8, lime, 27.1, water, 4.3. 



5. Carbonates. Sulphates. 



69. (22.) Calcite, or Carbonate of Lime. — Crystals rhombohedral or 

 hexagonal, with perfect cleavage parallel to the faces of a rhombo- 

 hedron the obtuse angle between whose faces (or R on E, fig. 52 a) 

 is 105° 5 / . Forms various, a few of which are shown in the figures. 

 Crystals often transparent ; massive kinds granular, as in statuaiy- 

 marble, or opaque and earthy, as in common limestones. Colors, 



Composition : 

















Analcime = 



Silica 



54.6, 



Alumina 



23.2, 



Soda, 



14.0, 



Chabazite 



" 



48.2, 





it 



20.0, 



Lime, 



10.8, 



Natrolite 



a 



47.4, 





a 



26.9, 



Soda, 



16.2, 



Scolecite 



u 



46.0, 





a 



26.1, 



Lime, 



14.2, 



Stilbite 



(I 



57.6, 





" 



16.3, 



" 



8.9, 



Heidandite 



" 



59.3, 





(( 



16.8, 



" 



9.2, 



