64 



STRUCTURAL GEOLOGY. 



Opal. — Un crystallized silica, a little less hard than quartz and of less density 

 (G=2-3), and having- usually a greasy or waxy luster. Colors, white to milky gray, red, 

 etc. ; when showing internal colored reflections it is the gem, opal. Upal is identical 

 with quartz in composition, yet commonly contains some water ; it dissolves more readily 

 in heated alkaline waters. Here belongs the material deposited by the hot waters of 

 geysers, making the geyser basins (sometimes called geyserite) ; also the siliceous 

 secretions of Sponges, and the shells of Hadiolarians, and of the minute microscopic plants 

 called Diatoms. 



Tridymite. — Pure silica of the density of opal, but occurring in minute thin glassy 

 hexagonal crystals, in obsidian and some other volcanic rocks. 



2. Alumina. 



Sapphire or Corundum. — Composition: Al203=oxygen 46-8, aluminium 53'2=:100, 

 The crystals are the hardest of gems next to the diamond ; the blue transparent crystals 

 are sapphire, the red crystals, oriental ruby ; and the coarser material when gi-ound 

 makes emery. 



o. 



Silicates of Aluminium and other Basic Elements. 



The Feldspars. — The feldspars are next in abundance to quartz. Luster nearly 

 like quartz, but often somewhat pearly on smooth faces. H = 6i-7, or very nearly as hard 

 as quartz. Specific gravity 2 •4-2-6. In general white or flesh-colored ; rarely greenish or 

 brownish. Crystals stout, never acicular. Differs from quartz in having a perfect cleav- 

 age in one direction, yielding under the hammer a smooth lustrous surface, and in another, 

 nearly as perfect a cleavage surface, the two inclined 84° to 90° to one another ; also in being 

 more or less fusible before the blow-pipe. Composition : Silica and alumina with either 

 potash, soda, or lime, or two or all of these combined. Contains, unless impure, no iron 



The group of feldspars includes several species differing in the proportion of silica 



(the acid) to the other ingredients (bases), and in the 

 particular alkali (potash, soda, or lime) predominant, 

 but they graduate to some extent into one another. The 

 kinds are as follows : — 



Orthoclase, or potash-ieldsTpar, is the most common. 

 The cleavage surfaces make a right angle with one 

 another, whence the name, signifying cleaving at a right 

 angle ; the form is monoclinic. Figs. 34, 35, 36, repre- 

 sent crystals of this species, the last a twin crystal ; cleavage takes place parallel to the 

 faces and ii. Composition : Silica 64-7, alumina 18-4, potash 16-9 = 100. 



The other kinds are triclinic in crystallization, and the cleavages make an oblique 

 angle with one another, of 84° — 89° 44', and hence they are sometimes called plagioclase, 

 from the Greek plagios, obliqtie. 



Microdine. — Like orthoclase in composition ; but the cleavage angle differs 16' from 

 90°. The chief distinctions are optical. 



Albite. — A soda feldspar, named from the Latin albus, white. "When albite and 

 orthoclase occur together, albite is usually the whiter. Composition : Silica 68-6, alumina 

 19-6, soda 11-8 = 100. A little more fusible than orthoclase. 



Oligoclase. — A soda-lime feldspar. Composition : Silica 61-9, alumina 24-1, lime 5-2, 

 soda 8-8 = 100. Fuses like albite. 



Labradorite. — A lime-soda feldspar. Composition : Silica 52-9, alumina 30-3, lime 

 12-3, soda 4-5 = 100. Fuses easily, named from Labrador. Andesite is a species between 

 oligoclase and labradorite in composition, named from the Andes. 



Anorthite. — A lime feldspar. Composition: Silica 43-1, alumina 36-8, lime 20-1 = 100. 



