SILICATES, TITANATES, ETC. 



407 



apatite, micas, pyroxene, or amphiboles ; or it may form a ground 

 in; i>s filling in the cavities between the crystals of earlier crystal li/: i- 

 tion. 



In many instances quartz and orthoclase crystallize simulta- 

 nci>u>ly and in euteetic proportions, forming intergrowths of the 

 two >prrirs, termed 

 pegmatites, which 

 when fine in struc- 

 ture are micropeg- 

 matites. Cleavage 

 is always present, 

 and at right angles 

 when the section is 

 perpendicular to 001. 

 Interference colors 

 are grays of the first 

 order, and relief is 

 not marked. The 

 acute bisectrix lies 



FIG. 457. Section of Orthoclase, showing the Low 

 Relief and Well-developed Basal Cleavage Cracks. 



in the large angle ft, 

 with extinction 5 

 from the good basal cleavage. This angle increases with the re- 

 placement of potassium by sodium, and in some specimens may be 

 as large as 10. 



Orthoclase is the important feldspar of granites, where it crys- 

 tallizes at about the same time as quartz. In the coarse crystal- 

 line pegmatites the two minerals are in eutectic proportion, two 

 molecules of orthoclase to one of quartz, or, in composition, about 

 35 per cent, quartz. 



Orthoclase is also formed as the result of metamorphic agencies 

 in both the schists and gneisses of thermal or aqueous origin. 

 Secondary orthoclase sometimes fills veins, and is also associated 

 with metalliferous fissure veins as a gangue mineral. It is easily 

 decomposed, yielding its alkali to solutions containing carbon 

 dioxide, especially under pressure, when, with the separation of some 

 silica and hydration, orthoclase forms kaolin. It is by this con- 

 tinuous decomposition of orthoclase that the available supply of 

 potassium for plant growth is principally sustained in the soil ; and 

 it is well known that soils yielded by the decomposition of granites 

 are the most fertile. Other decomposition products of orthoclase 

 are muscovite, biotite, and gibbsite. 



