sil.K Alls, TITANATES, ETC. 419 



all had their potassium replaced at least in part in this way. In 

 other inMaiires leucite has produced feldspars or micas which have 

 n-t a iiu -i 1 the er\>tallinr form, producing pseudomorphs. Ulti- 

 mately leucite decomposes, forming kaolin. 



Artificially leiieite has been produced by simply fusing its con- 

 stituents in an open crucible and cooling the melt slowly; also by 

 fusing mixtures of natural minerals, as microcline and biotite, or 

 from the fusion of museovite alone. When leucite is heated to 

 I 1 .).") in a solution of sodium chloride orcarbonate, analcite isformed. 



PYROXENES 



The pyroxenes are the most important ferromagnesian rock-form- 

 ing minerals. They are essentially metasilicates of the general 

 formula, R"SiO 3 in which R" may be Mg, Ca, Fe, Mn, Zn, Na 2 , 

 Li 2 , or the triad elements Al, Fe'", or Mn'", which enter with a sec- 

 ond more or less complex, as R" R"' SiO 6 . Again silica may be 

 replaced in part by Zr, Ti, or Cb. 



The series in symmetry runs through the orthorhombic, mono- 

 elinie, and triclinic systems. Magnesium metasilicate is ortho- 

 rhombic, calcium metasilicate is monoclinic, manganese metasili- 

 cate is triclinic, and the ferrous metasilicate has not been found 

 unmixed with other silicates as a pyroxene, but as an amphibole. 

 The members of the series are isomorphous, and have many char- 

 acteristics in common, which may be seen by an inspection of the 

 table which follows. They have two cleavage directions at an angle 

 near S7, which serves to distinguish them from the closely related 

 forms of amphibole. In crystalline habit they are usually short, 

 stout, prismatic, or columnar. 



ORTHORHOMBIC PYROXENES 



The orthorhombic pyroxenes are mixtures of the metasilicates 

 of iron and magnesium ; some calcium may enter the molecule, 

 though always in small proportions. The two species enstatite 

 and hypersthene in composition grade into each other, the varie- 

 ties depending upon the percentage of ferrous iron present ; when 

 this amounts to 10 to 12 per cent, the mineral is known as bronzite ; 

 when much greater, hypersthene ; when much less, enstatite. 



