268 A TREATISE ON METAMORPHISM. 



. — The rhombic pyroxenes are common pyrogenic constituents 

 of igneous rocks rich in magnesium. They are common in the normal 

 diabases, gabbros, and basalts, and are abundant in the norites, peridotites, 

 etc. They also occur in the intermediate, basic, and ultrabasic volcanic 

 rocks, including both lavas and tuffs. A very common associate of the 

 rhombic pyroxenes is olivine. The rhombic pyroxenes also occur in the 

 schists and gneisses, especially those derived from eruptives. In such 

 rocks they are frequently associated with the monoclinic pyroxenes. They 

 further occur as vein materials and are found in meteorites. 



As metamorphic minerals, enstatite is derived from pyrope and 

 hypersthene from almandite, biotite, and common garnet. 



Alterations. — The most frequent alteration of the rhombic pyroxenes is 

 to talc (orthorhombic or monoclinic; sp. gr. 2.7-2.8). The less frequent 

 alterations are to serpentine (monoclinic; sp. gr. 2.50-2.65), bastite (ortho- 

 rhombic; sp. gr. 2.50-2.75), actinolite (monoclinic; sp. gr. 3-3.2), and 

 anthophyllite (orthorhombic; sp. gr. 3.1-3.2). 



For the sake of simplicity it is assumed that where pure talc or 

 serpentine is produced these materials are derived from enstatite; and that 

 where bastite, actinolite, and anthophyllite are produced these minerals are 

 derived from bronzite or hypersthene. Of course, serpentine or talc may 

 be produced from bronzite or hypersthene, the iron separating as oxide or 

 carbonate. One such possible alteration is written. However, the ordinary 

 alterations of the ferriferous pyroxenes are to bastite, which is iron-bearing-. 



The change of enstatite to talc is as follows : 



(1) 4MgSi0 3 +C0 2 +H 2 = H 2 Mg 3 Si 4 12 +MgC0 3 -k. 



Supposing the magnesium carbonate to be dissolved, the increase in 

 volume is 9.93 per cent. If a ferriferous pyroxene be supposed to alter 

 to talc, iron oxide, must separate. Supposing - this to be in the form of 

 magnetite (isometric; sp. gr. 5.1,74), and supposing that the magnesium 

 is to the iron as 3:1, or that the mineral is intermediate between bronzite 

 and hypersthene, the reaction may be written: 



(2) 3Mg 3 FeSi 4 12 +3H 2 0+0=3H 2 Mg 3 Si 4 1 2+Fe 3 O i -:-k. 



Similar equations may be written by which, instead of magnetite, 

 hematite (rhombohedral; sp. gr. 5.225) or limonite (amorphous; sp. gr. 

 3.80) is produced, in which case the expansion of volume would be 



