218 A TREATISE ON METAMORPHISM. 



As a metamorphic mineral, quartz is derived from actinolite, anorthite, 

 anorthoclase, anthophyllite, augite, biotite, bronzite, chalcedony, curnming- 

 tonite, diopside, enstatite, epidote, garnet, grossularite, hornblende, hypers- 

 thene, microcline, olivine, opal, orthoclase, plagioclase, prehnite, pyrope, 

 sahlite, scapolites, serpentine, tridymite, and zoisite. 



Modifications. — The most frequent and important modification of quartz is 

 by recrystallization. Crystallized quartz is dissolved under conditions of 

 weathering, as are all other minerals. This process is, however, exceed- 

 ingly slow. As a result of solution the quartz crystals may be corroded. 

 Such corrosion has been described by Hayes." In the belt of cementation, 

 and especially adjacent to trunk channels of circulation, quartz may be ex- 

 tensively dissolved from veins and from the wall rocks. (See pp. 848-849.) 



Granulation and recrystallization of quartz occur on a most extensive 

 scale in all quartzose rocks which are subjected to mass-mechanical action 

 or other favorable conditions in the zone of anamorphism. These changes 

 involve no heat and volume reactions so far as the quartz itself is concerned, 

 except that as the original minerals may be strained, or the new grains are 

 imperfectly adjusted, the change may involve a slight expansion. But 

 such expansion is followed by an equal contraction when the material is 

 recrystallized into quartz free from strain. In the recrystallization many 

 small individuals may be merged into one large individual. In some 

 instances of recrystallization, where large grains are produced from smaller 

 ones, the large individuals may average more than a million times as g*reat 

 as the small individuals from which they are derived. (See p. 695.) In 

 the production of a comparatively few large individuals from a multi- 

 tude of small individuals there is probably a release of energy. (See 

 p. 771.) During recrystallization the material taken into solution may 

 be deposited practically in situ or may travel far and be extensively 

 deposited elsewhere. Often quartz deposited in situ, or nearly so, can not 

 be discriminated from quartz deposited from solutions coming from distant 

 sources, as above described. 



A second modification of quartz only less important than that of 

 recrystallization is silication by the union with bases united with other 

 acids, thus forming silicates. Of such acids carbonic is by far of 



"Hayes, C. W., Solution of silica under atmospheric conditions: Bull. Geol. Soc. America, vol. 

 8, 1897, pp. 213-220. 



