OPAL. 221 



in the changes of tridymite than with quartz. In the change of tridymite 

 to quartz there is a diminution of volume, amounting to 14.24 per cent, and 

 there is also probably liberation of heat. Energy is therefore potentialized 

 in tridymite as compared with quartz. The change is one which is particu- 

 larly likely to occur in the zone of anamorphism, where pressure is the 

 dominant factor. In the fact that quartz is a denser mineral than 

 tridymite we probably have a reason not only for the passage of tridymite 

 into quartz in the lower zone, but for the absence of tridymite as an 

 original pyrogenic constituent in the plutonic igneous rocks which crystal- 

 lized originally in this zone. Under its conditions the denser mineral, 

 quartz, formed. 



OPAL. 



Opal: 



Si0 2 .nH 2 (H 2 2 to 13 per cent; but mostly 3 to 9 per cent, ) 



Amorphous. 



Sp.gr. 2.1-2.2. 



occurrence. — Opal, like most other hydrous minerals, is a product of the 

 zone of katamorphism. Opal is a direct deposit from hot springs. In the 

 sedimentary rocks it is abundantly formed from the siliceous skeletons of 

 certain animals and plants, such as racliolaria, sponges, and diatoms. Opal 

 is plentifully deposited in cavities in rocks by subterranean waters. Its 

 most common places of occurrence are the limestones, where it is largely 

 of organic origin, and the porous igneous rocks, especially as amygdules of 

 the amygdaloids, where it is a chemical precipitate. 



In general, as a metamorphic product opal may be derived from the 

 same minerals as quartz. 



Modifications. — The most frequent change of opal is to quartz (rhombohe- 

 dral; sp. gr. 2.652-2.654). Frequent intermediate products are chalcedony 

 and chert, which appear to be partly crystalline substances. (See p. 222.) 

 In the passage of opal into quartz, the changes are three : dehydration, 

 reduction of volume, and recrystallization. Supposing the composition of 

 the opal is Si0 2 .|H 2 0, which would be about 6 per cent of water, the 

 decrease of volume would be 22.81 per cent. The change from opal to 

 quartz above given is commonly accomplished by solution and redeposition 

 or recrystallization. When the material is taken into solution this silica may 

 be .deposited near by or transported elsewhere. It may unite with free 

 bases, producing silicates ; it may displace other acids combined with bases, 



