324 A TREATISE ON METAMORPHISM. 



formed in connection with the contact action of such rocks as granites, 

 granulites, diabases, and gabbros. In such positions the formation of 

 axinite is usually regarded as assisted by fumarole action. 



Alterations. — Apparently the alterations which axinite undergoes in 

 rocks have not been worked out, as they are not recorded in the standard 

 text-books. 



PKEHNITE. 

 Prehnite: 



H,Ca,Al 2 Si 3 12 . 

 Orthorhonibic. 

 Sp. gr. 2.8-2.95. 



occurrence. — Prehnite is almost identical in its occurrence with the 

 zeolites (see pp. 331-333). It is therefore especially prevalent in the basic 

 and intermediate rocks, such as anorthosite, basalt, diabase, gabbro, 

 andesite, diorite, and syenite; also it occurs to some extent in granites and 

 gneisses, where it may be associated with epidote. In the igneous rocks it 

 is especially prevalent in the volcanics, since these are usually more 

 porous. Like the zeolites, it is a very frequent occupant of amygdaloidal 

 cavities, and also of cracks and crevices in the rocks. As already inti- 

 mated, the most constant associates of prehnite are the zeolites. Prehnite 

 occurs to some extent in the schists and gneisses, including those derived 

 from igneous rocks, such as the amphibolites, and from aqueous rocks, 

 such as the marbles. In some cases it is found in cavities in sedimentary 

 rocks which have been metamorphosed by granitic or granulitic intrusions. 

 As a secondary mineral prehnite is often derived from anal cite, laumontite, 

 mesolite, natrolite, the plagioclases, and scolecite. Fused prehnite yields 

 wollastonite and ankerite. 



Alterations. — The only alteration which I have been able to note in refer- 

 ence to prehnite is to chlorite (mouoclinic ; sp. gr. 2.71-2.725). This 

 change requires the substitution of magnesium for calcium. Supposing 

 the chlorite were amesite (hexagonal plates; sp. gr. 2.71), the change might 

 be expressed: 



H 2 Ca 2 Al 2 Si30 1 ,+2MgC0 3 +H 2 0=H 4 M S2 Al 2 SiO, J +2Si0 2 +2CaC03+k. 



Ignoring the carbonates, the increase of volume of the chlorite and quartz 

 (rhombohedral; sp. gr. 2.6535) as compared with the prehnite is 3.27 per 

 cent. The change is one of hydration and desilication, and would be 

 expected to take place in the zone of katamorphism. 



