OCCURRENCE AND ALTERATION OF IOLITE. 291 



IOLITE (CORDIERITE). 



Iolite (cordierite) : 



H 2 (MgFe) 4 Al 3 Si 10 O 37 . 

 Orthorhombic. 

 Sp. gr. 2.60-2.66. 



occurrence. — Iolite occurs in a great variety of schists and gneisses. In 

 some cases it is so abundant as to make the rock a cordierite-gneiss. It is 

 associated with the very heavy metamorphic minerals, such as tourmaline, 

 andalusite, sillimanite, garnet, etc, Iolite occurs, likewise, in ejected frag- 

 ments of volcanoes and as a contact mineral in connection with dikes ; also 

 rarely as an original mineral in igneous rocks. 



Alterations. — The most common alteration is simple hydration. Further 

 changes may remove some of the ferrous iron or introduce alkalies, or 

 both, forming pinite (massive; sp. gr. 2.775). Simultaneously with this an 

 isotropic substance is said to be formed. Iolite sometimes passes into a 

 chlorite similar to talc. 



By the hydration of iolite, according to Clarke, chlorophyllite (crystal- 

 lization not given; sp. gr. 2.77) is formed." Supposing the Mg and Fe to 

 be in the same proportions both in the iolite and in the chlorophvllite, the 

 reaction is simple : 



(1) H 2 (MgFe) 4 Al 8 Si 10 O 37 +3H 2 O=H 8 (MgFe) 4 Al 8 Si 10 O 40 +k. 



If it be supposed that the Mg : Fe : : 3 : 1 in both compounds, the equation is — 



(2) H 2 Mg 3 F e Al a Si 10 O 3 H-3H 2 O=H 8 Mg 3 FeAl 8 Si 10 O 40 +k. 



The decrease in volume is 0.86 per cent. 



The reaction being hydration, one would expect it to involve increase 

 of volume, but the chlorophyllite produced is enough heavier to compensate 

 for this. One would expect the reaction to take place in the zone of kata- 

 morphism, but observations on this point are not known to me. 



The character of the product which forms simultaneously with pinite 

 being unknown, and the character of the chlorite which forms as a sec- 

 ondary product not being ascertained, it seems hardly worth Avhile to 

 attempt to write equations for these alterations, for they would be largely 

 conjectural. 



ffClarke, F. W., The Constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1S95, p. 83. 



