OCCURRENCE AND ALTERATIONS OF STAUROLITE. 327 



passes into borax, and that the excess of alumina separates as gibbsite 

 (monoclinic; sp. gr. 2.35), the reaction is — 



4NaH 5 Mg 4 Al 5 B 3 Si 6 O sl +4K 2 C0 3 +lSra. 2 C0 3 = 



8HKMg 2 Al 2 Si 3 12 +3Na 2 B 4 7 +4Al(OH) 3 +5C0 2 +k. 



The decrease in volume of the biotite as compared with the tourmaline is 

 6.75 per cent; but if the gibbsite be included the increase of volume 

 is 3.96 per cent. 



STAUROLITE. 

 Staurolite: 



HFeAl 5 Si 2 13 

 Orthorhombic. 



Sp. gr. 3.65-3.77. 



occurrence.— Staurolite is similar in its occurrence to garnet, but apparently 

 requires more intense metamorphic action for it to begin to form. Its most 

 widespread occurrence is in the schists and gneisses of sedimentary origin. 

 It also develops in profoundly metamorphosed rocks of eruptive origin, but 

 it is not known as an original constituent in any eruptive rock. Like 

 garnet, it may be abundantly developed in the zone of anamorphism in 

 rocks which are cut by intrusives. The conditions favorable to its formation 

 are therefore similar to those which produce garnet (see pp. 300-302) and 

 such minerals as tourmaline, andalusite, sillimanite, and cyanite, with 

 which it is associated. It is evidently a mineral which derives its materials 

 from various other minerals, the elements being recombined into the more 

 compact form of staurolite under deep-seated conditions. 



Alterations. — The only alterations recorded for staurolite are to talc 

 (orthorhombic or monoclinic; sp. gr. 2.75), to chlorite (monoclinic; sp. gr. 

 2.71-2.725), and to muscovite (damourite) (monoclinic; sp. gr. 2.76-3.0). 

 The first two minerals are essentially magnesian ones, although if the 

 chlorite be aphrosiderite (massive; sp. gr. 2.90) a considerable amount of 

 iron may be present. It is therefore clear that in the change to talc and 

 chlorite magnesium must be derived from some other compounds. As the 

 alterations are those which occur in the zone of katamorphism, it may be 

 supposed that the magnesium is in the form of carbonate, since magnesium 

 carbonate is an almost universal constituent of ground waters in the upper 

 physical-chemical zone. The alteration to muscovite requires an entire loss 

 of the iron and the addition of potassium. It is therefore clear that some 



