326 A TREATISE ON METAMORPHISM. 



TOURMALINE. 



Tourmaline is a complicated aluminum silicate, which may be of any 

 one of four different types or intermolecular growths of these types. 

 According to Clarke, the formulae for these types are — ° 



Tourmaline: 



NaHR 3 Al 8 B 3 Si 6 31 (R in some cases being lithium and hydrogen). 



NaH 3 Mg 2 Al,B 3 Si 6 3l (Mg frequently being replaced by Fe). 



NaH 4 Mg 3 Al 6 B 3 Si 6 31 . 



NaH 5 Mg 4 Al 5 B 3 Si 6 31 . 



Rhombohedral. 



Sp. gr. 2.98-3.20. 



• — Tourmaline rather frequently occurs in the marbles and in 

 the calcareous schists. It also has a rather widespread occurrence, although 

 generally not as an abundant mineral, in granites, gneisses, schists, and 

 granulite. In these rocks it frequently occurs in such relations to dikes of 

 igneous rocks, especially of pegmatites, as to suggest that its development 

 is promoted by contact action. Because of the boron, tourmaline has 

 generally been regarded as evidence of fumarole action. Certain it is that 

 boron is not usually a constituent of the ordinary sediments, and to 

 account for this element, especially where the tourmaline is abundant, as it 

 occasionally is in the schists, would seem to require its introduction from 

 an outside source, either by gaseous or by aqueous solutions. 



Alterations. — Mineral specimens of tourmaline are recorded as altering 

 into mica, chlorite (monoclinic; sp. gr. 2.71-2.725), and steatite (massive; 

 ;sp. gr. 2.794). However, in rocks tourmaline is one of the more permanent 

 minerals, and the chemical additions and subtractions which occur in the 

 alterations are so little known, and the exact nature of the tourmaline from 

 which individual minerals are derived is so uncertain, that it is not thought 

 advisable to attempt to write all the reactions representing these changes. 

 If one assumes a definite tourmaline and a definite mica as being produced 

 from it, it is easy to write a reaction. For instance, supposing that normal 

 biotite (monoclinic; sp. gr. 2.90) is derived from a tourmaline of the 

 composition of the last of the four formulae given, that the additional 

 alkalies are added in the forms of carbonates, that the free boric acid 



"Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, 

 pp. 56-57. An alternative form has been proposed by Penfield. 



