156 NEW YORK STATE MUSEUM 



that are bordered at times by fine-grained aplitic granite. Peg- 

 matites of this nature have no economic importance as sources of 

 feldspar or quartz, as the minerals are not sufficiently large or 

 segregated to admit their easy separation. 



The feldspar minerals. The general mineral composition of 

 pegmatites has been given on pages 66 and 67 of this report. It 

 should be noted, however, that feldspar, the principal economic 

 product of the local pegmatites, is not a definite mineral species, 

 but rather a mineral group, the members of which vary among 

 themselves in chemical and physical properties, as well as in their 

 industrial uses. The requirements for pottery spar, for which a 

 fairly large and steady market exists, are such as to exclude all but 

 a few varieties, and similarly there are certain restrictions generally 

 upon the kinds that find use in other industries. It is therefore 

 highly essential to ascertain the nature of the feldspar in pegmatite 

 and its adaptability for different purposes before undertaking the 

 development of a deposit. 



The feldspar minerals are composed of silica and alumina with 

 one or more of the bases — potash, soda and lime. It is usual to 

 class them in two principal groups, the potash feldspars and the 

 lime-soda feldspars, according to the nature of the bases present. 



The potash feldspars correspond chemically to the formula, 

 KA1SL0 8 or K 2 0.A1 2 3 . 6Si0 2 ; accordingly when pure they should 

 contain silica 64.7 per cent, alumina 18.4 per cent and potash 16.8 

 per cent. As a matter of fact, the potash seldom reaches the theo- 

 retical proportion, being partially replaced by soda which enters 

 into the chemical structure or is contained in another kind of feld- 

 spar intergrown with the potash variety. The amount of soda 

 present may range from 1 to 5 or 6 per cent. The potash feldspars 

 include orthoclase and microcline, the former monoclinic and the 

 latter triclinic in crystal form. Their distinction requires accurate 

 measurements of the cleavage or interfacial angles, or a study of 

 their optical properties under the miscroscope. Microcline is the 

 more common variety in New York pegmatites. There is no 

 difference in their value for pottery or other uses. 



The lime-soda group of feldspars, or the plagioclases, consists of 

 a continuous series that ranges from the pure soda variety, or 

 albite, at the one end to the lime feldspar, anorthite, at the other. 

 The composition of albite is represented by the formula NaAlSi 3 O s 

 or Na 2 O.Al 2 3 .6Si0 2 , corresponding to the following individual per- 

 centages : silica 68.7; alumina 19.5; soda 11.8. Anorthite has the 

 composition CaAl 2 Si 2 8 or CaQ.AL0 3 .2Si0 8 and contains in per- 



