OUARRY MATERIALS OF NEW YORK 



157 



centages : silica 43.2; alumina 36.7; lime 20.1. The intermediate 

 members are mixtures of the two in various proportions which can 

 be expressed in general terms as Ab^An . They include oligoclase, 

 andesine, labradorite and bytownite, named in order from the soda 

 to the lime end of the series. The feldspars with high percentages 

 of soda are called the acid series on account of their relatively 

 large proportion of silica in contrast with those high in lime, which 

 are relatively low in silica. The identification of the different 

 members requires accurate crystal measurements (all belong to the 

 triclinic system but differ individually in form), or optical study, 

 or chemical analysis, and the methods need not be explained here. 

 The plagioclases commonly exhibit a striated appearance on certain 

 faces which arise from minute parallel lines that mark the contact 

 of lamellae in reversed or twinned position. This characteristic is 

 not common to the potash feldspars. 



The color of the feldspars exercises no influence upon their use, 

 except as it may be due to the presence of iron stain or iron-bearing 

 impurities. The potash feldspars commonly are light yellow, pink, 

 red or gray in color. The color of plagioclase varies from pure 

 white, most often seen in albite, to gray, brown or greenish, and 

 less commonly reddish. The variations in natural color disappear 

 when the feldspar is fused, the melt being usually white. 



The use of feldspar in pottery and generally for glazing purposes 

 is conditioned by the chemical composition which determines the 

 temperature of fusion. The potash varieties, orthoclase and micro- 

 cline, and the soda variety, albite, have the lowest melting tempera- 

 tures. According to the more recent work of Day and Alien,^ who 

 carried out a very extensive series of experiments on the subject, 

 these varieties do not melt at a definite point, but their fusion 

 extends over a range of temperatures. In finely powdered micro- 

 cline there was evidence of sintering at 1000° C, but the material 

 was not actually fused until the temperature reached about 1300°. 

 Albite fused at a somewhat lower point, but still above 1250° C. 

 The lime-soda varieties melt at temperatures between 1340°, the 

 fusing point of oligoclase, and 1532°, which is the melting point of 

 anorthite. .^7^'^W' 



Besides their lower fusing point, the feldspars that contain high 

 percentages of alkalies possess a further important feature, namely, 

 that on melting they yield a translucent glass. The varieties high 



1 Carnegie Institute Publications No. 31, Washington, 1905, p. 13-75; also 

 Amer. Jour. Sci. 4th ser. v. ig. 1905, p. 93-142. 



