merwin: quaetz and fluorite as standards 59 



The analysis of the blue crystals leads to the formula CuO.- 

 3AI2O3.2P2O5.9H2O, which, as none of the water is given off below 

 200°, can be interpreted as CuOH.6[Al(OH) 2 ].H B .(P0 4 ) ) . 



A comparison with published analyses shows a very close 

 agreement with the composition calculated from this formula. 

 The formula of chalcosiderite should be written analogously with 

 that of turquoise, as CuO.3Fe2O3.2P2O5.9H2O instead of with 

 8H 2 0. 



MINERALOGY. — Quartz and fluorite as standards of density 

 and refractive index. H. E. Merwin, Geophysical Labora- 

 tory. Communicated by A. L. Day. To appear in the 

 American Journal of Science. 



The two minerals, quartz and fluorite, in clear colorless varieties, 

 appear to be the only minerals of wide distribution that have 

 density constant to =±= 0.C01 and refractive index constant to 

 ±O.OC01, regardless of locality. These minerals may therefore 

 be used as standards. The density of quartz at 20° is 2.6495 and 

 its refractive index w at 20° for sodium light is 1.54425. The 

 corresponding values for fluorite are 3.180 and 1.43385. 



MINERALOGY. — The temperature stability ranges, density, chem- 

 ical composition and optical and crystallographic properties of 

 the alkali feldspars. H. E. Merwin, Geophysical Labor- 

 atory. Communicated by A. L. Day. To appear in the 

 American Journal of Science. 



Changes in the double refraction of sections of the alkali feld- 

 spars, observed during and after heating in the thermal micro- 

 scope, indicate that both albite and orthoclase appear in two forms, 

 the transition temperature of the low temperature 0-form into the 

 high temperature a-form being in both cases about 900°. Sani- 

 dine has the optical properties of a-orthoclase and adularia those 

 of /3-orthoclase. Microcline appears to be stable to its melting 

 point. Anorthoclase behaves like microcline when heated, ex- 

 cept that a slight change in double refraction near 900° has been 

 observed in specimens intergrown with albite. The inversion in 

 both orthoclase and albite is very sluggish even at 1100°. The 



