Washington and Wright — Feldspar from Linosa. 55 



the extinction angles on ( 001 ) would indicate a feldspar com- 

 position ranging from about Ab^Aiig to Ab 21 An 20 , while from 

 those on (010) a composition from Ab n An 5 to Ab 13 An 10 and 

 higher might be inferred. The values, of the optic axial angle 

 alone indicate variations from Ab 37 An 20 to Ab 6 An 5 , according to 

 the most accurate measurements of the different members of 

 the plagioclase series. As a whole, therefore, the optic charac- 

 ters would show that we have an andesine feldspar of a composi- 

 tion somewhat more sodic than A^An^ on an average about 

 Ab 3 An 2 , while the specific gravity is almost exactly that of a 

 labradorite of the composition AbjAn^ or (corrected) of an 

 andesine Ab 5 An 4 . 



Chemical Composition. 



The material used for the chemical analyses was obtained by 

 coarsely crushing several of the crystals and fragments and 

 very careful hand-picking under the lens. It was found to be 

 impossible to separate the portions contaminated with adherent 

 basaltic scoria by means of heavy solutions. Because of the 

 similar specific gravities, particles with and without attached 

 scoria floated and sank together. In the heavy solution the 

 material used for analysis seemed to be homogeneous. 



All the material analyzed was perfectly fresh, colorless, and 

 water-clear, and consisted of but one mineral, so far as could be 

 ascertained by examination under the microscope. While the 

 greater portion was entirely free from inclusions, it was 

 impracticable not to use some fragments containing inclusions, 

 and as these have a bearing on the chemical discussion, they 

 may be briefly described here. Apart from them the material 

 was perfectly pure and admirably suitable for chemical 

 analysis. 



The inclusions are never very abundant, especially in the 

 fragments used for the analyses. They are very minute in 

 size, the largest being 0'5 mm long by 0'l mm wide, and the great 

 majority are much smaller. They are of uniform character, 

 in the form of narrow, spindle-shaped bodies or thin cylinders 

 with rounded ends. They are black, with metallic luster, and 

 perfectly opaque, so that they may be regarded as essentially 

 magnetite, a conclusion also indicated by the results of the 

 analyses. 



The feldspathic mineral is only slightly acted on by hot, 

 dilute hydrochloric acid, even after prolonged treatment, so 

 that the main portion was brought into solution by fusion with 

 mixed sodium and potassium carbonates. In this, as in other 

 respects, the analyses were made by the methods advocated by 

 Hillebrand and by Washington, the alkalies being determined 

 by the Smith method. 



