WASHINGTON AND MERWIN! NEPHELITE CRYSTALS 389 



MINERALOGY. — Nephelite crystals froin Monte Ferru, Sardinia. 

 H. S. Washington and H. E. Merwin, Geophysical 

 Laboratory. 



A trachytic phonolite forms the upper part of a rounded hill 

 immediately south of Monte Enzu, the culminating point of 

 Monte Ferru. 1 This rock is dense, light gray, aphyric, and 

 composed essentially, as shown by the microscope, of tables of 

 soda-orthoclase, with interstitial nephelite, and small amounts 

 of minute diopside crystals. There are numerous small miaro- 

 litic cavities, in which are found small (1-2 mm.) well-formed 

 nephelite crystals, the walls being lined with minute tables of 

 orthoclase and crystals of pyroxene and titanite. 



More than one hundred crystals were picked out for examina- 

 tion, twenty-five being selected for the crystallographic and optical 

 examination, and the balance for chemical analysis, the former 

 being undertaken by Merwin and the latter by Washington. 



The nephelite crystals are stout, almost equant, prisms, 

 from 1 to 2 mm. in diameter and weighing, on the average, 3 mg. 

 each. They are fresh and clear, except for inclusions of minute 

 augite and titanite crystals. 



Usually not more than 4 faces giving satisfactory signals 

 were found on a crystal. The prism appeared vertically striated; 

 the base in all cases gave multiple signals, possessed a pearly 

 luster, and showed grooves parallel to the edges in some cases. 

 The unit pyramid (1011), and the pyramids (1012) and (2021) 

 gave good signals. 12 angles (1010) A (1011) from excellent 

 signals varied between 45°39' and 45°58', average 45°49'. 13 

 other angles from less accurate settings varied between 45°30' 

 and 46°15', average 45°51'. 7 angles (1010) A (1012) varied be- 

 tween 26°55' and 27°18', average 27°10'. Between (1010) 

 and (2021) 4 angles varied between 63°30' and 64°20', average 

 64°10'. The complements of these observed angles and the 

 corresponding calculated crj^stallographic constants appear be- 

 low. With them are placed those for nephelite from Monte 



^ For a description of the lavas of this volcano, see H. S. Washington, Am. J. 

 Sci., 39: 513. 1915. 



