Rogers — Eglestonite from San Mateo County. Cat. 49 



prove them to be isometric, with the following forms : a {100} , 

 tf{110}, o{101}, and n\t\\\. These include four out of the 

 seven common forms for eglestonite given by Schaller.* Four 

 distinct habits were recognized, as represented, in figures 1 to 4. 

 The most common type of crystals is that with dominant 

 cube, modified by the dodecahedron and very small faces of the 

 trapezohedron {211} (lig. 1). Fig. 2 represents another type, 

 dominant octahedron with prominent cube faces and nar- 

 row dodecahedron. Dodecahedral habit with narrow trapezo- 

 hedral {211} faces is represented by fig. 3. Acicular crystals 

 proved to be distorted dodecahedrons with striations parallel 

 to the intersection edges are represented by fig. 4. Acicular 

 crystals of this type were described by Schaller, but the cubic 

 and cubo-octahedral habits are not known for the Terlingua 

 eglestonite. The identification of the forms is based upon the 

 following average measurements of four crystals : 



Meas. Calc. 



110^101 (1) 59° 50' 60° 0' 



211^110 (7) 30 1 30 



112^101 (2) 30 12 30 



IOOaOIO (1) 90 5 90 



Meas. Calc. 



001^211 (1) 65° 32' 65° 54' 



211/«,211 (1) 48 11 48 \\\ 



100^110 (5) 45 13 45 



The images were all sharp though often faint. Angles of 30° 

 for two different zones, as given above, is proof of the isomet- 

 ric system. 



Physical properties. — The luster is adamantine, the crystals 

 though minute being very brilliant. The color varies from 

 yellow or orange-yellow to brownish-yellow, being lighter than 

 the Terlingua specimens. It becomes dark on exposure to light. 

 Some crystals in a tube lying on a table for several weeks 

 became black but retained their adamantine luster. The streak 

 is yellow, with a greenish tinge. The mineral has no cleavage 

 and is very brittle. Fragments examined under the micro- 

 scope are irregular, perfectly isotropic, and have a high index 

 of refraction (greater than sulphur in methylene iodide, 1*795, 

 as tried by the Becke test). 



Pyrognostic tests. — In the closed tube with soda or lime 

 it gives a mercury sublimate ; in the closed tube alone turns 

 orange-colored, then black, and finally dark red (HgO), giving 

 a white sublimate with mercury beyond. It volatilizes com- 

 pletely and gives no water in the closed tube. 



Wet tests. — The eglestonite is decomposed by hot hydro- 

 chloric acid, becoming gray superficially. It is also decomposed 

 by cold nitric acid, turning white. Hydrochloric acid added 

 to the nitric acid solution gives a white precipitate, which 



* Bull. 405 U. S. G. S., p. 148, 1909. 



Am. Jour. Sci. —Fourth Series. Vol. XXXII, No. 187. 

 4 



-July, 1911. 



