276 F. E. WRIGHT OBSIDIAN FKO.M ICELAND 



porous and exposed portions of the obsidian. As noted above, Ijoth 

 livalite and ahmite were deposited from these solutions on the walls of 

 bubble cavities. In specimen 88434 the gas bubbles adjacent to the 

 spherulites are usually coated with minute water-clear crystals of a sub- 

 stance which is evidently a secondary mineral introduced liy circulating 

 solutions after the solidification of the obsidian; this mineral agrees in 

 its optical properties with alunite. The largest crystals measure less 

 than half a millimeter in diameter and are bounded by the basal pinacoid 

 and l)y rhombohedral faces, which are triangular in shape. Basal cleav- 

 age is distinct and gives rise to a distinct semipearly luster on the basal 

 ]nnacoi(l. As a result of this cleavage, it is an easy matter to obtain 

 sections normal to the optical axis, on which then the uniaxial, optically 

 positive interference figure of a mineral of medium to fairly strong bire- 

 fringence is visible, lihombohedral cleavage is also present, l)ut is poorly 

 developed. The refractive indices were measured by the immersion 

 method: w about 1.575, e about 1.595: birefringence about 0.020. Plard- 

 ness apparently 3 to 4. Slightly soluble in hydrochloric acid, but To a 

 greater degree in sulphuric acid. In tlie HCl solution potassium w;is 

 round to be present; also sulphuric acid. On heating in a closed tu1)e. 

 the mineral decrepitates and emits a white cloud of sulphurous fumes. 

 Tliis material heated on charcoal before the blow-pipe gives, after mois- 

 tening with dilute co])alt nitrate solution, the characteristic l)lue color 

 test for aluminum. The density was found by immersion of a clear 

 crystal in Klein's solution to be 3.73. These properties agree with those 

 of alunite, and the determination as such may be considered reasonably 

 certain. The alunite appears to have been formed during the later stages 

 of precipitation of the hyalite. Compared with hyalite, it is present in 

 small amounts. The small geodes of alunite, when examined under high 

 powers,^- glisten and sparkle with the crystal faces of this mineral and 

 are exceedingly beautiful. The same mineral occurs in the more com- 

 pletely crystallized rhyolite of specimen 88434, which is likewise lianded 

 and full of gas-bubble pores. 



It is of interest to inquire into the character and the temperature of 

 the solutions from which the hyalite and alunite were deposited. In this 

 connection one feature is of special interest : The obsidian fragments and 

 blocks which are associated witb tlie hvalite and alunite occurrences are 



^ For the examination of extremely minute crystals in the hand specimen, the follow- 

 ing method has l:)een found satisfactory : Use a binocular magnifying glass (magnifica- 

 tion. 65 X ) and view object illuminated by a strong electric light, partly inclosed in a 

 brass holder mounted on a universal arm, which is attached to binocular stand and can 

 be moved in any direction, thus enabling the observer to illuminate at will any particu- 

 lar crystal from any desired direction. 



