152 



GERSDORFFITE. 



quently containing crystals of Quartz, Cal- 

 cite, &c., coating their interior. Of such a 

 kind are the geodes of common occurrence 

 in the New Red Marl of Somersetshire and 

 Gloucestershire, to which the name of "po- 

 tato-stones" has been locally given, from 

 their external resemblance to the root of 

 that name. Specimens of these from the 

 neighbourhood of Bristol will be found in 

 the Upper Gallery of the Museum of Practi- 

 cal Geology. See Wall-case 44, Nos. 35 

 to 37. 



Gersdorffite, Haid'inger. Cubical ; pyri- 

 tohedral. Occurs in octahedrons, sometimes 

 with the faces of the pentagonal dodecahe- 

 dron, and cubo-octahedron. Colour tin- 

 v/hite inclining to lead-grey ; often with a 

 grey or greyish-black tarnish. Lustre 

 metallic. Streak greyish-black. Fracture 

 uneven. H. 5-5. S.G. 67 to 6-9. 



Comp. N"i, S^-t-Ni, As = nickel 35-54, 

 arsenic 45*18, sulphur 19'28^100. 



Analysis, from Schladming, by Pless : 

 Arsenic . . . .39-40 



Sulphur .... 16-91 

 Nickel . . . .28-62 



Iron ..... 12-19 

 Cobalt ..... 2-88 



100-00 



Decrepitates strongly when heated in a 

 flask. Heated to redness, yields a strong 

 sublimate of fused, yellowish-brown sul- 

 phide of arsenic, while a mass like copper- 

 nickel is left behind, (Berzelius.) 



Dissolves in nitric acid, depositing sulphur 

 and arsenious acid. 



Localities. Loos, in Helsingland, Sweden. 

 Albertine mine, near Harzgerode in the 

 Harz. Schladming in Styria. Hamsdorf, near 

 Lobenstein in Thuringia. Near Ems, in 

 tine crystals. (See also Amoibite.) 



Brit. Mus., Case 6. 



Geyserite. a loose hydrated form of 

 silica. It is held in solution by the hot 

 water of the Geysers of Iceland, and de- 

 posited by them on the ground around in 

 light, porous, concretionary or cellular 

 masses, somewhat resembling cauliflowers 

 in appearance. 



Gibbsite, Torrey, Cleveland, Phillips, 

 Nicol. (See also Hydrargillite.) Hexa- 

 gonal. In small crystals with the lateral 

 edges replaced, and a perfect basal cleavage. 

 Generally occurs in aggregations of irregular 

 stalactites, or small mammillary incrusta- 

 tions, v/ith smooth surfaces. Structure in- 

 distinctly fibrous, the fibres radiating from 

 the centre. Colour greyish-, greenish-, or 

 reddish-white. Translucent. Lustre faint. 



GIESECKITE. 

 When breathed on gives otf a strong argil- 

 laceous odour. Tough, but easily reduced 

 to powder. H. 3 to 3-75. S.G. 2-3 "to 2-4. 



Comp. Terhydrate of alumina or Al, H^ 

 = alumina 65-56, water 34-44 = 100. 



Analysis, from Richmond, U.S., by Smith 

 ^ Brush : 



Alumina .... 64-24 

 Water .... 33-76 



Silica 1-33 



Phosphoric acid . ., . 0-57 

 Magnesia .... O-lO 

 Protoxide of iron . . trace 



100-00 



In a matrass yields much water. 



BB alone infusible, but becomes white : 

 on charcoal decrepitates, becomes opaque, 

 crystals exfoliate ; phosphoresces. 



Entirely soluble in concentrated sulphuric 

 acid. 



Localities. Stalactitic, at Richmond, Massa - 

 chusetts, U.S., in a bed of Limonite; and at 

 the Clove Mine, Duchess co. New York, Cry- 

 stallized (Hydrargillite) in the Schischim- 

 skian Mountains, near Slatoust in the Ural. 

 Gumush-dagh in Asia Minor, with Corun- 

 dum. Unionville, Pennsylvania, U. S. 

 Brazil, resembling Wavellite. 



Name. After Colonel George Gibbs. 



Brit. Mus., Case 19. 



GiBSONiTE, Haidinger. A mineral crystal- 

 lizing in right rhomboidal prisms, partly 

 aggregated in little kidneys, and bearing 

 some resemblance to Prehnite. Colour rose- 

 white or pale rose. 



Locality. Hartfield in Renfrewshire. Fig. 

 209. 



Fig. 



GiESECKiTE ; GiSECKiTE, Stromeyer. A 

 pseudomorphus form of Elseolite, from which 

 it chiefly differs in containing 4-88 per cent, 

 of water. It occurs in regular six-sided 

 prisms of a brownish colour externally; in- 

 ternally greenish and blackish-green inter- 

 mixed, Thp colour of the Diana specimens 

 varies from pea- green to leek-green. Lustre 

 greasy. Opaque, but translucent in small 

 fragments. Structure granular, sometimes 

 waxy ; bearing a greater resemblance to a 

 pseudomorphous steatitic mineral than a 

 crystalline substance. Yields to the knife, 

 but scratches glass. Affords a white powder, 

 H. 3 to 3-35, S.G. 2-73 to 2-85. 



