188 ICE-SPAR. 



BB melts to a dark pearl. 



Locality. Montalban, near Toledo, in 

 Spain. 



Name. From Iberia, the ancient name 

 of Spain. 



Ice-spar. Is the name sometimes given 

 to the transparent variety of Orthoclase 

 (Glassy Felspar) found in Vesuvian lavas ; 

 and also to pellucid varieties of other species 

 of' Felspar. 



It occurs on Monte Somma, near Naples, 

 with Nepheline, Mica, Meionite and Horn- 

 blende. The name has reference to the re- 

 semblance of the mineral to ice, both in ap- 

 pearance and in brittleness. 



Brit. Mus., Case 30. 



Iceland Agate of many mineralogists. 

 See Obsidian. 



Iceland Spar. The name applied to 

 transparent Calc Spar, the finest specimens 

 of which are found in Iceland. 



Brit. Mus., Case 43. 



M.P.G. Horse-shoe Case, No. 382. • 



ICHTHYOPHTHALME. D' Androda. ICTHY- 



ophthalmite, or Fish-eye stone ; (from 

 tx^-js, a fish, and cipda.kf/,i; an eye), a name 

 for Apophyllite, from its white pearly lustre, 

 resembling that of a fish's eve. 



Brit. Mus., Case 27. 



Idocrase, Ha'dy, PhilUps. Pyramidal : 

 primary form a right prism with a square 

 "base. Occurs crystallized and massive. 

 The general form of the crystals is that of 

 a rectangular prism terminated by planes, 

 and the edges of the prism are often re- 

 placed. Colour brownish and yellowish- 

 green, sometimes sulphur- yellow, orange 

 and also blue, rarely black ; sometimes green 

 when viewed in the direction of the axis, 

 and pistachio-green in a transverse direc- 

 tion. Lustre vitreous, often inclining to 

 resinous. Generally translucent, sometimes 

 nearly transparent. Exhibits double re- 

 fraction. Streak white. Fracture imper- 

 fect-conchoidal ; small-grained, uneven. H. 

 6-5. S.G. 3-349 to 3-45. 



Fig. 240. 



a 



rfrli 



Comp. 3(Ca, Mg, M;i, Fe), (Fe Al) 



3Si = Ca3 Si^ + Al iii (Gmelin). According 

 to Hermann, its composition is represented 



by the general formula (K^, fe) Si + ti K H. 



IDRIALINE. 



Analysis, from Vesuvius, by Karsten : 



Silica 37-.50 



Alumina .... 18-50 

 Peroxide of iron . . . 6-25 



Lime 33-71 



Magnesia . . . .3-10 

 Protoxide of manganese . 0-10 



99-16 



BB swells up and fuses readily, forming 

 a yellowish -green or brownish glass. Dis- 

 solves easil}'- in borax and microcosmic salt, 

 forming a glass coloured by iron ; the glass 

 formed with microcosmic salt likewise con- 

 tains a skeleton of silica, and becomes opa- 

 lescent on cooling. 



Partly decomposed by muriatic acid ; but 

 after it has undergone fusion, it is com- 

 pletely decomposed by that acid with the 

 separation of gelatinous silica. 



Localities. The limestone quarries at Glen 

 Cairn in Aberdeenshire. Between Broadford 

 and Killride in the Isle of Skye. Irish. 

 Donegal ; in prisms of a hair-brown colour 

 at Derryloaghan, and at Bunbeg near 

 Gweedore, &c. 



The principal foreign localities are Vesu- 

 rins ( Vesuvian), where crystals of a hair- 

 brown or olive-green colour line the cavities 

 of volcanic rocks, and are associated with 

 Glassy Felspar, Garnet, Melanite, Mica and 

 Nepheline. The finest specimens, however, 

 come from Ala, in the Val di Brozzo, in 

 Piedmont ; these are in general semi-trans- 

 parent, and of fine olive-green and hair- 

 brown colours, and in some rare instances, 

 perfectly black. Near Lake Baikal and on 

 the banks of the Wiloui in Siberia ( Wiluite). 

 Egge near Christiansand in Norway. Czi- 

 klowa in the Bannat. Monzoni in the Fassa 

 Vallejs in sulphur-yellow cr3'Stals. Frugard 

 in Finland {I'rugardite), Gokum (Gokumite 

 and Loboite). Haslau near Eger in Bohemia 

 (Egeran). Near Tellemarken in Norway, 

 lOyprine), of a fine. smalt-blue colour. 



Name. From i'i^'->, to seem, and ^sgao-/,-, a 

 mixture, in allusion to its crystaline forms 

 being mixed figures, wdiich have often been 

 mistaken for those of other minerals. 



Idocrase is cut into ring-stones and other 

 ornaments at Naples and Turin, and sold 

 under various names, as Chrysolite,Hyacinth, 

 &c., according to the colour. 



Brit. Mus.,"Case 35. 



M.P.G: Horse-shoe Case, No. 882 ; Upper 

 Gallery, A¥ali-case A, in recess 4, Nos. 112 to 

 118. 



Idrialine, Brooke §• Miller, Dufrenoy. 

 Idrialite, Sclu-otter. A kind of Bitumen 

 found mixed with Cinnabar at the quick- 



