336 FELSPAR 



is a flesh-coloured felspar, from Kilpatrick, containing 3 per cent, of magnesia ; whilst 

 the felspar called hyalophane is notable for containing a large percentage of I'.iryt.i. 

 Loxocfasc and Chcsterlite, are American varieties of orthoclaso ; whilst Valcncianite is 

 the name given to a Mexican adularia. 



SANIDINE, or glassy felspar, is a transparent potash-soda-folspar, regarded often ;is 

 a distinct species. It occurs in tabular, vitreous crystals, embedded in trachyte 

 and other volcanic rocks; and much resembles the artificial felspar obtained in 

 certain metallurgical operations. The well-known glassy felspar from the Drachen- 

 fels consists of silica, 66*6; alumina, 18'5 ; potash, 8'0 ; soda, 4*0; lime, I'O ; per- 

 oxide of iron, 0'6; = 987 (Berthier). Rhyacolite is a name proposed by Kose for a 

 Vesuvian variety of glassy felspar, which appears, however, to be only a mixture of 

 felspar and nepheline. The transparent felspars occurring in the lavas of Vesuvius 

 are sometimes termed ice-spar. 



II. ALBITB, or Cleavclandite known also as pericline and tetartine is a soda- 

 felspar first described by G-. Rose. It consists of silica, 6875; alumina, 1879; 

 soda, 10*90; potash, 1-21; lime, 0*51; magnesia, 0'09 ; peroxide of iron, 0*54 ; = 

 10079 (Weiss). Albite differs from orthoclase in the form of its crystals, which are 

 much less symmetrical, and belong to an essentially distinct system (triclinic). It 

 was at one time supposed to be a frequent constituent of granite, and syenite, but it is 

 now known that much of the felspar formerly mistaken for albite is really oligoclase. 

 Peristerite is a Canadian albite, remarkable for the beauty of its opalescence. 



Perthite is a flesh-red potash-soda-felspar from Perth in Canada East, notable for 

 being a mixed mineral made up of alternating bands of orthoclase and albite. 



OJLIGOCLASE, or soda-spodumene, is composed of silica, 62'54; alumina, 22'49 ; 

 soda, 7'84; potash, 4*54; lime, 2-18; magnesia, 0'41 ; = 100 (Devttle). This is an 

 analysis of the mineral from Teneriffe, where it occurs in trachyte ; but it is found 

 also in granite, syenite, serpentine, and basalt. The name sun-stone, occasionally 

 applied to certain varieties of orthoclase, is properly restricted to a Norwegian 

 oligoclase, presenting an aventurine appearance, which has been referred to the 

 presence of minute crystals of either specular iron, or gothite. 



III. LABRADORITE, or Labrador felspar, is a soda-lime-felspar, consisting of silica, 

 5575; alumina, 26*50; lime, 11-0; soda, 4-0; peroxide of iron, 1-25; water, 0'50; 

 = 99 (KlaprotK). This beautiful mineral was first observed by the Moravian 

 missionaries, on the shores of St. Paul's Isle, off the coast of Labrador, where it occurs 

 associated with hypersthene, hornblende, and magnetic iron-ore. It forms a common 

 constituent of most basalts and melaphyres, and occurs also in many recent lavas, as 

 well as in certain meteoric stones. 



From the beautiful chatoyant reflections which this mineral exhibits when polished, 

 it is highly valued for purposes of ornament. The parts exhibiting the play of 

 colours are disposed in irregular spots and patches, and the same spot, held in different 

 positions, displays various tints. The cause of this play of colours has not been 

 satisfactorily determined, but it is commonly supposed to proceed from minute internal 

 fissures or cleavage-planes in the stone ; whilst the aventurine-liko appearance is due 

 to certain microscopic embedded crystals, regarded variously as gothito, specular iron 

 ore, or magnetic pyrites. See LABRADORITE. 



Delcsse has applied the name of Vosgite to a French labradoritc. rendered hydrous 

 by partial alteration. 



ANDESITE is a felspar occurring in the trachyte of the Andes, *n the Vosges and 

 elsewhere. Its composition is silica, 60-26 ; alumina, 25'01 ; soda, 774 ; lime, 6'87 ; 

 potash, 0'84; magnesia, 0'14; = 100*86 (Rammelsberg}. 



IV. ANORTHITE, a lime-felspar, composed of silica, 43'96 ; alumina, 35*30 ; lime, 18'98 ; 

 soda, 0-47; magnesia, 0'45 ; potash, 0*39 ; peroxide of iron, 0'63 ; =100'18 (Abich). 

 It occurs in the ejected limestone-blocks on Monte Somma ; and has been found also 

 in the island of Procida, in the Bay of Naples ; and in the syenite of Carlingford, in 

 Ireland. 



Amphodelite is a Scandinavian anorthito, whilst Thiorsaurite is an Icelandic variety. 

 Barsowite and Bytownite are two closely allied minerals ; the former from Siberia, 

 and the latter from Canada. In the species called Danburitc a mineral frequently 

 classed with the lime-felspars a largo proportion of the silica is replaced by the 

 analogous compound, boric acid. 



Although in many cases it is by no means easy to distinguish the several species of 

 felspar one from another, yet a broad distinction may bo readily drawn between those 

 felspars which crystallize in the monoclinic (singly-oblique) and those which crystallize 

 in the triclinic (doubly-oblique) system. The former present two cleavage pianos at 

 right angles to each other, and are hence called orthoclastic ; whilst the latter have 

 their directions of cleavage oblique to each other, and are therefore called plapioclastic 

 felspars. The triclinic or plagioclastic species are characterised by fine striae on the 



