FELSPAR 335 



crystalline form, to admit of division into several well-defined species. The felspars 

 uro, in all cases, anhydrous double silicates, consisting of a silicate of alumina, com- 

 bined with a silicate of a protoxide which may bo either potash, soda, or lime ; but as 

 these oxides are capable of mutually replacing each other, it commonly happens that 

 two or even more of them coexist in the same' species. In all the members of the 

 felspar-group, there exists the simple relation of one atom of the sesquioxide, alumina, 

 to one of the protoxide base or bases ; but the proportion of silica varies considerably 

 in the different species. The amount of silica bears a definite relation to the geolo- 

 gical character of the rock in which the felspar occurs, and to the minerals with which 

 it is associated; the highly-silicatod species, as orthoclase, albite, and oligoclase, 

 occurring in granites, trachytes, and other so-called acid rocks ; whilst the less sili- 

 cated felspars, as labradorite, are confined to basalts, certain lavas, and other basic 



Exposed to a high temperature, the felspars fuse, with greater or lees difficulty, 

 to a vitreous enamel possessing considerable hardness, a property which has led to 

 their use in glazing porcelain. All the felspars are sufficiently hard to scratch glass, 

 but are themselves scratched by quartz; and this hardness^ combined with their 

 crystalline form, often serves to distinguish them from other minerals. Their specific 

 gravity varies from about 2*5 in orthoclase to iipwards of 27 in labradorite; and by 

 its constancy in the different species, becomes a useful characteristic. The lustre is 

 commonly vitreous, passing into pearly on the more perfect cleavage planes ; whilst 

 certain of the rarer varieties enjoy peculiar opalescent and iridescent properties, of 

 which the jeweller takes advantage, to a limited extent. With the exception of labra- 

 dorite, all the felspars are either unacted, or imperfectly acted, on by acids. 



According to the nature of their basic constituents, the felspars admit of classifica- 

 tion into four groups, viz. : 



I. Potash-felspar (often containing soda) ; orthoclase or common felspar ; and sani- 

 dine, or glassy felspar. 



II. Soda-felspar (often containing potash) ; albite, and oligoclase, or soda-spodumene. 



III. Soda-lime-felspar (often containing potash) ; labradorite, or labrador-felspar ; 

 and andesine. 



IV. Lime-felspar; anortkite. 



With regard to this arrangement it may be remarked that some of the species 

 here enumerated appear to shade insensibly one into another, so that doubts have not 

 unnaturally been entertained by certain mineralogists as to the real specific distinctions 

 between such minerals, Of late years, Tschermak, an Austrian mineralogist, has 

 specially applied himself to this question, and has arrived at the conclusion that there 

 are only three really distinct species : namely, a potash-felspar (orthoclase'), a soda- 

 felspar (albite}, and a lime- felspar (anorthite) ; all the other so-called species being 

 merely mixtures in various proportions of these three extreme types. But as ortho- 

 clase crystallises in the monoclinic, or singly-oblique system, whilst albite and anorthite 

 crystallise in the triclinic, or doubly-oblique system, it is difficult to conceive how 

 orthoclase can be isomorphous with the other felspars. Hence Tschermak has been 

 led to recognise two distinct kinds of mixed felspars ; those that contain both potash 

 and soda being regarded as mechanical mixtures of orthoclase and albite, like perthite, 

 whilst those containing both soda and lime are true isomorphous mixtures of albite 

 and anorthite. Although much theoretical interest attaches to these views, it will be 

 convenient in this place to adhere to the old-established, classification cited above. 



I. OBTHOCLASE (Ortkose, Fr. ; OrthoJclas, Ger. ; Ortosa, Ital.) This is the most 

 ordinary species, and that which in popular language is called' simply ' felspar.' It 

 occurs as a colourless, grey, or flesh-red mineral, crystallising in oblique rhombic 

 prisms, and frequently presenting the peculiar forms called twin-crystals. The crystals 

 always exhibit two well-marked directions of cleavage at right angles to each other, 

 whence the name of the species (opObs, straight ; K\dca, to cleave). Orthoclase is a 

 potash-felspar, of which the following is an analysis from the Baveno granite : silica, 

 6572; alumina, 18'57 ; potash, 14-02; soda, 1-25; lime, 0-34; magnesia, 0'10; = 100 

 (Abich). This mineral is an important constituent of granite, gneiss, syenite, peg- 

 matite, fel stone, and trachyte : of granite it ordinarily composes from 40 to 45 per 

 cent. The species orthoclase has been subdivided into a great number of varieties, 

 frequently founded on very insufficient grounds. The purest and most transparent 

 varieties are distinguished as adidaria ; and when exhibiting a faint bluish opal- 

 cscence are used in jewellery under the name of Moon- Stone. Other opalescent varieties 

 are the microcline, from the zircon-syenite of Norway ; and the Murchisonite from 

 Dawlish, and from Heavitree, near Exeter a mineral named after the late Sir E. 

 Murchison. A bright green orthocLise. coloured by a trace of oxide of copper, occurs 

 near Lake Ilmen, in Siberia, and is employed for ornamental purposes under the 

 uanie of Amazon-stone, from having been first found in the River Amazon, Erythrite 



