432 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



quite equalling this stone in beauty. A polished plate of labradorescent felspar is repre- 

 sented in Plate XVI., Fig. 3 ; it is rarely used for gem purposes, since it is surpassed in this 

 ■direction by the abundantly occurring labradorite, but the whole rock mass is frequently 

 worked and utilised for such objects as gravestones and the ornamental facings of shops 

 and public buildings. 



LABRADORITE. 



Labradorite, labrador-spar, or labrador-felspar, so named because of its occurrence on 

 the coast of Labrador, is the most magnificent of all the felspars. It has an extraordinarily 

 brilliant play of colours combined with an intense metallic lustre ; it thus forms an extremely 

 effective material for many decorative purposes, although the stone in itself is of a dingy 

 grey colour. 



Labradorite from the locality mentioned, like oligoclase from Tvedestrand in Norway, 

 the so-called sun-stone, is a lime-soda-felspar, in which, however, there is more lime than 

 :soda. The result of an analysis is given below : 



Per cent. 

 Silica (SiOj) 55-59 



Alumina (AJjOs) 

 Ferric oxide (FejOj) 

 Lime (CaO) . 

 Soda (NajO) . 

 Potash (KjO) . 



25-41 

 2-73 



11-40 

 4-83 

 0-30 



100-26 



All lime-soda-felspars having a composition approaching that represented in the 

 -above analysis are referred to the mineralogical species labradorite, whether they come from 

 Labrador or elsewhere. 



The mineral is found but rarely in regular, well-developed crystals belonging to the 

 triclinic system. More usually it occurs in irregular masses, and this is especially the case 

 with that found on the coast of Labrador. As in all other felspars there are two well- 

 defined cleavages, which in this species make an angle of 94! degrees, one of which is less 

 perfect than the other. The more perfect cleavage surfaces have a distinctly pearly lustre, 

 and moreover exhibit the same striations due to twinning as were described in connection 

 with the sun-stone from Tvedestrand, only in labradorite the striations are usually wider 

 apart and less regular and numerous. A very similar series of straight lines or striations, 

 also due to twinning, is to be seen in labradorite on the less perfect cleavage surface. 



The other characters of labradorite are essentially the same as in other felspars. Thus 

 the hardness is 6 ; the mineral is fusible before the blowpipe, but with some difficulty ; the 

 specific gravity is low, though rather higher than that of potash-felspar, being 2-70. Other 

 features which distinguish labradorite from potash-felspar, or orthoclase, are the presence of 

 the twin striations, and the fact that it is moderately easily decomposed by hydrochloric 

 acid with the separation of gelatinous silica. 



Labradorite is perfectly opaque, and of a rather dark smoke- or ash-grey colour. 

 Naturally occurring specimens have little lustre, but take a good polish. When a fragment 

 of this mineral is turned about in the hand, so that the angle at which light strikes it is 

 varied, in one particular position the dull smoke-grey surface of the stone will be suddenly 

 lit up with a magnificent display of colours. This happens when rays of light are reflected 

 from the less perfect cleavage face, or from a certain other face, but in the latter case the 



