22 
PLATE VI. 
Figs. 1—3.— Potash Felspar, Common Prismatic 
Felspar, Orthoclase Petrilite. 
The primary form is an oblique rhombic prism of 
118° 48', as seen in Fig. 1; the crystals may be split in 
the direction of the terminal planes, and of the planes of 
truncation of the acute lateral edges, and less distinctly in 
the direction of the lateral planes. 
Truncation of the obtuse angles, simple and double 
truncation of the lateral edges occur; also twins, quadru¬ 
plets (as in Fig. 4, etc.), and very frequently compact 
masses or foliated granules. The hardness = 0*0, the 
specific gravity =2*50—2*62. Transparent, translucent, 
opaque, sometimes with play of colours and iridescence. 
The lustre is externally vitreous, internally in the fracture 
pearly. The colourless transparent crystals, such as occur 
in the Zillerthal and at St. Gothard, are called adular ; 
when polished, they occasionally present a peculiar splen¬ 
dour, and opalescent felspars are then called moon-stones , 
and if they contain red lustrous dots, sun-stones. The 
vitreous, transparent, fissured, and striated felspar, as it 
principally occurs in the trachyte of the Siebengebirge, at 
the Kaiserstuhl, Etna, etc., is called sanicline ; on the other 
hand, those from the Laacher sea, in Prussia, and from 
the Somma, in which the silicate of soda preponderates, 
are called rhyacolyte , or ice-spar. The clouded apple- 
green variety (Fig. 2) from Labrador and from the Urals, 
is called amazon-stone , but if it is grey, milky-white or red¬ 
dish, common felspar. The latter appears in thick masses 
of compact structure, and is called compact felspar ; if it 
is granularly crystalline, it receives the name of granulite. 
The chemical constituents are neutral silicate of alu¬ 
mina, and neutral silicate of potash, which are sometimes 
replaced by silicates of soda and lime. Thus, for example, 
the adular of St. Gothard, according to Abich, contains 
17*97 of alumina, 13*99 of potash, 1*01 of soda, 1*34 of 
lime, and 65*69 of silica, the formula being K Si + A1 Si 3 . 
Before the blowpipe it melts in splinters into a some¬ 
what blebbv, glassy pearl, and colours the blue flame dis¬ 
tinctly violet. Insoluble in acids. Dim and clouded fel¬ 
spars frequently contain 1—8 per cent of water. 
The felspars are found principally in the primary rocks, 
and form an important constituent in granite, gneiss, syenite, 
diorite, and in most porphyries (Fig. 6—8); in the granites 
(Figs. 16 and 17) they generally appear in greyish-white 
and red granules of varying size and form ; they are, how¬ 
ever, easily distinguished by their peculiar crystalline frac¬ 
ture, their hardness, and ready fusibility from the resinous 
lustres of quartz, and the finely granular, almost metallic 
lustre of mica. Fine crystals are principally found in the 
clefts of transported primitive rocks; sometimes, however, 
also imbedded in granite ; as, for instance, at Baveno, at the 
Fichtel-gebirge, at the Schwarzwald, in the neighbourhood 
of the Feldberg, in Bohemia, etc. Also in many porphy¬ 
ries, as in the Tyrol, at the foot of the Feldberg, opposite 
the Wiesenthal, in Sweden and in Greece. The glassy 
felspar is found in very fine crystals in some trachytes at 
Drachenfels, in the Sieben-gebirge, above Bonn. 
The potash felspar serves chiefly as the ingredient 
which gives tenacity to masses of porcelain, and various 
enamels, and affords, by its decomposition, the purest 
porcelain (kaoline) ; as, for instance, at Aue, not far from 
Schneeberg in Saxony, at Limoges in France, and other 
localities. Kaoline is a fine triturable earthy substance, 
somewhat unctuous to the touch. It forms, with water, 
a kneadable mass, and at a white heat melts imperfectly, 
and becomes so hard as to give off sparks when struck by 
steel. The amazon-stone is polished for boxes and other 
small ornaments ; the adulars are sometimes used as orna¬ 
mental stones. 
Fig. 5.— Soda-felspar, Albite, Pericline. 
The primary form is an oblique rhomboidal prism of 
120° 18'; it may be split in the direction of the terminal 
planes ; less distinctly in that of the lateral planes. Trun¬ 
cations of the acute angles occur, and of the basal and 
lateral edges, especially in pericline, each by itself or with 
others, and very frequently twins, which can be recognised 
even in the fracture. The specific gravity of pericline is 
always somewhat under 2*60, that of albite is 2*61—2*62. 
The latter is always more or less transparent; the former, 
on the other hand, is of a milky-white colour. In reference 
to colour and lustre, all that has been said in reference to 
the potash felspars applies here also. 
The chemical constitution is neutral silicate of soda 
and neutral silicate of alumina, combined in equal quanti¬ 
ties (Na Si +A1 Si 3 ), a little lime or potash is, however, 
sometimes substituted for the soda. This is especially the 
case in the vitreous albites of many trachytic and dioritic 
porphyries, as, for example, in the so-called andesine 
from the Cordilleras. Insoluble in acids. Before the 
blowpipe, it melts somewhat more readily than potash 
felspar (4*0—4*5), and colours the flame distinctly yellow; 
if it contains a small proportion of lime, the flame has a 
reddish colour, the splinters give a somewhat blebby, 
transparent pearl. 
The Oligoclase, or Soda-spodumene , of a specific gra¬ 
vity from 2*64—2*69, is also obliquely rhomboidal, of dis¬ 
tinctly foliated fracture, and belongs to the series of 
soda-felspars containing lime and potash. It is found of 
a yellow-green at Bodenmais in Southern Norway, and at 
Stockholm. 
The soda felspars form an element of many granites, 
porphyries, syenites and trachytes, also of many old lavas 
and clink-stones. 
Figs. 6—8.—Porphyry. 
Those volcanic rocks are so called which inclose in a 
primary homogeneous mass crystalline grains of felspar, 
quartz, and the like, and principally enter into the com¬ 
position of the primary, or older stratified rocks, as filling 
up crevices, in layers or eruptive masses. Those are 
called felspar porphyries , the primary mass of which is 
formed by compact felspar, and as these are, for the most 
part, coloured, we again distinguish between green (Fig. 
6), black or brown (Fig. 7), and red (Fig. 8) porphyries. 
