532 



MINERALOGY. 



Oryctogno- Naples ; also in fissures of basalt at Capo di Bove, near 

 ,_^y - ^_^ Rome. It is mentioned also as a production of the 

 "" v ~" Isle of Bourbon. 



Prismato- !! PR1SMATO- PYRAMIDAL FELSPAR OR MEIONITE, 



Pyramidal Jameson. Prismato-Pyramidischer Feldspath, Mohs. 

 l-'slspar. Meionite, Haiiy and Werner. 



Pyramid = 136 22', 63 22'. Cleavage in the di- 

 rection of the diagonals of the prism. Hardness r= 

 5.5. Sp. gr. = 2.5 - 3.7. 



External Characters. Colour greyish-white. Oc- 

 curs sometimes massive, but more frequently crystal- 

 lized. The following are the secondary figures : 



1. Rectangular four-sided prism, flatly acuminated 

 with four planes, which are set on the lateral edges. 



2. The preceding figure, truncated on the lateral edges. 



3. No. 1. bevelled on the lateral edges, and the edges 

 of the bevelment truncated ; and the edges between the 

 acuminating planes and the lateral planes also truncated. 



Externally crystals smooth and splendent, internally 

 splendent and vitreous. Generally transparent or semi- 

 transparent, seldom translucent. 



Chemical Characters. It is easily fusible before the 

 blowpipe ; intumesces during fusion, and is converted 

 into a white vesicular glass. It has not hitherto been 

 analysed. 



Geognostic and Geographic Situations It occurs, 



along with ceylanite and nepheline, in granular lime- 

 stone, at Monte Somma, near Naples. It is said also 

 to occur in basalt, along with augite and leucite, at 

 Capo di Bove, near Rome. 



Prismatic III. PRISMATIC or COMMON FELSPAR, Jameson. 



or Common Prismatischer Feldspath, Mohs. 



Felspar. Prism = 120. The most distinct cleavage is that 



parallel with the terminal planes of the prism ; other 

 two, less distinct, parallel with the lateral planes. 

 Hardness = 6.0. Sp. gr. = 2.52.8. 



This species is divided into nine Subspecies, viz. 

 1. Adularia, 2. Glassy-Felspar, 3. Ice-Spar, 4. Com- 

 mon Felspar, 5. Labrador Felspar, 6. Compact Felspar, 

 7. Clinkstone, 8. Earthy Common Felspar ; and, 9. 

 Porcelain Earth. 



FIRST SUBSPECIES. 



Adularia. ADULARIA, Jameson. Adular, Werner. 



External Characters. Principal colour greenish- 

 white, which sometimes passes into greyish-white and 

 milk-white, and even inclines to asparagus-green. It 

 is frequently iridescent; and the milk-white varieties, 

 in thin plates, when held between the eye and the 

 light, sometimes appear pale flesh-red. Occurs mas- 

 sive, and this variety is composed of granular and thick 

 lamellar concretions ; and frequently crystallized. The 

 following are the most frequent secondary figures : 



1. Oblique four-sided prism, flatly bevelled on the 

 extremities, and the bevelling planes set on the obtuse 

 lateral edges. 2. Broad six-sided prism, flatly be- 

 velled on both extremities, and the bevelling planes 

 set on the lateral edges, which are formed by the 

 smaller lateral planes. 3. Rectangular four-sided prism, 

 in which the terminal planes are obliquely bevelled. 



Lateral planes of the prism longitudinally streaked. 

 Externally splendent ; internally cleavage splendent, 

 and the fracture shining and glistening. Lustre inter- 

 mediate between vitreous and pearly. Semi-transpa- 

 rent, sometimes inclining to transparent, or is translu- 

 cent. Translucent varieties, when viewed in a certain 

 direction, sometimes exhibit a silvery or pearly light. 

 Refracts double. 



Chemical Characters. It melts before the blowpipe, 

 without addition, into a white-coloured transparent glass. 



Constituent Paris. Silica, 6* 

 Alumina, 20 

 Lime, 2 



Potash, 14 100 Vanquelin. 



Geognostic Situation. It occurs in contemporaneous 

 veins in drusy cavities in granite and gneiss. 



Geographic Situation It occurs in the granite of 

 the island of Arran ; and in the granite and gneiss 

 rocks of Norway, Switzerland, France, and Germany. 



Uses The variety of adularia which exhibits the 

 bluish pearly light, is valued by jewellers, and is sold 

 by them under the name Moonstone. Another variety 

 of adularia, found in Siberia, is known to jewellers 

 under the name Sunstone. It is of a yellowish-grey 

 colour, and numberless golden spots appear distributed 

 throughout its whole substance. 



SECOND SUBSPECIES. 



GLASSY FELSPAR, Jameson. Glasiger Feldspath, Glassy Fel- 

 Werner. s P ar - 



External Characters. Colour greyish-white, some- 

 times passing into grey. Occurs always crystallized, 

 in broad rectangular four-sided prisms, bevelled on 

 the extremities. These crystals are often very much 

 cracked ; they are generally small, seldom middle-sized, 

 and always imbedded. Internally splendent, and lus- 

 tre vitreous. Fracture uneven, or small and imperfect 

 conchoidal. Transparent. 



Constituent Parts. Silica, . 68.0 



Alumina, . 15.0 



Potash, . 14.5 



Oxide of iron, 0.5 98.0 



Klaproth, Beit. b. v. s. 1 8. 



Gcognoslic and Geographic Situations. It occurs 

 imbedded in pitchstone-porphyry in Arran and Rum ; 

 in a porphyritic rock in the Siebengebirge; also in a 

 rock composed of white felspar, and very small black- 

 ish-brown scales of mica, and fine disseminated mag- 

 netic ironstone, in the Drachenfels on the Rhine. 

 THIRD SUBSPECIES. 



IcE-SpAR, Jameson. Eispath, Werner. Ice-spar. 



External Characters Colour white. Occurs mas- 

 sive, cellular, and porous ; also in large granular con- 

 cretions, which are composed of thin and straight 

 lamellar concretions. Frequently crystallized in the 

 form of small thin longish six-sided tables, in which 

 the shorter terminal planes are bevelled. Externally 

 crystals shining, and sometimes splendent ; internally 

 shining, and lustre vitreous. Massive and other varie- 

 ties strongly translucent ; crystals transparent. 



Geognoslic and Geographic Situations. It occurs, 

 along with nepheline, meionite, mica, and hornblende, 

 at Monte Somma, near Naples. 



FOURTH SUBSPECIES. 



COMMON FELSPAR, Jameson. Frischer Gemeiner Common 

 Feldspath, Werner. Felspar. 



External Characters. Most frequent colours white 

 and red, seldom grey, and rarely green and blue. Oc- 

 curs most frequently massive and disseminated, seldom 

 in blunt angular rolled pieces and grains, and frequent- 

 ly in granular distinct concretions, from the smallest 

 to the largest size ; and sometimes crystallized, in the 

 same form nearly as adularia. Internally cleavage 

 shining, and sometimes splendent. Fracture glisten- 

 ing, and frequently not more than feebly glistening. 

 Lustre intermediate between vitreous and pearly, but 

 inclining rather more to the former than to the latter. 

 Fracture uneven or splintery. Fragments rhomboidal, 

 and have only four splendent shining faces. Translu- 

 cent, or only translucent on the edges. 



