344 



DESCRIPTIVE MINERALOGY. 



MESOTYPE. 



Zeolite. Cleavelani. — Prismatic Zeolite, or Mesotype. Jameson. — Prismatischer Euphon-Spath. Moh. — 

 M^otype. Haiiy, Beudanl, Shepard and Dana. — MesoUte. T%omson. 



Description. Colour white, red, yellow and j'ellowish brown. It 

 occurs crystallized, also pulverulent and fibrous. Primary form a right 

 rhombic prism. Fig. 326. M on M' 91° 10' {Brooke), 91° 40' {Beu- 

 dant). Cleavage parallel with M, perfect. Fracture uneven, in some 

 varieties coarse earthy. Lustre vitreous. Varies from nearly transpa- 

 rent to opaque. Brittle. Hardness from 5.0 to 5. 5. Specific gravity 

 from 2.14 to 2.23. Before the blowpipe, it froths, gives a phosphorescent light, and melts 

 into a white semi-transparent enamel ; with borax it fuses with difficulty into a transparent 

 colourless glass. It is soluble in, and forms a thick jelly with, acids. It becomes electric by 

 heat. 



Its power of forming a jelly with acids, and of becoming electric by heat, will distinguish 

 mesotype from several minerals which it resembles. Prehnite is harder, and does not form a 

 jeUy with acids. 



Varieties. The name Natrolite is applied to the yellow and brown varieties, in which the 

 fibrous and granular concretions are intersected by laminas. The fibrous variety consists of 

 Boinute crystals aggregated in a radiating or stellular form. When it is very soft, and has an 

 ejirthy fracture, it is sometimes called Pearly zeolite. 



Composition. There is some diversity in the analyses of this mineral. According to Smith- 

 son, a specimen from the Faroe islands contained silica 49.00, alumina 27.00, soda 17,00, 

 water 9 . 60. 



Geological Situation. It occurs in cavities and veins in greenstone and trap, and also in 

 the gneiss bordering on these rocks. 



Fig. 327. 



Fig. 328. 



M 



M 



m' 



localities. 



Fig. 329. 



New- York County. Minute but 

 well defined crystals of mesotype are 

 found in veins in gneiss at Harlem, 

 where it is associated with apophyllite 

 and stilbite. Figs. 327, 328 and 329 

 represent the forms which have thus 

 been observed. M on o 116° 32'; 

 o on 144° 16'. 



This mineral is said also, by Dr. Gale, to ocgur in the greenstone bowlders on the island. 



