ART. 16 ZEOLITES FROM OREGON HEWETT, SHANNON, GONYER H 



In the specimens at hand the mesolite is confined to on© lot and is 

 neither so abundant nor so varied in habit and associations as pseudo- 

 mesolite. The specimen illustrated in Plate 1 is very similar to the 

 one from wliich material for analysis was taken. In the illustrated 

 specimen the broad crude crystals of calcite are coated with a com- 

 pact layer of the white mesolite about 2 millimeters thick, consisting 

 mainly of fibers with elongation Y and an index of refraction of 

 about 1.520. Mixed with this is more or less material with the 

 optical properties of the pseudomesolite. The central part of the 

 specimen is a cavity loosely filled with free fibers of pseudomesolite 

 with almost zero birefringence, positive elongation, and an index 

 of 1.510. These fibers also have a narrow border of some mineral 

 of high birefringence and higher refractive indices. Much of the 

 mesolite appears to be in polysynthetic twins. 



Other specimens show mesolite in fibrous masses, the fibers up to 

 1 centimeter long, radiating from sharp scaienohedral crystals of 

 calcite, and terminating in free fibrous crystals in the cavities. The 

 fibrous crystals are truncated by a bright basal plane, but it was 

 found impossible to obtain any signals from them on the goniometer. 

 One specimen shows considerable amounts of chabazite, in colorless 

 masses with only a suggestion of crystal form, which is believed to 

 have been deposited later than the mesolite. Adjacent small amyg- 

 dules are filled with granular chabazite and contain none of the 

 fibrous zeolites. Another specimen has the two fibrous zeolites over- 

 lain by a considerable amount of stilbite. In this specimen the zeo- 

 lites preserve molds of rhombohedral calcite crystals 5 centimeters 

 on an edge. 



The mesolite is soluble in 1 : 1 hydrochloric acid and gelatinizes. 



PSEUDOMHSOLITH 



As stated above under the description of mesolite there occurs inti- 

 mately intergrown with that mineral in some specimens a second 

 fibrous zeolite distinguished by distinctly lower birefringence, 

 slightly lower index of refraction, and positive elongation, for which 

 the name pseudomesolite is here used. The two minerals may be 

 readily identified by the difference in index of refraction when 

 immersed in an oil of index 1.515. The difference in specific gravity 

 of the two minerals is sufficient to permit satisfactory separation 

 with heavy solutions, the mesolite being the heavier, owing perhaps 

 to its denser and more compact texture. The birefringence of pseu- 

 domesolite is extremely low and the thinner fibers are almost invisible 

 between crossed nicols. The free fibers loosely occupy cavities in the 

 mesolite specimens, as shown in the specimen illustrated in Plate 1. 

 The pseudomesolite occurs in parallel position as a continuation of 



