ART. 16 ZEOLITES FEOM OREGON" HEWETT, SHANNON", GONYER 13 



THOMSONITB 



The most abundant and, in some respects the most attractive 

 mineral from the locality is thomsonite. This zeolite, comparatively 

 imcommon else-where in the United States, makes up the larger part of 

 the specimens collected, and in form and appearance is somewhat 

 unlike the thomsonite from any other American locality. Its char- 

 acteristic mode of occurrence here is as the lining of amygdaloid cavi- 

 ties, often several inches across, as shown in Plate Ic. The lining 

 itself is seldom more than 4 millimeters thick and consists of radiating 

 blades of snowy color and pearly luster, which terminate in the sur- 

 face of the crust as chisel-shaped groups of dull, lusterless knife edges, 

 without definite crystal form and often soiled and dirty. 



The thomsonite is usually a more or less solitary mineral. It rests 

 upon a very thin skin of a whitish zeolite described under heulandite. 

 Very rarely there rest upon the thomsonite fine cottony tufts of fibers 

 of pseudomesolite. In one set of specimens the thomsonite layer rests 

 on a thin layer of chabazite and is overlain by crystals of stilbite, 



Tlie matrix of the thomsonite specimens is a slaggy basaltic lava 

 which, as already stated, is highly porous and vesicular. The smaller 

 vesicles are lined only with a thin slrin of supposed heulandite although 

 an occasional one is filled solidly with thomsonite blades. Much of 

 the matrix basalt is altered to greenish clayey material which is quite 

 soft when moist. 



The specimen which was analyzed was purified as far as possible 

 by cracking up and hand picking the pearly fragments of the crust. 

 The fragments were then crushed and screened between 60 and 100 

 mesh sieves, and run several times through a bromoform-carbon tetra- 

 chloride heavy solution. The resultant sample was apj)arently pure 

 and all of the same specific gravity. The analysis gave the follow- 

 ing results : 



Analysis of thomsonite 



Constituent: Percent Ratios 



Si02 37.84 0.630 0.630 0.630 0.315X2 1.01X2 



AI2O3 31.72 .311 .311 .311 .311X1 1. OOXl 



CaO 12.20 .2181 ^qqi 



MgO .84 .021/ •^•^y 



NazO 4.08 .0661 „^„ • ^^^ .316X1 1.00X1 



K2O .96 .011/ -^^^^ 



H20abovellO°C. 12. 561 ^26 726 726 /• 290X| . 93X- 



K2O .96 .011/ 



H20abovellO°C. 12. 561 ^or 



H2O below 1 10° C. .52/ ' '^^ • '^" • '^" 1.363X2 1.17X2 



100. 72 



The results agree very well with the formula for thomsonite given 

 by Dana, which is (Na2,Ca)O.Al203.2Si02.2y2HoO., the ratio 

 Ca : Nao being 3 : 1. The composition calculated from this formula 

 is: Sid,,37.0; ALO3, 31.4; CaO, 12.9; NaA 4.8; H.O, 13.9 = 100.0.' 



