Thomsonite

Occurrence
Thomsonite has been found at Franklin only within the last few years and was described by Gordon (227) as calciothomsonite. Radial aggregates of fine needles are associated with garnet, axinite barite, and datolite and form a fanlike radiation on the face of the specimen, measuring 4 inches from tip to circumference. The specific gravity is 2.45. Thomsonite is optically positive; 2V = 48° ; a = 1.530, b = 1.532, g = 1.542, all ±0.001 (measured in mercury light).

Composition
The following analysis shows the composition of calciothomsonite:

***Analysis of calciothomsonite***
(J. E. Whitfield (227), analyst)
	Percent	Molecular ratio
SiO₂	36.44	0.607	= 4 x 0.150
Al₂O₃	30.34	0.298	= 2 x 0.149
CaO	15.94	0.284*	0.343 = 2.3 x 0.150
MgO	Trace
Na₂O	3.50	0.056*
K₂O	0.18	0.002*
H₂O +	13.26	0.751	= 5 x 0.150
H₂O -	0.26

	99.92

[* Figures reflected in 0.343 value shown.]

Gordon suggests that the varieties of the thomsonite series are best explained as consisting of mixed crystals, with the end members: H₆CaAl₂Si₂O₁₁ (calciothomsonite) and H₂Na₂Al₂Si₃O₁₁ (natrolite—H₂O). The sodium end member can enter the mixed crystal only to a maximum of about 50 percent.


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	This page created: August 12, 2006 7:17 PM