14 PEOCBEDINGS OF THE NATIONAL MUSEUM vol.73 



There has been considerable discussion for some time regarding the 

 composition of thomsonite.^ It was hoped that the present material 

 might throw some new light upon this problem, but optical examina- 

 tion proved the sample analyzed to have little value for this purpose. 



Under the microscope the grains of the sample all lie on a perfect 

 cleavage plane and show two other cleavages at right angles to this 

 and to each other. Dana makes the perfect cleavage of thomsonite 

 &(010) with <z(100) less perfect and c{001) in traces. The elonga- 

 tion is parallel to the c axis. All of the grains lying on the perfect 

 (010) cleavage show an excellent biaxial positive interference figure, 

 with the axial plane across the laths or parallel to the plane c(OOl), 

 and the acute bisectrix Z perpendicular to (010). In an oil of index 

 1.525, however, the sample is found to be slightly inhomogeneous in 

 index. A part of the grains lying on (010) cleavage have a small 

 axial angle and strong dispersion r<v with the refractive indices: 

 a= 1.522, /?= 1.524. Another part of the grains has a little higher 

 birefringence when lying on this plane, slightly larger axial angle, 

 and the indices: a= 1.526, /3= 1.529, y= 1.533. The dispersion and 

 optical orientation are the same in both and it is evident that the 

 sample consists of two varieties of thomsonite differing slightly in 

 composition. There seems, however, to be no graduation in indices 

 between these two and they are so similar in specific gravity that 

 repeated attempts to separate them by the use of heavy solutions 

 were unsuccessful. Most of the free grains were composed of a 

 single one of the varieties but a coarser sample which was examined 

 showed an occasional core of the variety of lowest indices bordered 

 by later growths of the material of higher indices. 



Aggregates of plates occasionally lie on the less perfect <2(100) 

 cleavage. These are groups of slightly divergent individuals grown 

 together approximately parallel to (010). They show brilliant in- 

 terference colors and essentially parallel extinction. Such groups 

 give the gamma (y) index across the length and show the emergence 

 of the obtuse bisectrix. They also show negative elongation, whereas 

 the elongation of blades lying on (010) is positive. The laths may 

 be rolled between crossed nicols and the change of elongation ob- 

 served with the gypsum plate. 



The collection contains one thomsonite specimen of somewhat dif- 

 ferent type. This is an irregular mass which represents a completely 



• S. G. Gordon. Calciotbomsonite from Franklin, N. J. Proc. Acad. Nat. Sci. Phila., 

 vol. 75, pp. 273-274, 1923. 



E. T. Wherry. Note on the composition of thomsonite. Amer. Min., vol. 8, p. 121, 

 1923. 



S. G. Gordon. The composition of thomsonite. Proc. Acad. Nat. Sci. Phila., vol. 76, 

 pp. 103-107, 1924. 



A. N. Winchell. The composition of thomsonite. Amer. Mineralogist, vol. 10, pp. 

 90-97, 1925. 



