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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 41. 



mixture is represented by the mineral barrandite, of wliich two 

 analyses are on record and in both of which the iron phosphate 

 predominates, so that barrandite, instead of being a distinct min- 

 eral species is, in reality, only an aluminous strengite.^ Now, in 

 strengite the axial plane is a (100) and 2E equals 60°, whereas in 

 variscite the axial plane is 6 (010) and 2E is nearly twice as large. 

 The isomorphous admixture of a small amount of variscite in streng- 

 ite would therefore tend to decrease the axial angle of strengite, and 

 for a certain amount of variscite the axial angle becomes zero. A 

 still greater amount of variscite in the mixture will give an increasing 

 axial angle, but with the axial plane now parallel to h (010) as in 

 pure variscite. Since the axial angle of strengite is much smaller 

 than that of variscite, an isomorphous mixture with an axial angle 

 of nearly 0° should contam more strengite than variscite.^ Both 

 the analyses of barrandite show an excess of the strengite molecule, in 

 one in the ratio 4 : 3 and in the second 5:2. In that of the last 

 ratio, it therefore becomes higlily significant to note that Lacroix 

 in describing ^ barrandite says that the optic axes are very close 

 together, so that the value of 2E is very smaU. In the analysis of 

 barrandite given by Lacroix the ratio of FcgOg to AlgOg is about as 

 2| : 1. An approximate calculation of the value of the axial angle 

 of such an isomorphous mixture shows that it should be very small 

 and approximately 10°. 



On the other hand, it must be stated that the density and mean 

 refractive index of barrandite, as given by Lacroix, are not con- 

 sistent with the values calculated from those of variscite and streng- 

 ite, as is sho^vn by the following table : 



Comparison of values for barrandite. 



* Barrandite consists of 5 parts strengite and 2 parts variscite. 



The wide discrepancies in the found and calculated values of the 

 density and refractive index, respectively, are not in accord with 

 the idea of the isomorphous character of barrandite as developed 

 for the optical orientation. 



1 Compare the "Conclusions" of the paper "A study of the rutile group," in Bulletin 509, U. S. 

 Geological Survey, entitled, " Mineralogieal Notes, Series 2." 



2 It is interesting to note that should an isomorphous mixture of variscite and strengite be found (without 

 definite crystals) in such proportions that the axial angle is 0°, the discoverer would be very likely to con- 

 sider the find as a distinct new species, as it would be strictly uniaxial, while both variscite and strengite 

 are biaxial. 



« Lacroix, A. Mineralogie de la France, vol. 4, 1910, p. 4S6. 



