THE CHEMICAL COMPOSITION OF HYDROTALCITE 

 AND THE HYDROTALCITE GROUP OF MINERALS. 



By William F. Foshag, 

 Of the Department of Oeology, United States National Museum. 



The hydrotalcite group as here considered comprises the minerals 

 hydrotalcite, pyroaurite, stichtite, and brugnatellite. Of these hydro- 

 talcite and pyroaurite have been long laiown but their true chemical 

 nature remained hidden. They have been considered hydrated oxides, 

 a considerable and rather constant carbon dioxide content having been 

 entirely disregarded. New analyses of hydrotalcite and stichtite are 

 here presented. Further analyses are necessary before the chemical 

 composition of this group can be definitely established. Before any 

 definite conclusions can be drawn from any analysis, however, the 

 homogeneity of the material, as determined under the petrographi- 

 cal microscope, must be established. 



HYDROTALCITE. 



HocHSTETTEE, Journ. Prakt. Chem., vol. 27, p. 378, 1842. 

 Hermann, Journ. Prakt. Chem., vol. 40, p. 11, 1847. 

 Hermann, Journ. Prakt. Chem., vol. 46, p. 257, 1849. 

 Shepard, Amer. Journ. Sci., vol. 12, p. 209, 1851. 

 Johnson, Amer. Journ. Sei., vol. 12, p. 361, 1851. 



Hydrotalcite was first described by Hochstetter from the Shi- 

 shimsk District in the Urals, where it occurred implanted on schist. 

 It was later described from Snarum, Norway, under the name volk- 

 nerite. Hydrotalcite from Kongsberg, Norway (U. S. Nat. Mus., 

 No. 13191), was selected for analysis. The material consisted of 

 curved lamellar masses of a white color and pearly luster. Under the 

 microscope the material is made up of basal cleavages with refractive 

 index about 1.510. Sections normal to the basal cleavage showed a 

 birefringence of low order. The material was very pure, only a few 

 grains of foreign material appearing. The results of an analysis on 

 this material and another partial analysis, together with the calcu- 

 lated ratios, are given in the following table : 



Proceedings U. S. National Museum, Vol. 58— No. 2329. 



