R. C. Wells — New Occurrence of Hydrogiobertite. 189 



Art. XVII. — A New Occurrence of Hydrogiobertite ; by 

 RoGEK C Wells. 



Some specimens of an evaporation deposit which appear to 

 consist chiefly of hydroo;iobertite were recently examined in 

 the chemical laboratory of the United States Geological Sur- 

 vey. Tliese specimens were collected by Mr. G. A. Waring 

 of the Survey in Chiles Valley, Napa Co., California, near 

 Phillips Springs. The deposit occurs in considerable quanti- 

 ties below the springs and rests upon and penetrates a friable 

 shale. In the vicinity above the springs are serpentinized 

 rocks which may have been the source of the magnesium. In 

 one specimen the incrustation covers fossil wood ; in other 

 specimens the surface of the shale is covered by spherulitic 

 growths of 1-2""" diameter, while the interstices of the shale 

 are completely tilled with the deposit. 



Examined under the microscope, the spherules are seen to 

 possess a banded structure, and to contain small amounts of a 

 white opaque substance. Under these circumstances, an exact 

 optical characterization could not easily be made. The spher- 

 ules are white externally, their broken surfaces grayish-brown. 

 G.240 = 2-152 (Thoulet solution). H.= 3-4. Streak white. The 

 substance effervesces with cold acid. Many of the spherules 

 contain a particle of shale in the center. 



Two samples of the incrustation were analyzed. The first, 

 A, was powdered as submitted, including considerable shale ; 

 the second, B, a small portion, had been picked as free from 

 the shale as possible. The results of two analyses of A and of 

 B are given in the first three columns. In the next two col- 

 umns are given the percentage of MgO, CO^ and H^O, reduced 

 to 100 per cent after subtracting the CO^ equivalent to the 

 CaO found. In the last column is given the theoretical per- 

 centage for hydrogiobertite, 2MgO.CO.,.3H20. 











Mean of 









Al 



A2 



B 



Al and A2 



B 



Theor 



Insohible . 



25-41 



25-33 



14-93 









(Al,Fe),03 



1-85 



1-90 



1-06 









CaO 



2-54 



2-60 



1-84 









Mi?0 







31-81 



36-40 



46-94 



45-80 



44-9 



CO, 



18-08 



18-06 



23-'71 



23-64 



28-00 



24-7 



IV)..-- 



19-81 



20-06 



2081 



29-42 



26-20 



30-4 



99-76 98-75 100-00 100-00 100.0 



The original hydrogiobertite described by Scacchi was found 

 in an igneous rock.* Later, a mineral provisionally designated 

 *E. Scacchi, Rend. Aecad. Sci. Napoli, Dec, 13, 1885. 



