398 ( 1 larhe — Pseudo-Serpentine from Washington. 



colorless, but between crossed nicols it exhibits brilliant colors. 

 The mineral b is pale green, and intermingled with a. Its 

 birefringence yields weak colors, quite characteristic of 

 chlorite. In quantity it is less than one-fourth of a. 



Mineral c is granular, in scattered grains and irregular groups 

 with a high index of refraction and a birefringence which sug- 

 gests a carbonate, but it does not effervesce with a dilute acid. 

 It amounts to not more than 5 per cent of the whole. Mineral 

 b is certainly chlorite, and a may be brucite or possibly ser- 

 pentine. 



By applying Mr. Diller's observations to the analysis of the 

 rock, the proximate composition of the latter may be deduced ; 

 although certain assumptions must be made. The carbonate 

 present is probably hydromagnesite, for that species is a fre- 

 quent associate of brucite. The composition of the chlorite 

 is unknown, but it may be interpreted as essentially clinochlore, 

 and proportional to the ferric oxide and alumina. So much 

 assumed, the analysis gives the following approximate results 

 expressing the composition of the specimen : 



Hydromagnesite 5*0 Extraneous water 1*0 



Chlorite 14-0 



Serpentine 20*0 100*0 



Brucite 60*0 



In this, four minerals appear instead of three, but the micro- 

 scopic examination did not attempt to discriminate between the 

 brucite and the serpentine. 



In order to obtain evidence confirmatory of the foregoing 

 conclusions, a few experiments were made, tending towards 

 fractional determinations. Upon digesting the powdered rock 

 for two hours with cold, dilute nitric acid (10 per cent by 

 volume), 1-32 per cent of Fe 2 3 + A1 2 3 and 47*29 percent 

 MgO went into solution. In a similar experiment with cold, 

 20 per cent acetic acid, 0*69 Fe 2 8 ,+ A1 2 3 and 45*64 MgO 

 were extracted. Brucite dissolves readily in acids of the indi- 

 cated strength, but some chlorite was evidently attacked as 

 well. I also found that ordinary serpentine was quite appreci- 

 ably acted upon by weak acetic acid. These experiments then 

 merely show that the rock contains a large amount of magne- 

 sium in a very easily soluble condition, the quantity equivalent 

 to 60 per cent of brucite being 41*4. The results are in 

 harmony with the conclusions already reached, and help to sup- 

 port them, although accurate fractional determinations can not 

 be made. The rock is unusual in character, and if the sample 

 examined is fairly characteristic of the entire deposit, the latter 

 should be carefully studied in reference to its origin and its 

 geological relations. 



U. S. Geological Survey, Washington, D. C. 



