A. J. Moses — Neio Mercury Minerals from Texas. 253 



Art. XXIY. — Eglestonite^ Terlinqiiaite and Montr oydite^ New 

 Mercury Minerals from Terlingua, Texas; by Alfred J. 



Moses. 



1. Eglestonite, an Isometric Oxy chloride of Mercury. 



The most abundant material in the specimens received from 

 Mr. 'B. F. Hill so resembled minute crystals of sphalerite that 

 the first test made was for a zinc coating. The material 

 occurs, so far as observed, only as crystals which rarely exceed 

 one millimeter in diameter and are sometimes isolated and at 

 other times united loosely in a crust which readily crumbles 

 under pressure into separate well developed crystals, evidently 

 isometric and with the dodecahedron the predominating form. 

 The associated minerals are the later described terlinguaite 

 and montroydite, calomel, native mercury and calcite. 



Crystalline Form of Eglestonite. — 

 The crystals are usualty sharply and 

 beautifully developed, but in some speci- 

 mens are pitted and the cavities tilled 

 with metallic mercury. The system is 

 isometric and the class hexoctahedral. 

 With the two-circle goniometer the forms 

 identified were « J 100 1 ; a 1 11 ; n \ 112 \ 



and s 1123? as shown 



m 



fio'. 1. The 



dodecahedral planes are the largest. 



The following table gives the angles 

 obtained from twenty-nine faces on one-half of a crystal, one 

 mm. in diameter. The calculated angles are also given. 



"I 



d 





Faces 





9 





P 





Eeflect- 

 ing. 











Form. 



Measured. 



Calculated. 



Measured. 



Calculated. 



i 001 



1 







0° 





( 010 



4 



0° 





90° 





\ 110 



4 



45° Of 



45° • 



90° 





( Oil 



4 



0° ^ 



0° 



44° 58' 



45° 



i 112 



4 



45° 02' 



45° 



35° 17' 



35° 16' 



(121 



7 



26° 30' 



26° 34' 



65° 50' 



65° 54' 



123 



2 



26° 32' 



26° 34' 



36° 45i' 



36° 42' 



132 



2 



18° 26^' 



18° 26' 



57° 40' 



57° 41' 



231 



1 



33° 35' 



33° 41' 



74° 22' 



74° 30' 



Chemical Ancdysis of Eglestonite. — The analyses here 

 recorded were made by Mr. J. S. McCord, Assistant in Miner- 

 alogy at Columbia University. The method used was chosen 

 after an attempt to obtain an electrolytic determination of the 

 mercury by dissolving -2465 gms. of very carefully picked mate- 

 rial in nitro-hydrochloric acid, precipitating as sulphide and 

 dissolving in hot solution of sodium sulphide. This solution 



