444 A. F, Rogers — Manganese Minerals. 



Description of the Minerals. 



Tephroite. — Tephroite as a megascopic constituent of 

 the ore is rare but in several specimens it was noted as 

 a massive grayish red mineral which was at first thought 

 to be rhodonite. It lacks, however, the perfect cleavage 

 of rhodonite, a hand lens showing but traces of imperfect 

 cleavage surfaces. It also has a faint resinous luster 

 different from the vitreous luster of rhodonite. The 

 specific gravity of an impure specimen was found by a 

 rough determination to be about 3-75, but carefully 

 selected fragments with only traces of visible impurities 

 gave a specific gravity of 4-010 (determined on 1-5 grams 

 by pyknometer at 25 °C). 



The tephroite is very easily soluble in dilute HC1 with 

 gelatinization and the solution gives an abundant test for 

 manganese and faint tests for iron, calcium, and mag- 

 nesium. 



The indices of refraction, determined by the immer- 

 sion method, are greater than 1-740. Thin sections show 

 almost colorless anhedra with interference colors rang- 

 ing up to bluish-green of the second order, which proves 

 that the double refraction is higher than that of rhodo- 

 nite (ny-na = 0-011) but lower than that of olivine (ny- 

 ^a= 0-036). Polysynthetic twinning is a prominent 

 feature in the thin sections, a property not previously 

 reported for tephroite. In some areas the twinning re- 

 sembles the albite twinning of plagioclase, but more often 

 it resembles an intergrowth of two minerals. The twin- 

 ning is recognized largely by differences in interference 

 colors rather than by extinction angles. 



Although not often visible in the hand specimens, 

 tephroite in isolated anhedra surrounded by alteration 

 products is seen in many thin sections. For this reason 

 it was probably present in abundance in one stage of the 

 history of the deposit, but on account of its ready altera- 

 tion only remnants of it are left. 



Hausniannite. — Hausmannite, on the other hand, can 

 be recognized in many of the hand specimens and in some 

 it is by far the most abundant mineral present. It 

 occurs in crystals which vary from euhedral to anhedral, 

 but are usually subhedral. A few small (1 to 2 mm. in 

 size) euhedral crystals were found and prove to have the 

 following forms : (001), (113), (111), (221), with the unit 

 tetragonal bipyramid (111) as the dominant form. The 

 following measurements were made with the reflection 

 goniometer : 



