290 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



SHOSHONE COUNTY 



A single peculiar occurrence of tremolite which has been deter- 

 mined in Shoshone County deserves record, although the material 

 is not of specimen value. The specimens are labeled as from the 

 dump of a shaft near the head of Big Creek and Silver Creek. They 

 consist of quartz containing large rhombic molds from the removal 

 of some rhombohedral carbonate, probably ankerite. These cavi- 

 ties are lined with a layer up to 3 mm. thick, of ocher-brown material 

 with a faintly fibrous structure and silky luster. Under the micro- 

 scope this is found to be a mixture of a bladed colorless mineral 

 with fine limonite. In hot 1 :1 hydrochloric acid the limonite is 

 dissolved while the bladed mineral is undissolved. The optical 

 properties of the blades are: Biaxial, negative, 2V medium large, 

 extinction 15°-17°, 13= 1.623. These optical properties identify 

 the mineral as tremolite. This amphibole was probably included 

 in the original carbonate which has been removed by weathering. 



ACTINOLITE 



Lime-magnesia-iron silicate, Monoclinic. 



Ca0.3(Mg,Fe)0.4Si0 2 . 



Actinolite differs from tremolite principally in containing more 

 iron. It is usually in fibrous or acicular aggregates and is commonly 

 deep green in color. The following occurrences have been noted in 

 Idaho. 



ADAMS COUNTY 



Actinolite is listed by Livingston and Laney 9 as an abundant and 

 widespread contact-metamorphic mineral in the Seven Devils region. 



CUSTER COUNTY 



Acicular actinolite occurs sparingly in the contact silicate rocks of 

 the Alder Creek district. Its most common occurrence is as finely 

 divided crystals in the white marble. 10 The mineral occurs also in 

 veins up to 2 inches wide consisting of fibrous or acicular crystals in 

 apparently unchanged blue limestone. 11 



IDAHO COUNTY 



A small amount of green actinolite was noted in association with 

 abundant anthophyllite in a specimen described above which was 

 labeled as from Orofino, but which probably came from one of the 

 Kamiah asbestus prospects. 



• D. C. Livingston and F. B. Laney. Idaho Bur. Mines and Geol., Bull. 1, p. 62, 1920. 



w J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 49, 1917. 



ii J. F. Kemp and G. C. Gunther. Trans. Amer. Inst. Mining Eng., vol. 38, p. 269, 1908. 



