THE MINERALS OF IDAHO 331 



KOOTENAI COUNTY 



A specimen from Post Falls contains prehnite associated with 

 stilbite, laumontite, etc., in a seam in a diabasic rock. The prehnite 

 forms minute tabular crystals lining cavities as well as white to 

 greenish granular masses. Optically the mineral of this occurrence 

 is biaxial positive with 2V approximately 60°. The acute bisectrix 

 is perpendicular to the tabular face of the crystals. The dispersion 

 r<v is weak and the anomalous twinned structure and unusual 

 optical effects often exhibited by this mineral were not noted. The 

 refractive indices are a = 1.614, /3 = 1.624, y = 1.640. 



The specimen, which was received from Mr. Henry Fair, of Spo- 

 kane, is from a claim upon which the owner has pretended to have 

 found rubies and diamonds, arousing local interest. 



CUSTERITE (413a) 



■Calcium fluosilicate, CaO.Ca(OH,F) 2 .Si0 2 Monoclinic. 



Some specimens collected by Dr. Joseph B. Umpleby from the 

 metamorphosed limestone of the Alder Creek (Mackay) district were 

 found, upon laboratory investigation, to be a new mineral. This 

 was named custerite after the county in which it occurs. 57 



CUSTER COUNTY 



The custerite occurs in finely granular masses of a dirty greenish- 

 gray to white color, intimately mixed with magnetite, in a group of 

 prospects a short distance beyond the crest of the high ridge north- 

 west of the Copper Bullion tunnel. It occurs in the inner edge of a 

 fringe of garnet-diopside rock which surrounds a large limestone 

 inclusion in the granite porphyry. 58 The mineral is fairly abundant 

 and specimens of the usual size can be obtained showing about 

 equal parts of custerite and magnetite (as shown in pi. 8, lower) . 



Schaller 59 has discussed the composition, properties, and relation- 

 ships of the mineral and his discussion is given below. 



The results of microscopic study indicate that custerite is mono- 

 clinic. Two cleavages were noted, one parallel to the base and the 

 other parallel to the prism. The hardness is 5 to 6, density 2.91. 

 The crystals show lamellar twinning like albite. The indices of 

 refraction for sodium light are a = 1.586, j8 = 1.589, 7=1.598, bire- 

 fringence 0.012, all ±0.005. The mineral is characterized, micro- 

 scopically, by its moderate index of refraction, low birefringence, 

 polysynthetic twinning, maximum extinction of twin lamellae of 

 6° to 7°, positive optical character, distinct dispersion of the optic 

 axes with r>v and three cleavages which intersect at high angles. 



« J. B. Umpleby, W. T. Schaller, and E. S. Larsen. Amer. Journ. Sci., vol. 36, p. 385, 1913. 

 «8 J. B. Umpleby. U. S. Oeol. Survey, Prof. Paper 97, p. 52, 1917. 

 59 Waldemar T. Schaller. U. S. Geol. Survey, Bull. 610, p. 152, 1918. 



