THE MINERALS OF IDAHO 333 



These results were verified by a partial analysis (water not being 

 determined) of a different somewhat less pure portion of the same 

 specimen of custerite. The results obtained are: SiO , 33.46, CaO 

 53.93, F 7.29, MgO 1.41, magnetite 2.13. Alkalies were not deter- 

 mined on any of the samples because of paucity of material. Some 

 of the whiter chalky-looking material gave less water than the fresJi 

 material, several different samples yielding 2 to 3 per cent H 2 

 instead of the 5 or 6 per cent given in the above analyses. Whethei 

 this represents an alteration of the custerite or merely a much impurer 

 sample could not be determined on the scanty material available. 



The formula derived for custerite is Ca 2 SiHF0 4 with some of the 

 fluorine replaced by water (hydroxyl). The composition may be 

 also expressed as a mixture of two compounds, 2Si0 2 . 4 CaO. 2H 2 

 and 2Si0 2 .4Ca0.4F, with the first one slightly in excess. The 

 relation of fluorine to water (hydroxyl) can be much better shown in 

 the empirical formula according to which the ratios reduce to Ca 4 - 

 Si,0 6 (OH, F) 4 with the ratio of hydroxyl (OH) to fluorine (F) as 

 2.48 : 1.79 or nearly 4 : 3. 



No water was given off when custerite was heated to 110°, indi- 

 cating that the water is an inherent part of the mineral. The 

 temperature at which the water does go off was not determined, but 

 the observation was repeatedly made that the phosphorescence 

 phenomenon displayed itself and was destroyed by heat before the 

 water was given off. Some powdered custerite, placed in a watch 

 glass with several cubic centimeters of water, immediately gives a 

 deep red color with a few drops of phenolpthalein. This reaction was 

 described by Clarke, who has suggested that it is indicative of the 

 presence of the univalent group CaOH. Custerite is related most 

 nearly to the minerals cuspidine, hillebrandite, and zeophyllite. 



CHONDRODITE (415) 



Magnesium fluosilicate, 4Mg0.2Si0 2 .Mg(F, OH) 2 . Monoclinic. 



Chondroclite is a mineral usually occurring in metamorphosed 

 limestone but is characteristic of limestones which have suffered 

 regional metomorphism rather than of the lime-silicate zones of igneous 

 contacts. The only occurrence of this mineral that has thus far 

 been recorded in Idaho is in Custer County, where a lens of chon- 

 droditic limestone was found by Prof. L. G. Westgate in mapping the 

 geology of the Hailey Quadrangle in 1914. 



CUSTER COUNTY 



Chondrodite occurs in abundance in crystalline limestone from a 

 locality near a small lake in a cirque at the head of Wildhorse Canyon 

 at the east side of Mount Ilvndman. The mineral forms rounded 



