Rogers — American Occurrence of Periclase. 583 



the brucite to the hydromagnesite could be determined. 

 Fig. 2 shows the general character of the calcite-brucite 

 rocks in thin sections. 



3. Calcite-Brucite Rock from Crestmore, California. 



A white crystalline limestone occurring in contact with 

 granodiorite at the Chino Hill quarry of the Eiverside 

 Portland Cement Company at Crestmore, about eight 

 miles from Riverside, California, contains a pale pinkish 

 gray mineral which has been identified as brucite by 

 Eakle. 4 The brucite occurs in crystalline aggregates like 

 those in the Eiverside rock just described. The indices 

 of refraction were found to be : n y = 1-583 ± -003 ; 

 na = 1-563 ± -003. The brucite is evidently an alteration 

 product of periclase though no trace of the latter mineral 

 was found. The form of the original periclase has been 

 preserved as rough equant crystals, which are oscillatory 

 combinations of the dodecahedron and octahedron, in 

 habit much like a diamond crystal illustrated by Fers- 

 mann and Golds chmidt. 5 



The limestone also contains small amounts of minute 

 colorless, rounded subhedral crystals of chondrodite. 6 

 They were isolated by dissolving the rock in dilute hydro- 

 chloric acid and were identified by the following indices 

 of refraction: n y = 1-643 ± -003; na =1-613 ± -003. 



The residue from the ' hydrochloric acid solution also 

 contains a colorless, optically isotropic mineral in the 

 form of rounded equant subhedral crystals which is 

 probably spinel (n = 1-715 ± -005 ) 7 and a few subhedral 

 prismatic crystals of apatite as a nitric acid solution of 

 the rock gives a faint test for the phosphate radical. 

 This mineral is not wilkeite, 8 which is found in the adjoin- 

 ing quarry for no sulphate test was obtained. 



On the exterior of some of the limestone specimens the 

 pinkish gray brucite gives place to an opaque white 

 mineral which is identified as hydromagnesite. It is 



4 Eakle, University of California Publications, Bull. Dept. GeoL, vol. 

 10, pp. 327-360, 1917. The analysis gives MgO 67-48, Fe»0, 0-55, H,0 

 31-73 = 99-76. 



3 Der Diamant, Atlas, Taf. 14, fig. 98, Heidelberg, 1911. 



Eakle (loc. cit. p. 333) includes chondrodite in his list of minerals 

 from this locality, but says that there is no well authenticated proof of its 

 existence in the quarries. 



7 Rankin and Merwin (Jour Am. Chem. Soc, 38, 512, 1916) find the 

 index of refraction of pure spinel (MgAl,0 4 ) to be 1-718 ± -002. 



s Eakle and Eogers, this Journal, 37, 262-267, 1914. 



Am. Jour. Sci. — Fourth Series, Vol. XLVI, No. 274. — October, 1918. 



