582 Rogers — American Occurrence of Periclase. 



colors, a peculiar reddish-brown hue taking the place of 

 the orange and red of the first order. 1 The indices of 

 refraction determined by imbedding fragments in index 

 liquids were found to be n 7 = 1-583 ± -003, and na = 

 1-567 ± -003. The brucite is clearly an alteration product 

 of the periclase. 



Besides calcite, periclase, and brucite, the other min- 

 erals present in the limestone are pyrrhotite, olive-green 

 spinel (n > 1-740), magnetite, antigorite, and a colorless 

 mineral occurring in rounded grains and subhedral crys- 

 tals which is identified as a member of the chondrodite 

 group by the indices of refraction n y = 1-637 ± -003 ; 

 n a = 1-607 ± -003. These maximum and minimum 

 values of the indices of refraction were determined by 

 the immersion method. A few of the chondrodite crys- 

 tals show poly synthetic twin lamellae with a maximum 

 extinction angle of about 30° and this distinguishes it 

 from humite and clinohumite. 



This occurrence is interesting not only on account of 

 the presence of periclase but also because of its bearing 

 on the origin and history of calcite-brucite rocks. 



2. The Origin of Calcite-Brucite Rocks.' ■ 



Calcite-brucite rocks were first described from Pre- 

 dazzo in Austrian Tyrol under the supposition that they 

 represented a distinct mineral with the composition 

 CaC0 3 .Mg(OH) 2 which was called predazzite. Damour 

 showed that the predazzite was a mixture of calcite and 

 brucite. His conclusion was accepted until Lenecek 2 in 

 1891 decided that the mineral associated with the calcite 

 is hydromagnesite instead of brucite and since that time 

 there has been some doubt as to the nature of predazzite. 3 



Besides the Eiverside occurrence the writer has studied 

 two other American occurrences of dedolomitized lime- 

 stones which contain but small amount of silicates and 

 finds brucite to be present in abundance. In one of these 

 occurrences hydromagnesite occurs and the relation of 



1 Weinschenk (Petrographic Methods, translation by Clark, p. 244) 

 speaks of ' ' tombac brown, anomalous interference colors, which indicate 

 a very low double refraction that approximates that of chlorite. ' ' The 

 last part of this statement is incorrect, for brucite has fair maximum 

 double refraction, about 0021. The interference color in a section about 

 0-032 mm. thick (determined by taking the highest interference color of 

 the chondrodite) reaches as high as blue of the second order. 



2 Min. petr. Mitt., 12, 429-442, 447-456, 1891. 



3 Kemp, for example, in the glossary of hi'- Handbook of Eocfcs says, 

 "It is partly calcite and partly brucite or hydi umagnesite. ' ' 



