THE MINERALS OF IDAHO 399 



According to Mr. Hewett's interpretation of the occurrence of the 

 mineral, it has formed by replacement of siderite along cracks by- 

 heated solutions which elsewhere deposited zeolites. When first ex- 

 posed the mineral is blood red in color, but upon drying it assumes 

 the usual pitchy appearance and brown color and becomes filled with 

 contraction cracks. 



IDDINGSITE (506a) 



Hydrous silicate of ferric iron, etc. (Mg, Ca)0. Orthorhombic 



Fe 2 3 .3Si0 2 .4H 2 0. 



The mineral iddingsite, long recognized microscopically as an alter- 

 ation product of olivine in igneous rocks, has recently been found to 

 be a well-defined hydrous ferric silicate with definite optical proper- 

 ties. The mineral has been found in the several Idaho localities 

 listed below and probably occurs elsewhere in basaltic or other ex- 

 trusive rocks. 



BOISE COUNTY 



Lindgren states that 10 brown-red products from the decomposition 

 of olivine occur in basalt interbedded with the Payette lake beds on 

 the north side of Moore Creek 1 mile above Idaho City. 



Iddingsite also occurs in specimens of basalt from Warm Springs 

 Creek in the Bear Valley quadrangle, which contain the zeolites, 

 chabazite, and thomsonite, already described. 



OWYHEE COUNTY 



Iddingsite occurs in very typical red-brown grains and patches up 

 to 3 mm. in diameter in specimens of basalt from Bernard's Ferry. 

 It is clearly derived by the alteration of olivine. A sample of pure 

 crystalline iddingsite separated from this rock by the use of heavy 

 solutions and an electromagnet was analyzed with the following 

 results : 



Analysis of iddingsite from Bernard's Ferry 



(E. V. Shannon, analyst) 



Per cent 



Silica (Si0 2 ) 40. 28 



Titanium dioxide (Ti0 2 ) . 12 



Alumina (A1 2 3 ) 3. 16 



Ferric iron (Fe 2 3 ) 29. 76 



Ferrous iron (FeO) 



Lime (CaO) 3. 00 



Magnesia (MgO) 10. 36 



Water (H 2 0) above 110° C 5. 28 



Water (H 2 0) below 110° C 8. 12 



Total 100. 08 



The iddingsite is dark reddish brown in thin section and occurs as 

 pseudomorphs after olivine. The larger grains are completely al- 

 io Waldemar Lindgren. 18th Ann. Rept., U. S. Qeol. Survey, pt. 3, p. 609, 1898. 



