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BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



thin section between crossed nicols, and the refractive index ranges 

 from 1.82 to 1.84. The most characteristic form of garnet in this 

 district is as a dense aphanitic rock of greasy to vitreous luster. In 

 many places in the workings along the Alberta level this type of rock 

 is continuous for scores of feet. In many thin sections nothing but 

 garnet and a chance grain or patch of residual calcite appear. 



Analyses of this massive garnet show the presence of from 35 to 

 45 per cent of the grossularite molecule. In all types of garnet in 

 the deposits the andradite molecule is an important constituent. In 

 the massive garnet it comprises about 50 per cent of the whole and 

 in the dark amber-colored garnet which formed subsequently it is 

 €ven more abundant. From the color of the garnet it seems possible 

 as a rule to tell whether it is grossularite or andradite. Both of these 

 varieties, however, as determined by index solutions, may be so nearly 

 the same shade of amber color that no difference between them can 

 be detected. The following table gives typical analyses of garnet 

 from the ores of the Alder Creek district: 



Analyses of garnet from, Mackay 



(l) 



(2) 



(3) 



Silica (SiOz) 



Alumina(Al2C>3) 



Ferric iron (Fe203) 



Ferrous iron (FeO) 



Magnesia (MgO) 



Lime(CaO)._._ 



Soda (Na 2 0) 



Potash (KoO) 



Water (H 2 0) above 110° C 

 Water (H2O) below 110° C 



Titanium oxide (TiOz) 



Carbon dioxide (CO2) 



Phosphoric acid (P2O;) 



Manganese ox ; de (MnO) . . 



Total 



Specific gravity (lump) 



36. 92 

 8.75 



16.85 

 .50 

 .17 



33.71 

 .31 

 .21 

 .39 

 .21 

 .26 

 .95 

 .30 

 .67 



36.57 

 7.56 



20.34 

 1.24 



37.07 

 17.42 

 10.81 



100. 20 



99.79 



3.647 



3.664 



(1) Massive garnet from 100 feet south of No. 5 crosscut, Alberta tunnel. Index of refraction 1.82-1.83. 

 Maximum birefringence 0.01 ± 0.003. Chase Palmer analyst. 26 



(2) Dark amber-colored garnet crystals from 1.500 feet northwest of the Copper Bullion tunnel. Re- 

 fractive index 1.840 ± 0.003. Chase Palmer, analyst. 29 



(3) Light amber-colored crystals. Cyril Knight, analyst. 26 



The garnet in the Mackay district is not confined to the lime- 

 stone but large masses have developed endomorphically in the granite 

 porphyry. Comparison of analyses of the garnet rock with those of 

 the unaltered limestone indicates that great quantities of material 

 were added, presumably by solutions emanating from the magma. 27 



A number of specimens of garnet from these deposits have been 

 examined, although none of them have been studied in detail by 

 the writer. A specimen from the mouth of the No. 8 crosscut, 



26 J. F. Kemp and C. G. Gunther. Trans. Amer. Inst. Mining Eng., vol. 38, p. 286, 1908. 



27 J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 58, etc., 1917. The above notes on the garnet 

 of the Alder Creek district are all abstracted from Umpleby's description. 



