﻿Penfield and Sperry — Psetidomorjyhs of Garnet. 307 



Art. XXXV. — On Pseud 'omorphs of Garnet from Lake Superior 

 d Salida, Colorado ; by S. L. Penfield and F. L. Sperry. 



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1. Garnets from Lake Superior. 



The decomposed garnets from the Lake Superior region form 

 quite common specimens in the mineralogical collections of this 

 country. A description and microscopical examination of these 

 garnet crystals have been made by Mr. Kaphael Pumpelly.* 

 According to Mr. Pumpelly, " these pseudomorphs of garnets 

 occur in abundance in a bed of chloritic schist just overlying 

 the great magnetite bed of the Spurr-Michigamme iron range." 

 He calls the decomposition product, and also the material in 

 which the crystals are imbedded, chlorite. But he gives no 

 analyses to indicate its relations to the garnet. 



Our examination of these pseudomorph garnets was made 

 on crystals from a suite in the mineralogical collection of Pro- 

 fessor Geo. J. Brush, to whom we take great pleasure in expres- 

 sing our thanks for the material kindly placed at our disposal. 

 The crystals are invariably dodecahedral. Many of them show 

 interesting distorted forms. We first took the specific gravity 

 of twenty of the best crystals, which were found to vary between 

 4*11 and 3*22. As would be expected, a close examination 

 proved the heaviest crystals to be nearly pure garnet, while the 

 lightest ones were almost wholly the decomposition product. 

 This wide variation in specific gravity not only indicates the 

 unequal extent to which the decomposition has gone on, but 

 also guided us in selecting material for analysis. 



In order to study the decomposition of the garnet, an analysis 

 of the pure garnet was made. The material was taken from 

 partially decomposed imbedded crystals. The heavier garnet 

 was obtained, nearly pure, by means of the cadmium-boro- 

 tungstate solution. An examination with the microscope 

 revealed some of the green decomposition-product still adher- 

 ing to and penetrating the garnet. The heavy powder, 

 thus separated, was boiled with sulphuric and hydrochloric 

 acids to decompose the chlorite and then with caustic potash to 

 dissolve any separated silica. The garnet obtained had a light 

 pink color and appeared to be very pure on examination with 

 the microscope. The analysis is given below under I. It will 

 be seen that it is essentially an iron alumina garnet, having 

 this formula — Fe 3 Al 2 Si 3 lo , part of the iron being replaced 

 by Ca, Mg and Mn. The ratio of Si0 2 : A1 2 3 : RO is 3 : -96 : 

 2*87; required, 3:1:3. 



* This Journal, III, x, 17. 



