﻿308 Penfeld and Sperry — Pseudomorphs of Garnet. 



The analysis of the decomposition product was made from a 

 selected fragmentary crystal having a specific gravity of 3*281. 

 It was part of a large dodecahedron. Along the fracture it 

 showed numerous little octahedrons of magnetite and a foliated 

 structure of the chloritic material. A microscopical examina- 

 tion revealed the presence of a very little unaltered garnet. A 

 mechanical analysis showed that there were 3*23 per cent of 

 unaltered garnet and 3*28 per cent of magnetite in this decom- 

 posed product. The magnetite was carefully extracted with a 

 strong magnet, and the garnet was obtained by a process simi- 

 lar to that mentioned above in the preparation of the pure 

 garnet. The analysis is given below under II after deducting 

 3 '23 per cent of garnet. A second decomposed crystal having 

 a specific gravity of 3*22 was also analyzed. The material was 

 obtained by powdering the crystal and sifting through a sixty- 

 mesh sieve. The material was then suspended in water. The 

 garnet was allowed to settle out and the light chlorite scales 

 decanted off and collected in a platinum dish. The material 

 was obtained in a nearly pure state. The analysis is given 

 under III after deducting *46 per cent of garnet. 



Analyses of garnet and decomposition product from Lake 

 Superior. 



II. III. 



I. 



Garnet. Decomposition products. 



Si0 2 = 38-03 per cent. 27'45 per cent. 29'08 per cent. 



A1 2 3 = 20-83 19-53 19-94 



Fe 9 3 = 6-26 3-91 



FeO = 36-15 29-42 30'48 



MnO = 2-14 -20 



MgO = -97 6-04 



O'O 1 



CaO = 2-73 -25 



Ka 2 = .... '42 -29 



K 2 = 2-64 3-66 



H 2 = 7-50 6*53 



100-85 99*26 99'90 



No. II had specific gravity of 3*21, calculating it free from 

 garnet and magnetite. 



On comparing the above analyses, it will be seen that the 

 following changes have taken place in the garnet: a slight 

 oxidation of the iron, a decrease of nearly 10 per cent in silica, 

 an almost total disappearance of manganese and calcium, and 

 an increase in magnesium, alkalies and water. With the 

 exception of the silica and water the change has not been very 

 great. As regards the nature of the decomposition product, 



