﻿Penfidd and &perrtf — Pseudomorphs of Garnet. 309 



it is not an ordinary magnesium chlorite, such as ripidolite or 

 penninite, but is essentially a ferrous chlorite, closely related to 

 prochlorite, or more nearly to Sandberger's aphrosiderite. We 

 find by calculating the ratios of analyses II and III that the 

 elements are united in the following proportions: 



SiO, : R 8 3 : RO : H 2 



II = 4 ' : 1 : 97 : 5-11: 3*49 

 III = 4 : 1-80 : 5-06: 3-00 or nearly 

 4:2- : 5- : 3- 



Groth* gives the ratio of aphrosiderite as 



SiO g : R 2 3 : Ro : H 2 

 4 : 2 : 6 : 5 



But analyses from different localities vary to quite a large 

 extent, and Sandberger's analysis shows less water than is 

 required by Groth's formula. A little admixture of ripidolite 

 having a ratio of 4 : 2 : : 6*66 : 6f would materially alter the 

 proportion of RO : H 2 in aphrosiderite, and a little admixture 

 of free silica would account for the excess of Si0 2 . It is not 

 probable that the potash belongs to muscovite, which is 

 mechanically mixed with the aphrosiderite. In the first place, 

 a microscopical examination reveals no muscovite, and secondly, 

 by calculating out 22 per cent of muscovite, the amount 

 required by the potash in analysis II and 31 per cent in analysis 

 III, the elements left would be too low in sesquioxides and 

 very much too high in protoxides to be referred to any known 

 species of chlorite. 



The matrix. — The material in which the garnets are imbedded 

 appears to be very homogeneous, when examined in thin sec- 

 tions under the microscope, appearing as a mass of minute, 

 pleochroic green scales in which an occasional grain of 

 magnetite is imbedded. An analysis shows that it is also a 

 ferrous chlorite differing from the alteration product of the 

 garnet and agreeing closely with thuringite. The analysis gave : 







I. 







II. 







Mean. 









Mol. ratio. 





Si0 2 



= 



22-45 



per 



cent. 



22-26 



per 



cent. 



22-35 



per 



cent. 



= 



■3725 = 



3- 



A1 2 3 



= 



25-23 







25-05 







25-14 







=r 



•244 = 



1-93 



FeO 



r= 



34-34 







3443 







34 39 







= 



• 44 U= 



■161 \ 





MnO 



= 



trace 

























5-15 



MgO 



= 



6-31 







6-51 







6-41 







= 





H 2 



= 



11-25 







11-25 







11-25 







= 



•625 = 



5-04 



99-58 99-50 99-54 



The ratio^SiO. : A1 2 S :EO:H 2 = 3 :2:5:5 does not quite 

 agree with Bammelsberg's ratio for thuringite, 3:2:4:4, but 



* Tabellarische Uebersicht der Mineralien. Zweite Auflage. 1882. 

 f Taken in this form for easier comparison. 



