Ford and Bradley — Margarosanite. 161 



which gave the characteristic reactions for manganophyllite. 

 In some cases the margarosanite was found lying immediately 

 upon barite. The cleavages and general characters of these 

 two minerals are so similar, that it became necessary to sepa- 

 rate the magarosanite with great care before analyzing it. This 

 was done by crushing the material and picking it over grain 

 by grain under a lens. In this way material of undoubted 

 purity was obtained for the analysis. 



The method of the analysis was simple and was briefly as 

 follows. The mineral -was decomposed by treatment with 

 nitric acid with the consequent separation of silica. The solu- 

 tion was evaporated to dryness and after taking the residue up 

 in dilute acid the silica was filtered off. The filtrate was again 

 evaporated to dryness to remove the nitric acid and the residue 

 treated with water to dissolve the nitrates. During this evapo- 

 ration a small amount of basic lead nitrate was formed which was 

 insoluble in water. This was taken up in a very little nitric 

 acid. The lead was precipitated and weighed as the sulphide. 

 The manganese was precipitated as a sulphide and, after pre- 

 cipitation as the basic carbonate, was weighed as Mn 3 4 . The 

 calcium was determined as usual and the amount of water 

 found by a direct determination, according to the Penfield 

 method. 



A number of complete and several partial analyses were 

 made by Bradley, the majority of which showed close agree- 

 ment with each other. Several of the better determinations 

 in each case are given below : 



1-00 

 •5986 1-07 



10066 



These results are in substantial agreement with those of the 

 partial analyses made earlier by Penfield and Warren. Since 

 the water is in small amount and since it is driven from the 

 mineral at alow temperature, it is thought to be hygroscopic in 

 character and is not considered in the calculations. The analy- 

 sis points clearly to a metasilicate formula for the mineral. 

 The various bivalent oxides may be considered as isomorphous 

 with each other and the formula given as RSiO s , or in view of 

 the ratio existing between the lead oxide and the calcium-man- 

 ganese oxide, it may be written as Pb(Ca,Mn) 2 (Si0 3 ) 3 . 



Below are given the theoretical composition of the last 

 formula and the results of recalculating the present analysis 











A 



verage 



Ratios 





Si0 2 .. 



. 3373 



33-73 



33-98 



33 39 



33-71 



•5581 



3-00 



PbO... 



.. 40-15 



43-30 



43-89 



4367 



43-50 



■1951 



1-04 



CaO... 



. 21-61 



21-62 



21 97 





2173 



•3873 | 



•4035 216 



MnO .. 



.. 098 



1-01 



1-29 



1-30 



1-14 



•0162 f 



H.O... 



.. 0-48 



0-68 







0-58 







