560 Intelligence and Miscellaneous Articles. 



impression upon them. In strong transmitted light, the thinnest 

 discs present a greenish colour. Before the blowpipe, on charcoal, it 

 becomes magnetic, but does not fuse even on the edges ; with double 

 its bulk of borax, it slowly dissolves into a dark iron-green glass. 

 Its composition, as stated by Dr. Thomson, is as follows : — 



Silica 38-40 



Alumina 23-68 



Peroxide of iron .... 17*52 



Magnesia 8'96 



Potash 6-80 



Water 4*80 



100-16 



Ottrelite was discovered by M. Desclozeaux, and analysed by M. 

 Damour in 1842. A full description of it is given in the Annates 

 des Mines for that year, vol. ii. p. 357. It occurs in small discs or 

 plates, of a grayish-black or greenish-black colour, with considerable 

 metallic lustre, disseminated through a gangue which appears like a 

 greenish argillaceous slate. These discs present no distinct form in 

 the specimens I have examined, their edges being rounded, as in the 

 case of the phyllite ; but Desclozeaux has referred them to a hexa- 

 gonal prism, or to an acute rhomboid deeply truncated by a plane 

 perpendicular to the axis, or deeply compressed in that direction. 

 He also obtained a cleavage parallel with that plane. Minute frag- 

 ments are translucent, and show a greenish colour by transmitted 

 light. Before the blowpipe, it fuses, alone, with difficulty, on the 

 edges, into a black, magnetic globule. It dissolves slowly in borax, 

 giving the reaction of iron, and with carbonate of soda, shows the 

 presence of manganese. 



Its constituents are as follows : — 



Oxygen. Ratio. Formulae. 



Silica 43-34 22-51 4 



Alumina 24-63 11'50 2 2AlSi + (Fe 3 ,Mn 3 .) Si 2 



Protox. of iron 1672 3*801 +Aq. 

 Protox. of man- >5 - 6'3 1 



ganese .. 8*18 1-83 J* 2Al 2 3 Si0 3 + (Fe 3 Mn0 3 .) 



Water 5-66 5-03 1 2Si0 3 4-3H0 3 



98-53 



Dr. Thomson's analysis affords a different formula, and, according 

 to his method of determining the atomic proportions, phyllite is a 

 simple silicate (the atoms of silica and bases being equal), consist- 

 ing of nine atoms silicate of alumina, three atoms silicate of peroxide 

 of iron, three atoms silicate of manganese, and one atom silicate of 

 potash*. The occurrence of so large a proportion of potash in the 

 mineral is not a little remarkable, and I would suggest whether it 



* Outlines of Mineralogy, &c, vol. i. p. 384. Dr. Thomson's atomic 

 weights, founded upon the idea of Prout, that they are all multiples of the 

 atomic weight of hydrogen vary somewhat from Berzelius's. 



