63 



100-16 



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

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

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

 or plates, of a greyish-black or greenish-black colour, with consider- 

 able 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 hexagonal prism, or to an acute rhomboid deeply truncated by 

 a plane perpendicular to the axis, or deeply compressed in that di- 

 rection. He also obtained a cleavage parallel with that plane. Mi- 

 nute fragments are translucent, and shew a greenish colour by trans- 

 mitted 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 shews 

 the presence of manganese. 



Its constituents are as follow :— • 



Oxygen. Ratio. formulae. 



5.63 



2 Al Si + (Fe, Mn.) Si^ 

 + A(i. 



98.53 



2 AlSi + (Fe,Mn3.)Si« 

 + 3H. 



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), consisting 

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

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

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

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

 may not have been derived from the gangue of slate, from which it 



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

 founded upon the idea of Trout, that they are all multiples of the atomic weight 

 of hydrogen, vary somewhat from Berzelius't?. 



