302 Alger's Localities of Rare Minerals, 
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, mg: 
6.80 
Water, 4.80 
100.16. 
Ottrelite was discovered by M. Desclozeaux, and analyzed 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 disks or plates, of a grayish-black or greenish-black 
color, with considerable metallic lustre, disseminated through a 
gangue which appears like a greenish argillaceous slate. These 
disks present no distinct form in the specimens I have exam- 
ined, 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 direction. He also 
obtained a cleavage parallel with that plane. Minute frag- 
ments are translucent, and show a greenish color by trans- 
mitted light. Before the blowpipe, it fuses, alone, with 
difficulty, on the edges, into a black, magnetic globule. k 
dissolves slowly in borax, giving the reaction of iron, and with 
ate of soda, shows the presence of manganese. 
Its constituents are as follow : 
Oxygen. Ratio. Formule. 
Silica, 43.34 22.51 4 B 
Alumina, 24.63 11.50 2 2AlSi-+ (Fe, Mn.) Si 
Protox. of iron, 16.72 2.80 -- Aq. 
Protox. of man- 5.63 1 
Water,” 566 508 1 2A Si-+ (Pe, Mat) SP 
: 3H. 
98.53 
Dr. Thomson’s analysis affords a different formula, and, 8c 
