Chap. III.] 
BRA UNITE. 
65 
G. Rose 1 took up this idea, and in view of the known presence of siHca 
in most specimens of braunite, attempted to show that Si02 and Mn02 
isomorphously replace one another in this mineral. He gives the fol- 
lowing as the most general formula of the mineral, allowing in it for 
the presence of small quantities of various protoxide constituents 
replacing the manganese protoxide, MnO : — 
MnO ^ MnO ■ 
I 
FeO I FeO 
l-SiOs + }-Mn02 
CaO I CaO I 
MgO J MgO J 
He then considers as derivatives of this formula the braunite of 
Elgersburg, which had not then been shown to contain silica, and the 
variety of this mineral from St. Marcel in Piedmont, called marceline by 
Beudant^ in 1832, and analysed by Damour 3. These two varieties 
would then have the formula) : — 
MnO ^ J 
Braunite [ Mn02. ' 
BaO ) 
rMnOo. 
Marceline MnO \ 
(.Si02 
Rammelsberg 4 then analysed the braunite of Elgersburg and found 
that it contained 8-6.3 per cent, of Si02. He considers that the 
mineral is an isomorphous mixture of manganic oxide, Mn203, and 
manganous metasilicate, MnSiOs, the various protoxides that often 
enter into the composition of the braunite replacing portions of the 
MnO of the MnSiOs. He gives as the formula : — 
I Mn ■) 
3 Mn203 + MnSiOs = \ 2O3. 5 
isi 3 
Except for the fact that Rammelsberg regards the oxide portion 
of the formula to be manganic oxide, Mn203, this is essentially the 
formula put forward by Rose, who, however, regards the oxide por- 
tion as composed of MnO.Mn02. Rammelsberg draws attention to 
the analogy between the formula of braimite as he has expressed it 
and that of ilmenite, which may be written as — 
mFePs -I- n{ Fe, Mg) O.TiO, = (Fe, Ti,Mg),03. 
1 Pogg. Ann., CXXI, pp. 318-325, (1864). 
2 'Traite elementaire de Mineralogie', II, p. 18P, (Dana). 
3 Annales des Mines, 4me. Ser., I, pp. 405. 406, (1842). 
4 Loc cit., p. 510. 
6 The analysis given by Rammelsberg agrees much better with the formula TJIn^Os + 
2MnSi03, than with the one given. 
