rHAP. TIT. ] 
VRROENBURGITE. 
.47 
specimen, the extra Fe203 being regarded as a further portion of the 
impurities. 
On the evidence of the foregoing calculations it might then be con- 
sidered as fairly certain that the formula of this mineral is really 3Mn304. 
2Fe203. At first sight, however, there might seem to be an objection 
to this formula in the fact that the mineral is so strongly magnetic that 
the iron must be supposed to be present as Fe304 rather than as Fe203. 
But this is not a real objection, because there are several minerals 
exhibiting magnetic properties — usually it is true, to a much smaller degree 
than magnetite — that do not contain their iron in the form of Fe304. Thus 
pyrrhotite or magnetic pyrites, a sulphide of the formula FeiiSi2, is 
often strongly magnetic. Ilmenite is often slightly magnetic. Its 
formula may be written either as FeTiOs, as (Fe,Ti)203, or as 
mFeTi03.»Fe203. Whichever interpretation be the correct it is seen that 
the iron is in the form of either FeO or Fe203. Since hematite (Fe203) 
is usually non-magnetic, it seems as if the magnetism of ilmenite must 
be due to the presence in it of titanium and not iron. In the same way 
all the Indian braunites are more or less magnetic. Although the 
formula of braunite is open to some doubt, as is shown in the section 
dealing with this mineral, yet no interpretation of it shows the presence of 
an R3O4 group. Now the Indian braunites usually contain an appre- 
ciable proportion of iron replacing a part of the manganese. Such 
iron must, therefore, be in the form either of Fe203 or, less likely, of FeO. 
It so happens, however, that some braunites are very low in their iron 
percentage, and yet are still magnetic. Hence we must suppose that, as in 
the case of the ilmenite the magnetic properties of the mineral are due 
to the titanium rather than to the iron, so in the case of braunite the 
magnetism is due, to the manganese rather than to the iron. Hence 
in the case of the mineral under consideration it does not necessarily 
follow that its magnetic properties are due to the iron, even though 
it is present in large amount. The manganese may be the cause of the 
magnetism ; but considering the strength of this property it seems 
more probable that the iron has something to do with the magnetism. 
It may be that this particular proportion of manganese and iron oxides 
gives rise to the magnetism. It will be seen from the foregoing that 
even if the magnetism be entirely due to the presence of the iron, it does not 
follow that it is present in the form of Fe304. It will be interesting, 
however, to see if the analysis can be interpreted on the supposition 
that the iron is present in the Fe304 condition. 
