Biotite

It has generally been called biotite, but tests made on several specimens from the Parker shaft and the Trotter mine show the presence of considerable manganese. Hence it seems probable that the black mica of those localities, at least, should be regarded as belonging in the variety manganophyllite, as Chester (161) assigned it.

Caswellite
An alteration of the black mica to a bronzy, dull copper-red substance, described by Chester (161) under the name "caswellite", was fairly common on the dump of the Parker shaft, especially in specimens containing much yellow garnet. It is micaceous, inelastic, and weakly doubly refracting and has a hardness of about 3. The analysis indicates a highly hydrous calcium-manganese silicate to which no definite formula can be assigned and which shows little relation in composition to the original mica. Specimens examined under the microscope by the author showed a diverse and complex structure composed of a mixture of substances, none of which could be identified. Hence it seems safe to conclude that the name "caswellite" cannot be used with any definite mineralogic significance.

***Analysis of caswellite from Trotter mine, Franklin***
(A. H. Chester (161), analyst)
SiO₂	38.74
Fe₂O₃	6.85
Al₂O₃	6.58
Mn₂O₃	15.95
CaO	22.30
MgO	5.52
Ignition	4.64
	100.58


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