1V14 MANGANESE DEPOSITS OF INDIA : MINERALOHV. [ PaRT T : 
Some varieties show a maximum absorption for rays with vibrations 
at right angles to c, unlike other micas ; but in those containing most 
manganese the absorption is normal, the pleochroism then being : — 
;l = red-brown, 
b and C = dark lirowii. 
Certain intermediate kinds show no pleochroism. The optic axial 
angle of this miea is not given, but is probably small or nil ; for biotite 
is usually practically uniaxial, although the optic axial angle may 
occasionally be as high as 50^. 
Casivellite is the name given to an altered mica of a light copper-red 
colour and a bronze-like lustre resembling clintonite (seybertite). 
The structure is micaceous, the lamina^ being inelastic. G. = 3-54. 
The mineral is not pleochroic and on analysis was foi^nd to be a silicate, 
chiefly of manganese and calcium, with smaller amounts of aluminium, 
iron, and magnesium. The one analysis published shows 15"95% of 
Mn203. It occurs with rhodonite, polyadelphite, and biotite, from 
which it is believed to have been derived, at the Trotter Mine, Frank- 
lin Furnace, New Jersey, United States of America. ^ 
Alurgite : — This is a name given to a mica found at the n.anganese 
mines at St. Marcel, Piedmont. It has been investigated in some 
detail by S. L. Penfield, 2 who finds that it is to be regarded as a 
distinct species of the formula HR2(A10H)Al(Si03)4 with R=MgOH, K. 
It is most closely related to lepidolite. The quantity of manganese 
in the mineral is small, namely 0-87% Mn203 and 0-18% MnO in the 
specimen analysed by Penfield. The mineral is monoclinic with the usual 
basal cleavage, whilst the laminae are flexible. H = 3. G=2'83 to 2"8.'>. 
In colour it is a deep brownish copper-red, whilst ' cleavage pieces have 
something of the colour of clear chips of almandine garnet '. The streak 
is rose-red to pale pink. The pleochroism is not very marked owing to 
the fact that the absorption is nearly the same in all directions. The 
pleochroism. is : — 
a and r — - red with a purplish cast, 
b = browiish red. 
The optic axial angle, 2E— 56° to 57°. Owing to twinning, however, 
the mineral is sometimes found to give a uniaxial figure. 
Manganchlorife is a manganiferous variety of the chlorite known 
as clinochlore, the formula of this mineral being HgMg5Al2Si30ig ; 
in manganchlorite there is 32-3% of MnO replacing a part of the MgO. 
1 Dana, Appendix I, p. 14. 
2 Amcr. Jour. Sc! XLVT. pp. 28S» to 291 . (1893). 
