clarke.] MICAS AND CHLORITES. 51 
From some points of view, and especially with reference to its crys- 
talline form, kaolin may be regarded as a member of the mica group. 
As such it would form a connecting link between the micas proper and 
the clintonite group, and with it the calcium mica, margarite, can be 
correlated. Furthermore, margarite yields an alteration product, dud- 
leyite, which falls into line with the other two species, thus : 
Kaolin. 
Margarite. 
Dudley ite. 
.OH 
-Si0 4 =H 3 
X)H 
Al-SK) 4 =CaH 
.OH 
Al-SiO 4 =0aH 
\si0 4 =Al 
\Si0 4 =(A10) 3 
\si0 4 =A10H.AlH 2 
These expressions are suggestive, but not altogether conclusive. They 
represent the known facts fairly well, however, and can serve their 
purpose for the time being. Several other micaceous minerals are 
altogether doubtful in character, and need only passing mention. Thus 
helvetan may be a trisilicate of the biotite type. Caswellite,* an altera- 
tion product of a manganesian biotite, may be written as a member of 
the clintonite group. Willcoxite seems to be an alkaline clintonite, and 
corresponds very closely to 
/°>Mg /°>Mg 
Al— O + Al— O 
\si0 4 ^ Al \ Si0 4 =NaH 2 
There are also several vermiculitic minerals which probably belong in 
the chlorite group. All of these supposed species need further investi- 
gation. 
In roscoelite, which is a vanadium mica, the ratios are quite uncer- 
tain. According to Genth, the mineral contains V 2 3 , while Eoscoe 
gives the vanadium as V 2 5 ; and in other respects the analyses are dis- 
cordant. If Genth's analysis is accepted, roscoelite has essentially the 
ratios of a phengitic muscovite, with half of the aluminum replaced by 
vanadium. This seems to be probable, but the species needs reanalysis. 
Ganophyllite, described by Hamberg in 1890, t and regarded by the 
discoverer as a manganese zeolite, should probably be classed with the 
micas. Like the micas, it is monoclinic, highly cleaveable, and gives a 
percussion figure consisting of a six-rayed star. In all its physical 
characteristics it resembles the micas, with one of which, manganophyll, 
it is associated. It loses water, however, in vacuo, regaining the amount 
lost on exposure to moist air, and on this account it may be compared 
either with the zeolites or with the vennieulite group. 
* Chester, Trans. N. T. Acad. Sci., XIII, p. 97, 1894. 
tGeol. Foren. Forhandl., XII, p. 586. 
