clarke.] MISCELLANEOUS SPECIES. 65 
In order to interpret the clintonite micas, the highly basic compound 
/°>Mg 
Al-O 
\si0 4 =Al 
was assumed. This formula also gives, empirically, the percentage 
composition of the species kornerupine or prismatine, which, unlike 
the micas, is orthorhombic, and also insoluble in acids. On these 
grounds it may be compared with andalusite and staurolite; and a clew 
to its constitution is found in the existence of an alteration product of 
prismatine, kryptotile, having the composition HAlSi0 4 . Tripling the 
formuhe, we have 
Andalusite. Kornerupine. Kryptotile. 
.Si0 4 =(A10) 3 Si0 4 =(A10 2 Mg) 3 / Si0 4= H 3 
Al-SiG 4 =Al Al-Si0 4 ^Al A1-SK) 4 ~A1 
\Si0 4 ^=Al \si0 4 =Al N %i0 4 =Al 
symbols which express the observed relationship between these and 
other species. Further than this the formuhe have no real significance, 
and alternative expressions are considered later in connection with the 
pyroxenes and amphiboles. Associated with kornerupine the still 
more basic sapphirine, Mg 5 Ali ? Si 2 2 7, is found. To the constitution of 
this species there is no good clew, and any attempt at structural formu- 
lation is useless. More than one structural formula is possible for 
sapphirine, but all are at present meaningless, except in so far as they 
show that the compound is stoichiometrically conceivable. 
A peculiarly difficult group of minerals to interpret constitutionally 
is the group of clays. One member of this group, kaolin, has already 
been considered, and this member has the advantage of being crystal- 
line. The other clays are amorphous, and of uncertain origin; they 
often occur in comx>licated mixtures, are difficult to identify with cer- 
tainty, and still more difficult to correlate with other species. They 
represent undoubtedly the breaking down of crystalline silicates, to 
which they are related somewhat as kaolin is related to the feldspars; 
but rarely, if ever, has their actual genesis been observed. Their for- 
niuke, therefore, are merely tentative, and serve only as a first step 
toward a better study of the several species. 
Upon comparing the formula of aluminum orthosilicate with that of 
kaolin, an indication of serial arrangement becomes evident, which may 
be writ-ten thus : 
Normal salt. H'estanite or ivoerthite. 
.Si0 4 — Al .OH 
Al— Si0 4 ^ Al Al— Si0 4 :EAl 
\Si0 4 =Al \Si0 4 HAl 
Kaolin. Montmorilloniie. Xewtonite. 
.OH .OH / OH 
Al— Si0 4 ^=H 3 Al— Si0 4 ^H 3 Al— OH 
^SiO, Al \si0 4 ^H 3 \si0 4 EEH 3 
Bull. 12 
