236 
W. F. Cole. 
In the case of tlie clay minerals the fact that they may be subdivided into 
three groups, each groTip being characterised by a particular large spacing, is 
made use of in their identification in soil colloids. 
In preparing the following particulars regarding the chemical composition 
of memb(n-s of tlic tlmee groups of clay minerals tlie author has consulted 
Hendricks and Ahixamler (10). 
(1) Kdolinifc Qrotip. — The kaolinite group includes the following min- 
erals : kaolinit(^ anauxite, nacrite, dickite, halloysite and hj^drous halloysite. 
These minerals, with the exception of hydrous halloysite whicli possesses two 
easily detachable molecules of water, have the ideal formula [AI 2 ] [Si 2 ] 05 ( 0 H)^ 
in whieli isomorphous re]>lacement is largely restricted to mutual substitution 
of A1 and 8i in [Si 2 ] positions. Tla^ characteristic large spacing for all members 
except hydrous halloysite occurs at apyjroximatoly 7 A. Hytlrous halloysite, 
which readily reverts to halloysite by the loss of two molecules of water, has 
a basal spadng of 10*3 A. The patterns of alt members of this group are 
destroyed by heating to r>00®C. 
(2) Monhnorillonite Group.^ — The montmorillonite group includes the fol- 
lowing minerals ; montmorillonite, saponite, nontronite and beidellite. Mont- 
morillonite has the ideal formula [Alg] [Si4]Oio(OH)2.xH20 in which extensive 
isomorphous replacement can take place. Substitution of [AI 2 ] by [Mgg] gives 
saponite [Mg 3 l[Si 4 ]Ojo(OH) 2 .xH 20 ; substitution of [Al 2 ]“by [¥o^] gives 
nontronite fKo 2 ] > substitution of Si by A1 in position 
together with the replacement of O by (OH) or the replacement of A1 in 
[Alg] position by other ions gives beidellite [AI 2 ] ri^i 3 Al] 09 (OH) 3 .xH 20 in which 
the Si02 ; I'atio lies close to 3 ; 1. The characteristic* large spacing 
for air dried Tnatc'rial of all ineinbc^rs occurs at 14-lo A. Members of this 
group show rc'vcrsible lattice shrinkage and expansion acco 7 *ding to tlu'ir water 
content(ll). Ujjon heating to o00°C. the 14-15A basal spacitig shrinks to 10 A, 
(3) Mtca Group.- — In this group no subdivision is at ))resent recognised. 
Gruner (12) however, has shown that the structures of glauconite and mica 
are almost ideiitical. Mica has the ideal formula KfAl 2 ] [Si 3 Al]Oj 9 (OH )2 in 
which extcnisive isomorphous replacement can take jilace. Replac^ejnent of 
K by H 2 O accomfianied hy substitution of Si for A1 in tetrahedral co-ordination 
or (OH) for O, together with Mg and Fe replacing [AI 2 ] Avith octaliodral eo- 
ordinatioii, results in a mineral of the glauconite type. The charactcaistic 
basal spacing for the mica group occurs at 10 A, No {'hango in pattern is 
produced by heating members of this group to 500T, 
As the minerals Avithin a group cannot readily be distinguished in the 
diffraction ]3attern of a soil colloid, the terms “■ kaolinite ” and ” montmorillon- 
ite ” A\ ill be used, for brcAuty, in the remainder of this paper to designate a 
mineral bcdonging to the kaolinite group and montmorillonite group respec- 
tively. 
In the draAving up of Table I. an attempt has been made to assign a pos- 
sible origin to all tlie obser\a*d lines. For montmorillonite and kaolinite the 
data published by Kelley et al. (13) Avas used. This data includes only spacings 
to whudi Miller indices related to a definite unit cell could be assignetl. The 
kaolinite spacings were those tabulated by Gruner (14) with 7’espect to the 
monoclinic unit cell a ^ 5*14 A, b = 8*90 A, c 14-51 A, /S 100° 12'. 
The montmorillonite spacings Avere selected from the data ofMaegdofrau and 
Hofmann (15) Avho referred the mineral structure to an orthorhombic cell AAuth 
dimensions a ^ 5*18 A, b = 8*97 A, AA'ith c variable according to the degree 
