『Abstract
A comprehensive study of clay minerals should include determination
of the vacancy pattern of the dioctahedral sheet. The purpose
of this report is to consider the advantages and limitations in
various diffraction and non-diffraction methods for the determination
of the layer types in clay minerals. Identification of trans-vacant
(tv) and cis-vacant (cv) clay minerals reported here is based
on powder X-ray diffraction (XRD) patterns calculated for different
polytypes consisting of either tv or cv layers, on the simulation
of experimental XRD patterns corresponding to illite or illite
fundamental particles in which tv and cv layers are interstratified,
and on the semi-quantitative assessment of the relative content
of the layer types in the interstratified structures by generalized
Mering（eの頭に´）'s rules. A simple and effective
method for identification of tv and cv layers in dioctahedral
2:1 layer silicates employs thermal analysis and is based on different
dehydroxylation temperatures for tv and cv illite and smectite
layers.
Crystal chemical analysis of various dioctahedral 2:1 layer silicates
consisting of tv and cv layers indicates that compositional control
is present in the distribution of octahedral cations over trans-
and cis-sites. In dioctahedral smectites the formation of tv and
cv layers is related to the layer composition and local order-disorder
in the distribution of isomorphous cations. Dioctahedral 1M micas
with abundant Fe3+ and Mg occur only as tv varieties.
In contrast, 1M-cv illite, as well as cv layers in illite fundamental
particles of I-S, can form only as Fe- and Mg-poor varieties.
In illites and illite fundamental particles of I-S consisting
of tv and cv layers, cv layers prevail when the amounts of Al
in octahedra and tetrahedra are ＞1.55 and ＞0.35 atoms per O10(OH)2, respectively.
The main factors responsible for the stability of cv and tv illites
have been established. Monomineral cv 1M illite, its association
with tv 1M illite, and interstratified cv/tv illite occur around
ore deposits, in bentonites, and in sandstones mostly as a result
of different types of hydrothermal activity. The initial material
for their formation should be Al-rich, and hydrothermal fluids
should be Mg- and Fe-poor.
Tv and cv smectites of volcanic origin differ in terms of octahedral
cation composition and distribution of isomorphos octahedral cations.
Mg-rich cv smectites have random distribution of isomorphos octahedral
cations, whereas in Mg-bearing tv smectites octahedral Mg cations
are dispersed so as to minimize the amount of Mg-OH-Mg arrangements.
Key Words: Cis-vacant layers; Illite; Illite-smectite; Smectite;
Thermal analysis; Trans-vacant layers; XRD』
Introduction
Methods for determining layer types
Identification of periodic cv and tv dioctahedral mica varieties
Diffraction methods for determining layer types in interstratified
cv/tv structures in illites and illite fundamental particles
Dehydroxylation and rehydroxylation behavior of dioctahedral
2:1 phyllosilicates: implications for determination of the vacancy
pattern of the octahedral sheet
Crystal-chemical factors and physicochemical conditions responsible
for the formation of cv layers in clay minerals
Structural and chemical factors favoring the formation of
tv and cv layers in illite and illite fundamental particles in
I-S
Potential role of crystal-chemical features in the formation
of tv and cv smectites
Physicochemical conditions and geological environments favorable
to the formation of cv illite and cv layers in illite fundamental
particles of I-S
Geological conditions favoring formation of tv and cv smectites.
Implications for indicator characteristics of I-S
Conclusions
Acknowledgments
References