1642
Crystal-Chemical and Analytical Insights into Phyllosilicate
Decomposition in Acid Leaching
Isabel Barton
University of Arizona Department of Mining &Geological Engineering
ABSTRACT: The breakdown of clays, micas, chlorites, and other phyllosilicates can cause long-term reagent
consumption, silica gel generation, and other serious problems in acid leaching. This talk describes how the
decomposition of these common gangue minerals depends on the crystal chemistry and nanoscale occurrence
patterns of metal ions in their lattices as well as on leaching conditions. Poorly crystallized phyllosilicates with
high contents of divalent Mg and Fe are most reactive, while well-crystallized Al- and Si-dominated minerals
are comparatively inert.
INTRODUCTION
Problem Statement
Phyllosilicate minerals cause a large number and wide range
of problems throughout the metal extraction process. In the
case of leaching, problems associated with phyllosilicates
can include heap-blinding, dust hazards, silica gel genera-
tion, preg-robbing, and reagent consumption (Graefe et al.,
eds., 2017). All in all, the metallurgical impact of phyllo-
silicates is out of proportion to their actual abundance in
most ores.
Factors usually considered include the commonness,
sheet structure, and grain size of most phyllosilicates. This
is correct but misses a crucial factor. Widespread occur-
rence, fine grain size, and sheet structure are important, but
equally consequential is the complex crystal chemistry of
phyllosilicates. Its influence on the incongruent dissolution
of the phyllosilicate lattice helps explain several problems
in leaching, as well as the variable behavior of the minerals
in the phyllosilicate group. Some phyllosilicates form silica
gels when they dissolve, others do not (Terry, 1983a,b)
some consume large amounts of acid, others little (Chetty,
2018) some contain valuable metals that leach, others
contain valuable metals that are unrecoverable under the
same leaching conditions (Drexler et al., 2023). Including
phyllosilicate crystal chemistry in considerations of leach-
ing mechanisms can help explain this variability.
Mineralogical background and previous work
The phyllosilicate group includes micas, chlorites, clays,
serpentine, talc, and other minerals with a characteristic
layered structure. This layered structure consists of sheets of
high-field-strength cations (usually Si with some Al) in tet-
rahedral coordination with O2– and sheets of lower-valent
cations (usually Mg, Fe, and Al) in octahedral coordina-
tion with O2–. The ratio and arrangement of tetrahedral to
octahedral sheets, and what other layers are present (e.g.,
alkali sheets), determine the type of phyllosilicate the lattice
represents. Figure 1 shows several common examples.
In general there are two types of phyllosilicate structure,
with countless minor variants in each class. Phyllosilicates
may be T-O, with a 1:1 ratio of tetrahedral (T) and octa-
hedral (O) sheets stacked in alternation, as in chlorite
(Figure 1). Or they may consist of T-O-T layers stacked
together, with a net 2:1 ratio of tetrahedral and octahedral
Crystal-Chemical and Analytical Insights into Phyllosilicate
Decomposition in Acid Leaching
Isabel Barton
University of Arizona Department of Mining &Geological Engineering
ABSTRACT: The breakdown of clays, micas, chlorites, and other phyllosilicates can cause long-term reagent
consumption, silica gel generation, and other serious problems in acid leaching. This talk describes how the
decomposition of these common gangue minerals depends on the crystal chemistry and nanoscale occurrence
patterns of metal ions in their lattices as well as on leaching conditions. Poorly crystallized phyllosilicates with
high contents of divalent Mg and Fe are most reactive, while well-crystallized Al- and Si-dominated minerals
are comparatively inert.
INTRODUCTION
Problem Statement
Phyllosilicate minerals cause a large number and wide range
of problems throughout the metal extraction process. In the
case of leaching, problems associated with phyllosilicates
can include heap-blinding, dust hazards, silica gel genera-
tion, preg-robbing, and reagent consumption (Graefe et al.,
eds., 2017). All in all, the metallurgical impact of phyllo-
silicates is out of proportion to their actual abundance in
most ores.
Factors usually considered include the commonness,
sheet structure, and grain size of most phyllosilicates. This
is correct but misses a crucial factor. Widespread occur-
rence, fine grain size, and sheet structure are important, but
equally consequential is the complex crystal chemistry of
phyllosilicates. Its influence on the incongruent dissolution
of the phyllosilicate lattice helps explain several problems
in leaching, as well as the variable behavior of the minerals
in the phyllosilicate group. Some phyllosilicates form silica
gels when they dissolve, others do not (Terry, 1983a,b)
some consume large amounts of acid, others little (Chetty,
2018) some contain valuable metals that leach, others
contain valuable metals that are unrecoverable under the
same leaching conditions (Drexler et al., 2023). Including
phyllosilicate crystal chemistry in considerations of leach-
ing mechanisms can help explain this variability.
Mineralogical background and previous work
The phyllosilicate group includes micas, chlorites, clays,
serpentine, talc, and other minerals with a characteristic
layered structure. This layered structure consists of sheets of
high-field-strength cations (usually Si with some Al) in tet-
rahedral coordination with O2– and sheets of lower-valent
cations (usually Mg, Fe, and Al) in octahedral coordina-
tion with O2–. The ratio and arrangement of tetrahedral to
octahedral sheets, and what other layers are present (e.g.,
alkali sheets), determine the type of phyllosilicate the lattice
represents. Figure 1 shows several common examples.
In general there are two types of phyllosilicate structure,
with countless minor variants in each class. Phyllosilicates
may be T-O, with a 1:1 ratio of tetrahedral (T) and octa-
hedral (O) sheets stacked in alternation, as in chlorite
(Figure 1). Or they may consist of T-O-T layers stacked
together, with a net 2:1 ratio of tetrahedral and octahedral