XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1643
sheets, as in micas. Between T-O or T-O-T units there may
be either an interlayer with nothing but water molecules,
or an alkali layer containing K, Na, or similar large and
low-charge ions.
The existence of multiple layers with varied ions and
coordination states means that phyllosilicates contain a
large number of different chemical bonds with different
strengths, with major implications for leaching. Quantifying
bond strength completely requires complex measurements
or calculations (described in Sun and Huggins, 1946).
However, for ionic crystals a useful proxy is the Pauling
bond strength, defined as the charge of the cation divided
by the number of anions it is coordinated to. For example,
Ca2+ in cubic (viii) coordination has a bond strength of
2/8, or ¼. Silicon as Si4+ in tetrahedral (iv) coordination
has a Pauling bond strength of 4/4, or 1. This measure cor-
relates well with measured actual bond strengths between
metals and the oxide anion (Figure 2).
Since dissolving a mineral means breaking its bonds
down to however it speciates in solution, these Pauling
bond strengths roughly represent to what extent each of
the mineral’s components will resist dissolution. In the
phyllosilicates, these bonds range from the extremely weak
alkali-oxide bonds to the very strong silicon-oxygen bond
(Table 1). The weakest bonds are the hydrogen or Van der
Waals bonds found in interlayers (if present), followed by
the bonds in the alkali layer (if present), then the octahedral
layer. The silicon-oxygen bond makes the tetrahedral layer
extremely strong. Three of the four oxide anions in each
silica tetrahedron are bridging oxygens that connect the tet-
rahedra into a sheet. The fourth connects the tetrahedral to
the octahedral layer (Figure 1).
The varying bond types in phyllosilicates have vary-
ing responses to dissolution (reviewed in part by Li et al.,
2017). Mineralogical studies, mainly applying transition
state theory, model the dissolution process in three steps:
1. the formation of precursor complexes on accessible
parts of the mineral’s surface via reaction with the
solution,
2. the activation of the complexes, and
Figure 1. Common varieties of the phyllosilicate structure, not to scale
sheets, as in micas. Between T-O or T-O-T units there may
be either an interlayer with nothing but water molecules,
or an alkali layer containing K, Na, or similar large and
low-charge ions.
The existence of multiple layers with varied ions and
coordination states means that phyllosilicates contain a
large number of different chemical bonds with different
strengths, with major implications for leaching. Quantifying
bond strength completely requires complex measurements
or calculations (described in Sun and Huggins, 1946).
However, for ionic crystals a useful proxy is the Pauling
bond strength, defined as the charge of the cation divided
by the number of anions it is coordinated to. For example,
Ca2+ in cubic (viii) coordination has a bond strength of
2/8, or ¼. Silicon as Si4+ in tetrahedral (iv) coordination
has a Pauling bond strength of 4/4, or 1. This measure cor-
relates well with measured actual bond strengths between
metals and the oxide anion (Figure 2).
Since dissolving a mineral means breaking its bonds
down to however it speciates in solution, these Pauling
bond strengths roughly represent to what extent each of
the mineral’s components will resist dissolution. In the
phyllosilicates, these bonds range from the extremely weak
alkali-oxide bonds to the very strong silicon-oxygen bond
(Table 1). The weakest bonds are the hydrogen or Van der
Waals bonds found in interlayers (if present), followed by
the bonds in the alkali layer (if present), then the octahedral
layer. The silicon-oxygen bond makes the tetrahedral layer
extremely strong. Three of the four oxide anions in each
silica tetrahedron are bridging oxygens that connect the tet-
rahedra into a sheet. The fourth connects the tetrahedral to
the octahedral layer (Figure 1).
The varying bond types in phyllosilicates have vary-
ing responses to dissolution (reviewed in part by Li et al.,
2017). Mineralogical studies, mainly applying transition
state theory, model the dissolution process in three steps:
1. the formation of precursor complexes on accessible
parts of the mineral’s surface via reaction with the
solution,
2. the activation of the complexes, and
Figure 1. Common varieties of the phyllosilicate structure, not to scale