1644 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
3. their detachment into the solution.
When dissolution is congruent, the different precursor
complex species (e.g., Al-OH, Mg-OH) form and activate
at the same rate, and the various steps can be combined
to yield a single overall rate equation for the dissolution
(Aagaard and Helgeson, 1982 Oelkers et al., 1994). In
these simple congruent cases, a chemical affinity model
describes dissolution behavior well.
Most phyllosilicates are notably not so simple, in
particular those containing Mg and Fe (Li et al., 2017
Mulders and Oelkers, 2020). In these, precursor complexes
form and/or activate at very different rates depending on
the strength of their bonds in the mineral. As a result, some
components dissolve much faster than others. This has
several follow-on effects on the overall dissolution process.
First, the dissolution of the more soluble components dis-
rupts the mineral lattice, causing partial detachment of some
of the more strongly bonded complexes and facilitating
their dissolution (Aagaard and Helgeson, 1982). Second,
as the reaction proceeds, a feedback develops between the
solution and the remaining mineral: the buildup of metals
in solution can cause saturation, inhibiting further dissolu-
tion of those metals. For example, in acid conditions such
as typical leaching environments, dissolved Al can limit the
dissolution rate of remaining parts of the aluminosilicate
lattice unless it is taken up in dissolved complexes or pre-
cipitates as Al salts (Oelkers et al., 2008). Both of these
factors lead different components in phyllosilicates to have
y =12430x -78.561
R² =0.957
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
0.00 0.20 0.40 0.60 0.80 1.00
Pauling bond strength
Figure 2. Correlation between calculated bond strength of metal-oxide bonds in Table
1 with Pauling bond strength. (Note that the y-axis shows bond strength calculated as
dissociation energies from solid metal oxides to gaseous species)
Table 1. Pauling bond strengths of common complexes found in phyllosilicates. Measured bond
strengths are approximated by Chetty (2018) from Sun and Huggins (1946)
Bond Found in
Coordination in
Phyllosilicates
Pauling Bond
Strength
Approximate
Measured Bond
Strength (kJ/mol)
K-O, Na-O alkali layers (micas) xii 1/12 ~1,300
Ca-O, Mg-O octahedral layers vi 1/3 ~3,660
Fe2+-O octahedral layers vi 1/3 3,845
Zn-O octahedral layers vi 1/3 3,940
Al-O octahedral layers vi 2/3 7,200
Al-O tetrahedral layers iv ¾ 7,860
Si-O tetrahedral layers iv 1 13,080
Measured
bon d
strength,
kJ/mol
3. their detachment into the solution.
When dissolution is congruent, the different precursor
complex species (e.g., Al-OH, Mg-OH) form and activate
at the same rate, and the various steps can be combined
to yield a single overall rate equation for the dissolution
(Aagaard and Helgeson, 1982 Oelkers et al., 1994). In
these simple congruent cases, a chemical affinity model
describes dissolution behavior well.
Most phyllosilicates are notably not so simple, in
particular those containing Mg and Fe (Li et al., 2017
Mulders and Oelkers, 2020). In these, precursor complexes
form and/or activate at very different rates depending on
the strength of their bonds in the mineral. As a result, some
components dissolve much faster than others. This has
several follow-on effects on the overall dissolution process.
First, the dissolution of the more soluble components dis-
rupts the mineral lattice, causing partial detachment of some
of the more strongly bonded complexes and facilitating
their dissolution (Aagaard and Helgeson, 1982). Second,
as the reaction proceeds, a feedback develops between the
solution and the remaining mineral: the buildup of metals
in solution can cause saturation, inhibiting further dissolu-
tion of those metals. For example, in acid conditions such
as typical leaching environments, dissolved Al can limit the
dissolution rate of remaining parts of the aluminosilicate
lattice unless it is taken up in dissolved complexes or pre-
cipitates as Al salts (Oelkers et al., 2008). Both of these
factors lead different components in phyllosilicates to have
y =12430x -78.561
R² =0.957
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
0.00 0.20 0.40 0.60 0.80 1.00
Pauling bond strength
Figure 2. Correlation between calculated bond strength of metal-oxide bonds in Table
1 with Pauling bond strength. (Note that the y-axis shows bond strength calculated as
dissociation energies from solid metal oxides to gaseous species)
Table 1. Pauling bond strengths of common complexes found in phyllosilicates. Measured bond
strengths are approximated by Chetty (2018) from Sun and Huggins (1946)
Bond Found in
Coordination in
Phyllosilicates
Pauling Bond
Strength
Approximate
Measured Bond
Strength (kJ/mol)
K-O, Na-O alkali layers (micas) xii 1/12 ~1,300
Ca-O, Mg-O octahedral layers vi 1/3 ~3,660
Fe2+-O octahedral layers vi 1/3 3,845
Zn-O octahedral layers vi 1/3 3,940
Al-O octahedral layers vi 2/3 7,200
Al-O tetrahedral layers iv ¾ 7,860
Si-O tetrahedral layers iv 1 13,080
Measured
bon d
strength,
kJ/mol