XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3923
breakage patterns, this definition is somewhat ambiguous.
Diamonds present with various morphologies, each with
its own internal weaknesses such as mineral inclusions
(e.g., sulphides, graphite, silicates), crystal distortions from
growth, natural cleavage planes (e.g., {111} for octahedra),
ruts and fractures. Breakage resulting from these relatively
weak zones may not necessarily be highly resorbed, compli-
cating differentiation. Below, we elaborate on the distinct
crystallographic factors influencing the type and extent of
diamond fracture. Specifically, we describe the common
breakage patterns observed across the primary and second-
ary morphologies identified in Figure 3, while also incorpo-
rating the severity of breakage scale employed by De Beers
since 1978 (Figure 2).
Octahedra. Primary octahedral crystals exhibit a
range in breakage patterns, from slightly chipped stones
with 10% missing to heavily broken varieties with 50%
missing. Chipping typically occurs at the vertices of small
and micro-diamond octahedra (Figure 4a) and octahedral
aggregates. Two distinctive chipping patterns emerge: (1)
where the corner is chipped off along an irregular fracture
surface aligning somewhat with the ‘cubic’ (100) plane of
the octahedral crystal (Figure 4b), or (2) where a small piece
is cleaved off along the (111) octahedral cleavage plane,
forming angular stepped features towards the vertices
(Figure 4c). Natural ‘serrated-lamellae’ resorption features
(Figure 4d, e) present on the newly cleaved surfaces pro-
vides evidence that cleavage occurred while in the mantle.
In contrast, the sharp fractured surface at the vertices is less
clearcut. Similar chipping patterns manifest on the verti-
ces of aggregated octahedral stones. It is noteworthy that
the aggregates do not break along crystal intergrowth mar-
gins because the covalent bonds binding adjacent diamond
crystals together may surpass the strength of the individual
stones—meaning that when external impact forces are
applied to a diamond aggregate, the diamonds are more
likely to fracture within the crystals rather than along the
boundaries between them (e.g., Figure 3c).
Heavily broken octahedrons, with 50% missing,
exhibit random breakage patterns typically surrounding
internal mineral inclusions. A specific example of this type
of breakage is depicted in Figure 4f -i. Here, the diamond
has broken around a remnant sulphide inclusion, that is
now significantly oxidised as shown by the russet colour of
its residue (Figure 4f, g). Moreover, the sulphide contains
elements, such as Fe, Ni±Cu, with heavier atomic numbers
than diamond, and thus appears brighter in the SE and
BSE images portrayed in Figure 4h, i. Diamond breakage
takes advantage of natural fracture propagation around the
inclusion due to a build-up of localised stress that can be the
result of the following factors: (1) a difference in hardness,
where the inclusion has a lower Moh’s scale hardness value
compared to the diamond (i.e., Moh’s scale hardness vales
of ~3.5 -4 for common pyrrhotite, pentlandite or chalco-
pyrite inclusions compared to the value of 10 for diamond),
(2) difference in thermal expansion, where changes in tem-
perature during interaction with mantle fluids or ascent can
cause differential expansion and contraction between the
inclusion and diamond, (3) crystallographic misfit, where
the lattice of the inclusion does not align perfectly with that
of the diamond, and (4) the inclusion may exert internal
pressure on the surrounding diamond due to differences in
composition and density (e.g., Angel et al., 2022 Harris
et al., 1972). Also, the heavily resorbed outer surfaces of
the stone combined with the sharp jagged surfaces exposed
around the inclusion, highlight the ambiguity surrounding
a clear mechanism for breakage in these instances. However,
in the case of this particular stone, we observe the presence
of ‘tram-line’ abrasion pits (Figure 4f), which are exclusively
associated with mechanical abrasion during the processing
of primary ore and not natural shearing. Fracture associated
with mechanical breakage encountered during the commi-
nution processes and transport of the arterial in the process
plant is discussed in the next section.
Macel twins. These ‘spinel-twinned’ stones are consis-
tently fractured (lightly broken, up to 25% missing) at the
corners of their triangular habits (Figure 4j -l). This type
of breakage exploits inherent inflection points of the twin
plane (Figure 4j), whereby the breakage leverages natural
re-entrant pits or facets where the two {111} planes do not
entirely match up (e.g., Hartman, 1956 Yacoot et al. 1998).
The conchoidal-like fracture also exhibits terracing along
the (111) cleavage plane (Figure 4k, l). Obvious internal
distortion at the concave fracture site (Figure 4l) suggests
that a significant compressive or impact force caused struc-
tural defects in response to the stress.
THH and Dodecahedra. These significantly resorbed
and rounded crystals seem relatively impervious to the type
of chipping and light breakage patterns described for pri-
mary octahedra and macels. Instead, when these second-
ary morphologies break, they break extensively (i.e., heavily
broken crystals, with 50 to 75% missing), leaving behind
remnant crystal faces. The broken stones often display
‘half’ or ‘quarter’ configurations, indicating breaks along
secondary resorption planes and sometimes the intrinsic
primary cleavage planes {111} of the original octahedral
crystal. For example, the dodecahedron fragment depicted
in Figure 4 m -p shows cleavage along the inherent (111)
octahedral cleavage plane (Figure 4 m), less obvious cleav-
age along the (110) dodecahedral plane (Figure 4n), and
breakage patterns, this definition is somewhat ambiguous.
Diamonds present with various morphologies, each with
its own internal weaknesses such as mineral inclusions
(e.g., sulphides, graphite, silicates), crystal distortions from
growth, natural cleavage planes (e.g., {111} for octahedra),
ruts and fractures. Breakage resulting from these relatively
weak zones may not necessarily be highly resorbed, compli-
cating differentiation. Below, we elaborate on the distinct
crystallographic factors influencing the type and extent of
diamond fracture. Specifically, we describe the common
breakage patterns observed across the primary and second-
ary morphologies identified in Figure 3, while also incorpo-
rating the severity of breakage scale employed by De Beers
since 1978 (Figure 2).
Octahedra. Primary octahedral crystals exhibit a
range in breakage patterns, from slightly chipped stones
with 10% missing to heavily broken varieties with 50%
missing. Chipping typically occurs at the vertices of small
and micro-diamond octahedra (Figure 4a) and octahedral
aggregates. Two distinctive chipping patterns emerge: (1)
where the corner is chipped off along an irregular fracture
surface aligning somewhat with the ‘cubic’ (100) plane of
the octahedral crystal (Figure 4b), or (2) where a small piece
is cleaved off along the (111) octahedral cleavage plane,
forming angular stepped features towards the vertices
(Figure 4c). Natural ‘serrated-lamellae’ resorption features
(Figure 4d, e) present on the newly cleaved surfaces pro-
vides evidence that cleavage occurred while in the mantle.
In contrast, the sharp fractured surface at the vertices is less
clearcut. Similar chipping patterns manifest on the verti-
ces of aggregated octahedral stones. It is noteworthy that
the aggregates do not break along crystal intergrowth mar-
gins because the covalent bonds binding adjacent diamond
crystals together may surpass the strength of the individual
stones—meaning that when external impact forces are
applied to a diamond aggregate, the diamonds are more
likely to fracture within the crystals rather than along the
boundaries between them (e.g., Figure 3c).
Heavily broken octahedrons, with 50% missing,
exhibit random breakage patterns typically surrounding
internal mineral inclusions. A specific example of this type
of breakage is depicted in Figure 4f -i. Here, the diamond
has broken around a remnant sulphide inclusion, that is
now significantly oxidised as shown by the russet colour of
its residue (Figure 4f, g). Moreover, the sulphide contains
elements, such as Fe, Ni±Cu, with heavier atomic numbers
than diamond, and thus appears brighter in the SE and
BSE images portrayed in Figure 4h, i. Diamond breakage
takes advantage of natural fracture propagation around the
inclusion due to a build-up of localised stress that can be the
result of the following factors: (1) a difference in hardness,
where the inclusion has a lower Moh’s scale hardness value
compared to the diamond (i.e., Moh’s scale hardness vales
of ~3.5 -4 for common pyrrhotite, pentlandite or chalco-
pyrite inclusions compared to the value of 10 for diamond),
(2) difference in thermal expansion, where changes in tem-
perature during interaction with mantle fluids or ascent can
cause differential expansion and contraction between the
inclusion and diamond, (3) crystallographic misfit, where
the lattice of the inclusion does not align perfectly with that
of the diamond, and (4) the inclusion may exert internal
pressure on the surrounding diamond due to differences in
composition and density (e.g., Angel et al., 2022 Harris
et al., 1972). Also, the heavily resorbed outer surfaces of
the stone combined with the sharp jagged surfaces exposed
around the inclusion, highlight the ambiguity surrounding
a clear mechanism for breakage in these instances. However,
in the case of this particular stone, we observe the presence
of ‘tram-line’ abrasion pits (Figure 4f), which are exclusively
associated with mechanical abrasion during the processing
of primary ore and not natural shearing. Fracture associated
with mechanical breakage encountered during the commi-
nution processes and transport of the arterial in the process
plant is discussed in the next section.
Macel twins. These ‘spinel-twinned’ stones are consis-
tently fractured (lightly broken, up to 25% missing) at the
corners of their triangular habits (Figure 4j -l). This type
of breakage exploits inherent inflection points of the twin
plane (Figure 4j), whereby the breakage leverages natural
re-entrant pits or facets where the two {111} planes do not
entirely match up (e.g., Hartman, 1956 Yacoot et al. 1998).
The conchoidal-like fracture also exhibits terracing along
the (111) cleavage plane (Figure 4k, l). Obvious internal
distortion at the concave fracture site (Figure 4l) suggests
that a significant compressive or impact force caused struc-
tural defects in response to the stress.
THH and Dodecahedra. These significantly resorbed
and rounded crystals seem relatively impervious to the type
of chipping and light breakage patterns described for pri-
mary octahedra and macels. Instead, when these second-
ary morphologies break, they break extensively (i.e., heavily
broken crystals, with 50 to 75% missing), leaving behind
remnant crystal faces. The broken stones often display
‘half’ or ‘quarter’ configurations, indicating breaks along
secondary resorption planes and sometimes the intrinsic
primary cleavage planes {111} of the original octahedral
crystal. For example, the dodecahedron fragment depicted
in Figure 4 m -p shows cleavage along the inherent (111)
octahedral cleavage plane (Figure 4 m), less obvious cleav-
age along the (110) dodecahedral plane (Figure 4n), and