XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3921
individual crystallographic forms and surface textures and
then, describe the various breakage patterns. This allows for
the differentiation between mechanically induced fracture,
and breakage controlled by crystallographic factors (e.g.,
cleavage). Breakage type versus frequency statistics can-
not be directly applied here, but continued research will
incorporate these findings into breakage modelling meth-
odologies which will be based on the much larger diamond
parcel.
METHODOLOGY
Microscopy and X-Ray Computed Tomography (CT)
The inherently high reflective index of diamond makes
it a suitable candidate for optical microscopy. A Leica
‘Gemological’ Optical Microscope integrated with a
digital camera was used to identify and photograph the
Roberts Victor micro-diamond morphologies and to scru-
tinize their surface features. Preliminary back-scattered
electron (BSE) and secondary electron (SE) microscopy
was conducted at the Central Analytical facilities (CAF)
at Stellenbosch University. A Zeiss Merlin FE-SEM was
employed, using an accelerating voltage of 20 kV, working
distance of 12.5 mm and variable magnification, bright-
ness, and contrast settings. X-ray computed tomography
(CT) was used to examine diamonds in three dimen-
sions (3-D). This method entails subjecting the diamond
to a collimated X-ray beam while continually capturing
attenuation measurements throughout a complete 360°
rotation of the sample. Subsequently, these measurements
are reconstructed into a three-dimensional image of its
volume, comprised of individually stacked 2D slices. The
3-D dataset can be reduced, manipulated, and exported as
images using Volume Graphics software (e.g., VGStudio
MAX or MgVGL). The CT scanning was conducted at
X-Sight X-Ray Services in Somerset West, South Africa.
A Metrology-CT (MCT225) system was used with an in-
house developed Nikon Metrology 225 kV micro-focus
X-ray source that enables around 150× magnification and
up to 2 µm voxel. A copper filter was used to prevent beam
hardening by removing the soft x-rays, and the measure-
ments were run at 100 kV. The system is pre-calibrated and
verified using the VDI/VDE 2630 guidelines for Computed
Tomography in Dimensional Measurement.
RESULTS AND DISCUSSION
Diamond Morphologies and Surface Features
Before assessing any breakage features, it is essential to
delineate the primary growth morphologies and secondary
resorption features of the Roberts Victor small and micro-
diamonds. These characteristics are detailed below and
summarized in Figure 3). In general, the diamonds exhibit
a range of colours from light-yellow to brown, and some
are heavily included with sulphide and graphite inclusions.
The fundamental growth forms of diamond are governed
by the isometric atomic arrangement of carbon within
the unit cell. Ideally, a primary octahedral habit emerges
as the crystal grows in all directions until eight triangular
faces, approximately equal in size, converge at sharp-edged
vertices (Figure 3a). The cubic symmetry of diamonds per-
mits common ‘spinel-type’ twinning about the {111} plane
forming triangular-shaped flattened octahedra called macels
(Figure 3b), as well as crystal intergrowth or aggregation
(Figure 3c) (see Harris et al. (2022) for macro-diamond
examples). Very few truly primary flat-faced, sharp-edged
octahedra exist in this parcel. Most exhibit some evidence
of surface growth features such as triangular plates (Figure
3d see triangular growth plates on the {111} face), imbri-
cated surfaces (Figure 3e see overlapping of {111} plates)
and step-faces (Figure 3f). Fully developed step-face terrac-
ing (striations) can also occur (Figure 3g).
During mantle residence, changes in growth condi-
tions such as temperature, pressure, redox, carbon availabil-
ity and the presence of other diamonds can distort primary
habits through elongation, flattening or further aggregation
(Figure 3h). Moreover, interaction with metasomatic fluids
or with the host magma during transportation to the sur-
face can trigger dissolution reactions resulting in distinctive
resorption features such as rounded edges (Figures 3i -l),
where octahedra may take on a more shield-shaped appear-
ance with the formation of ditrigonal crystal faces (Figure
3i) and macels a rounded ‘pincushion’ appearance (Figure
3l), as well as new surface features. For example, the stones
may develop serrated lamellae (Figure 3i), triangular fea-
tures called trigons (Figure 3j, k) on the {111} or all planes,
and surface ribbing (Figure 3l).
Further dissolution of primary octahedra can result
in complete transformation into secondary morpholo-
gies. Specifically, the octahedra undergo a staged evolu-
tion through various transitional forms that culminate in
rounded stones with either 24 curved surfaces (tetrahexahe-
dra Figure 3 m, n) or 12 curved surfaces (dodecahedrons
Figure 3o, p). The tetrahexahedral form is distinguished
from the dodecahedrons by the presence of notable medial
lines positioned along the short diagonals of each rhom-
bic ‘face’ which divides it into two distinct planes. These
rounded secondary morphologies exhibit a variety of addi-
tional surface features, including terraces surrounding triad
apices or {111} directions (Figure. 3 m, n) which are inter-
preted as exposed intermittent growth layers of the original
octahedron, common surface smoothing (i.e., Figure 3o),
individual crystallographic forms and surface textures and
then, describe the various breakage patterns. This allows for
the differentiation between mechanically induced fracture,
and breakage controlled by crystallographic factors (e.g.,
cleavage). Breakage type versus frequency statistics can-
not be directly applied here, but continued research will
incorporate these findings into breakage modelling meth-
odologies which will be based on the much larger diamond
parcel.
METHODOLOGY
Microscopy and X-Ray Computed Tomography (CT)
The inherently high reflective index of diamond makes
it a suitable candidate for optical microscopy. A Leica
‘Gemological’ Optical Microscope integrated with a
digital camera was used to identify and photograph the
Roberts Victor micro-diamond morphologies and to scru-
tinize their surface features. Preliminary back-scattered
electron (BSE) and secondary electron (SE) microscopy
was conducted at the Central Analytical facilities (CAF)
at Stellenbosch University. A Zeiss Merlin FE-SEM was
employed, using an accelerating voltage of 20 kV, working
distance of 12.5 mm and variable magnification, bright-
ness, and contrast settings. X-ray computed tomography
(CT) was used to examine diamonds in three dimen-
sions (3-D). This method entails subjecting the diamond
to a collimated X-ray beam while continually capturing
attenuation measurements throughout a complete 360°
rotation of the sample. Subsequently, these measurements
are reconstructed into a three-dimensional image of its
volume, comprised of individually stacked 2D slices. The
3-D dataset can be reduced, manipulated, and exported as
images using Volume Graphics software (e.g., VGStudio
MAX or MgVGL). The CT scanning was conducted at
X-Sight X-Ray Services in Somerset West, South Africa.
A Metrology-CT (MCT225) system was used with an in-
house developed Nikon Metrology 225 kV micro-focus
X-ray source that enables around 150× magnification and
up to 2 µm voxel. A copper filter was used to prevent beam
hardening by removing the soft x-rays, and the measure-
ments were run at 100 kV. The system is pre-calibrated and
verified using the VDI/VDE 2630 guidelines for Computed
Tomography in Dimensional Measurement.
RESULTS AND DISCUSSION
Diamond Morphologies and Surface Features
Before assessing any breakage features, it is essential to
delineate the primary growth morphologies and secondary
resorption features of the Roberts Victor small and micro-
diamonds. These characteristics are detailed below and
summarized in Figure 3). In general, the diamonds exhibit
a range of colours from light-yellow to brown, and some
are heavily included with sulphide and graphite inclusions.
The fundamental growth forms of diamond are governed
by the isometric atomic arrangement of carbon within
the unit cell. Ideally, a primary octahedral habit emerges
as the crystal grows in all directions until eight triangular
faces, approximately equal in size, converge at sharp-edged
vertices (Figure 3a). The cubic symmetry of diamonds per-
mits common ‘spinel-type’ twinning about the {111} plane
forming triangular-shaped flattened octahedra called macels
(Figure 3b), as well as crystal intergrowth or aggregation
(Figure 3c) (see Harris et al. (2022) for macro-diamond
examples). Very few truly primary flat-faced, sharp-edged
octahedra exist in this parcel. Most exhibit some evidence
of surface growth features such as triangular plates (Figure
3d see triangular growth plates on the {111} face), imbri-
cated surfaces (Figure 3e see overlapping of {111} plates)
and step-faces (Figure 3f). Fully developed step-face terrac-
ing (striations) can also occur (Figure 3g).
During mantle residence, changes in growth condi-
tions such as temperature, pressure, redox, carbon availabil-
ity and the presence of other diamonds can distort primary
habits through elongation, flattening or further aggregation
(Figure 3h). Moreover, interaction with metasomatic fluids
or with the host magma during transportation to the sur-
face can trigger dissolution reactions resulting in distinctive
resorption features such as rounded edges (Figures 3i -l),
where octahedra may take on a more shield-shaped appear-
ance with the formation of ditrigonal crystal faces (Figure
3i) and macels a rounded ‘pincushion’ appearance (Figure
3l), as well as new surface features. For example, the stones
may develop serrated lamellae (Figure 3i), triangular fea-
tures called trigons (Figure 3j, k) on the {111} or all planes,
and surface ribbing (Figure 3l).
Further dissolution of primary octahedra can result
in complete transformation into secondary morpholo-
gies. Specifically, the octahedra undergo a staged evolu-
tion through various transitional forms that culminate in
rounded stones with either 24 curved surfaces (tetrahexahe-
dra Figure 3 m, n) or 12 curved surfaces (dodecahedrons
Figure 3o, p). The tetrahexahedral form is distinguished
from the dodecahedrons by the presence of notable medial
lines positioned along the short diagonals of each rhom-
bic ‘face’ which divides it into two distinct planes. These
rounded secondary morphologies exhibit a variety of addi-
tional surface features, including terraces surrounding triad
apices or {111} directions (Figure. 3 m, n) which are inter-
preted as exposed intermittent growth layers of the original
octahedron, common surface smoothing (i.e., Figure 3o),