3920 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
to the diamond ore should not be large enough to break
individual stones but instead gradually wear down the sur-
faces resulting in the rounding of stone edges (Daniel and
Morley, 2010).
For economic reasons, breakage descriptions are typi-
cally focused on macro-diamonds—conventionally defined
as those that do not pass through a 0.85 mm square mesh
screen (Canadian National Instrument 43-101 reporting
standards), with small diamonds (5 mm) and micro-
diamonds sensu-stricto (1 mm) receiving much less atten-
tion. This, however, does not negate their vital importance
in the diamond industry. Micro-diamond recovery is inte-
gral to the diamond exploration process and is often the
first confirmation that a specific kimberlite deposit contains
diamonds. Moreover, small diamond populations exist in
overwhelmingly larger concentrations than macro-dia-
monds, and lognormal size-frequency-distribution (SFD)
curves prove crucial in diamond size projections (Sasman
et al., 2018). The smaller stones are also useful proxies for
preservation of macro-diamonds because their bigger sur-
face area to volume ratio leads to a faster rate of reaction
(absorption), when exposed to destructive fluids or melts
during mantle residence and transport to the surface. Thus,
we may anticipate the detection of macro-diamonds by
observing an abundance of small stones.
Diamond morphology studies indicate that small and
micro-diamond populations have different proportions of
specific habits, such as a higher concentration of sharp-edged
octahedra, compared with larger populations (Hagerty,
2019). However, understanding the way each morphology
responds to mechanical breakage could be used as a proxy
for breakage modelling at all sizes. Recognizing that small
diamond particles typically demonstrate greater resistance
to mechanical breakage (i.e., despite withstanding lower
loads, smaller diamond particles exhibit higher breakage
strengths under compressional forces Field in 2012) sug-
gests that significant breakage in micro-diamonds may
serve as an early indicator of potential severe breakage in
larger stones. A comprehensive characterization scheme for
the differentiation of natural versus mechanical diamond
breakage is lacking in the public domain. Moreover, a
notable absence of morphological descriptions alongside
the severity of breakage calculations excludes important
crystallographic factors that may significantly influence the
type and extent of breakage. Moreover, it remains unclear
whether fracture properties in small and micro-diamonds
are comparable to larger stones and if they can be used for
extrapolation of breakage dynamics at larger sizes.
Roberts Victor Mine: Case Study
The Roberts Victor micaceous kimberlite (now classified
as a carbonate-rich olivine lamproite or Kaapvaal lam-
proite Pearson et al., 2019 Scott Smith et al., 2018) was
discovered in 1905 in the Boshof district of the Free State
province, South Africa. After ~40 years of sustained activ-
ity, mining was discontinued in the late 1990s. The geol-
ogy comprises a NE-SW trending dyke and blow system,
erupted at ca. 127 ± 5 Ma (Smith et al., 1985 Gurney
and Kirkley, 1996). The mine was renowned for relatively
high-quality macro-diamonds (mostly pure-white and
blue-white octahedra) that came from a small ~2 ha area of
the dyke sequence called “the jewel box.” According to the
Kjarsgaard et al. (2022) “Tiered structure” approach for the
classification of primary magmatic-hosted diamond mines,
Roberts Victor ranks as Tier 4 ‘marginal’, with an average
grade of 0.45 ct/t (30 to 60 cpht), current stone value of
100 US$/ct, ore value of 45 US$/t and an overall produc-
tion value of ~44 million US$ over the life of mine. Little
is published about the morphological characteristics of the
macro- and micro-diamond complement at Roberts Victor,
and nothing is mentioned about mechanical breakage.
The objective of this paper is to demonstrate that small
(5 mm) and micro-diamonds (1 mm) can be used to
understand mechanical diamond breakage systematics at all
sizes through a conventional processing circuit. A subset of
43 small diamonds (2 mm) were selected from a larger his-
torical Roberts Victor parcel because they exhibited obvious
signs of breakage. This preliminary work uses non-destruc-
tive imaging technologies, including optical and electron
microscopy as well as 3D micro-CT, to first characterize
Figure 2. Diamond breakage classification developed by De Beers
in 1978 and adapted from Chele (2021)
to the diamond ore should not be large enough to break
individual stones but instead gradually wear down the sur-
faces resulting in the rounding of stone edges (Daniel and
Morley, 2010).
For economic reasons, breakage descriptions are typi-
cally focused on macro-diamonds—conventionally defined
as those that do not pass through a 0.85 mm square mesh
screen (Canadian National Instrument 43-101 reporting
standards), with small diamonds (5 mm) and micro-
diamonds sensu-stricto (1 mm) receiving much less atten-
tion. This, however, does not negate their vital importance
in the diamond industry. Micro-diamond recovery is inte-
gral to the diamond exploration process and is often the
first confirmation that a specific kimberlite deposit contains
diamonds. Moreover, small diamond populations exist in
overwhelmingly larger concentrations than macro-dia-
monds, and lognormal size-frequency-distribution (SFD)
curves prove crucial in diamond size projections (Sasman
et al., 2018). The smaller stones are also useful proxies for
preservation of macro-diamonds because their bigger sur-
face area to volume ratio leads to a faster rate of reaction
(absorption), when exposed to destructive fluids or melts
during mantle residence and transport to the surface. Thus,
we may anticipate the detection of macro-diamonds by
observing an abundance of small stones.
Diamond morphology studies indicate that small and
micro-diamond populations have different proportions of
specific habits, such as a higher concentration of sharp-edged
octahedra, compared with larger populations (Hagerty,
2019). However, understanding the way each morphology
responds to mechanical breakage could be used as a proxy
for breakage modelling at all sizes. Recognizing that small
diamond particles typically demonstrate greater resistance
to mechanical breakage (i.e., despite withstanding lower
loads, smaller diamond particles exhibit higher breakage
strengths under compressional forces Field in 2012) sug-
gests that significant breakage in micro-diamonds may
serve as an early indicator of potential severe breakage in
larger stones. A comprehensive characterization scheme for
the differentiation of natural versus mechanical diamond
breakage is lacking in the public domain. Moreover, a
notable absence of morphological descriptions alongside
the severity of breakage calculations excludes important
crystallographic factors that may significantly influence the
type and extent of breakage. Moreover, it remains unclear
whether fracture properties in small and micro-diamonds
are comparable to larger stones and if they can be used for
extrapolation of breakage dynamics at larger sizes.
Roberts Victor Mine: Case Study
The Roberts Victor micaceous kimberlite (now classified
as a carbonate-rich olivine lamproite or Kaapvaal lam-
proite Pearson et al., 2019 Scott Smith et al., 2018) was
discovered in 1905 in the Boshof district of the Free State
province, South Africa. After ~40 years of sustained activ-
ity, mining was discontinued in the late 1990s. The geol-
ogy comprises a NE-SW trending dyke and blow system,
erupted at ca. 127 ± 5 Ma (Smith et al., 1985 Gurney
and Kirkley, 1996). The mine was renowned for relatively
high-quality macro-diamonds (mostly pure-white and
blue-white octahedra) that came from a small ~2 ha area of
the dyke sequence called “the jewel box.” According to the
Kjarsgaard et al. (2022) “Tiered structure” approach for the
classification of primary magmatic-hosted diamond mines,
Roberts Victor ranks as Tier 4 ‘marginal’, with an average
grade of 0.45 ct/t (30 to 60 cpht), current stone value of
100 US$/ct, ore value of 45 US$/t and an overall produc-
tion value of ~44 million US$ over the life of mine. Little
is published about the morphological characteristics of the
macro- and micro-diamond complement at Roberts Victor,
and nothing is mentioned about mechanical breakage.
The objective of this paper is to demonstrate that small
(5 mm) and micro-diamonds (1 mm) can be used to
understand mechanical diamond breakage systematics at all
sizes through a conventional processing circuit. A subset of
43 small diamonds (2 mm) were selected from a larger his-
torical Roberts Victor parcel because they exhibited obvious
signs of breakage. This preliminary work uses non-destruc-
tive imaging technologies, including optical and electron
microscopy as well as 3D micro-CT, to first characterize
Figure 2. Diamond breakage classification developed by De Beers
in 1978 and adapted from Chele (2021)