DEYSEL, K. Leucoxene study: a mineral liberation analysis (MLA) investigation. The 6th International Heavy Minerals Conference ‘Back to Basics’, The
Southern African Institute of Mining and Metallurgy, 2007.
Leucoxene study: a mineral liberation analysis (MLA)
investigation
K. DEYSEL
Richards Bay Minerals, Natal, South Africa
Determination of mineral composition is of significant importance for the mineral sands industry
as this information is required for orebody evaluation, the control of mineral processing plants and
the determination of product quality. Mineral processing is made more difficult by the presence of
pseudorutile, leucoxene, altered-ilmenite and particle inclusions. The paper investigates the
properties of such elements from a single product stream using an MLA approach.
Introduction
Richards Bay Minerals, situated just north of Richards Bay,
is currently mining unconsolidated aeolian deposits in a
coastal dune cordon. Economic heavy minerals recovered
include ilmenite, rutile and zircon, of which ilmenite is the
abundant ore mineral present and is the largest global
resource of titanium (Hugo and Cornell, 1991). Ilmenite has
a wide range of chemical compositions because Mg2+, Mn2+
and Fe3+ can substitute for Fe2+ in the ilmenite structure
(Hugo, 1993). The mineral may also contain small
quantities of Cr, Zn, Cu, Al, Si and Ca. High levels of these
elements lower the quality of an ilmenite concentrate
because they decrease the grade of TiO2 and represent
unwanted impurities in products. The mineral also forms a
large variety of intergrowths with other iron-titanium
oxides as a result of exsolution, oxidation or hydrothermal
processes. Ilmenite ‘locked’ with these phases has different
chemical compositions and physical properties to
homogeneous ilmenite (Hugo, 1993). The alteration process
of ilmenite in the deposit involves the removal of iron from
the crystal lattice, resulting in a TiO2 enriched product
(Bailey et al., 1956; Temple, 1966). The alteration process
does not only affect the overall grade, but changes the
magnetic susceptibility (Temple, 1966; Frost et al., 1986)
and the density of the mineral (Temple, 1966), thereby
affecting the mineral behaviour in a specific plant
environment. Studies have indicated that SiO2 and Al2O3
impurity levels in the grains increase with increasing
alteration and therefore have a direct bearing on the quality
and recoverability of ilmenite (Frost et al., 1986; Hugo and
Cornell, 1991). This paper will focus on the properties of
leucoxene species present in a single RBM product stream
using an MLA approach.
Provenance of coastal heavy minerals
The abundance and distribution of heavy minerals is related
to the geology, physiography and coastal dynamics of the
region. These factors have resulted in the detritus from
various rock types being transported by rivers, dispersed
along the coast by currents and concentrated on beaches
and dunes by wave action and aeolian processes
respectively (Ware, 2003). The mineralogy of coastal heavy
minerals indicates that the provenance is mainly from rocks
of the Karoo Igneous Province and the KwaZulu-Natal
basement. The sandstones of the Vryheid Formation, and to
a lesser extent the unconsolidated sands of the Berea and
Port Durnford Formations, are important sources of
recycled ilmenite (Hammerbeck, 1976).
The primary source of ilmenite is from rocks belonging to
the basement rocks of the Kaapvaal Craton and the Natal
Metamorphic Province, as well as Karoo and post-Karoo
volcanics (Hammerbeck, 1976; Fockema, 1986; Hugo,
1990). Studies done by Hugo (1990 and 1993) have
indicated that rutile is solely derived from the Natal
Metamorphic Province. Zircon is thought to have primarily
been derived from the Natal Metamorphic Province and the
Kaapvaal Craton (Fockema, 1986). Work done by
Whitmore et al. (2002) suggested three distinct zircon
populations including Natal Metamorphic Province,
650–500 Ma Pan-African belt, and zircons from a unknown
provenance.
Nomenclature
No standard nomenclature exists for the ilmenite alteration
products, and the following nomenclature will be used:
• Ilmenite—homogeneous, hexagonal-trigonal minerals,
unaltered ilmenite grains with a composition close to
the theoretical formula (FeTiO3)
• Pseudorutile—a deformed hexagonal mineral formed
by the alteration of ilmenite, whose composition
approximates Fe2Ti3O9 (Deer, Howie, and Zussman,
1992)
• Leucoxene—an industrial term used for the alteration
products of all titanium-bearing minerals. These
leucoxene species will contain very fine to fine
intergrowths of pseudorutile or rutile with quartz and
other silicates, which could include clays (such as illite
or kaolinite, and at times possibly smectite). A whole
host of minerals can be deposited in the ‘open’
structure of pseudorutile/secondary rutile
• Rutile—optically homogeneous mineral, with
composition of essentially pure TiO2 (Deer, Howie, and
Zussman, 1992).
The alteration process
Most of the ilmenite in Holocene dunes along the east coast
of South Africa occurs as homogeneous, unaltered grains,
but evidence of three alteration mechanisms has been
studied (Hugo, 1988):
LEUCOXENE STUDY: A MINERAL LIBERATION ANALYSIS (MLA) INVESTIGATION
167
• Type I—the gradual weathering of ilmenite to
leucoxene via hydrated ilmenite and pseudorutile,
occurring in a groundwater environment
• Type II—the direct weathering of ilmenite to leucoxene
in sediments above the water table and
• Type III—the alteration of ilmenite to hematite plus
rutile in source rocks.
For the purpose of this study, the focus will be on Type I
and Type II alteration mechanisms.
Type I alteration
The alteration, as described by various authors, begins as
irregular patches of hydrated ilmenite along the grain
boundaries and weakened areas within the grain, or as
orientated stringers along the basal cleavage planes of the
ilmenite (Hugo and Cornell, 1991). Various types of
analyses have indicated that the altered areas within the
ilmenite have variable TiO2 contents and that the increasing
TiO2 content corresponds with a decrease in iron content.
Increases in the Al2O3 and SiO2 content are also noted with
the alteration process (Hugo and Cornell, 1991).
The second stage of alteration is also marked by the
development of leucoxene. Leucoxene in most instances
develops from hydrated ilmenite or pseudorutile but may
also replace the ilmenite. This alteration type may be
explained by the two-stage model of Grey and Reid (1975),
where the stage involves the oxidation of all the ferrous
iron and the leaching of one third of the ferric iron from the
ilmenite lattice by electrochemical corrosion and is
considered to occur in a mildly acidic groundwater situation
(Hugo and Cornell, 1991):
[1]
The second stage of alteration beyond pseudorutile occurs
via a dissolution reprecipitation process whereby both the
iron and titanium are dissolved but the iron is leached while
the titanium is redeposited. This leads to the formation of
rutile in beach deposits and is believed to occur in the nearsurface regions of a deposit (Grey and Reid, 1975):
[2]
In accordance with [1], where hydrated ilmenite and
pseudorutile develop from ilmenite in the Zululand
deposits, these phases appear to be readily displaced by
leucoxene (Hugo and Cornell, 1991). This suggests that
pseudorutile is unstable in the deposit and alters readily to
leucoxene, according to process [2].
Type II alteration
Altered ilmenite and ilmenite are observed altering directly
to leucoxene in this type of alteration (Hugo and Cornell,
1991). The alteration occurs from the grain boundaries
along weaknesses or, in some cases, the leucoxene may
grow as replacement fronts across grains. The boundary
between the ilmenite and leucoxene may consist of porous
areas of microcrystalline leucoxene, and some of these
pores indicate silica and aluminium contents (Hugo and
Cornell, 1991). This style of ilmenite alteration to
leucoxene or rutile may be expressed as:
[3]
The paragenesis of ilmenite alteration is significant as the
site of alteration will affect the distribution and proportions
of various types of alteration in the deposit. A fairly random
168
distribution of altered grains is expected if the alteration
process occurred before final deposition (Hugo and Cornell,
1991). However, if alteration occurred after deposition,
altered grains may be concentrated in areas of the deposit
where conditions are most conducive to alteration. Various
ilmenite alteration products are undesirable as they
contribute to the impurity levels and ultimately affect the
local grade and volume of recoverable material. As
illustrated, strongly magnetic ilmenites have significantly
higher TiO2 content, whereas magnetic leucoxenes tend to
have relatively higher iron content, while non-magnetic
leucoxenes have relatively higher SiO2 content.
It is found that the RBM deposit contains a varied and
petrographically complex suite of altered ilmenite grains.
The Type I alteration process appears to be the dominant
mechanism of alteration although all three types of
alteration mechanisms are evident. This, together with other
petrographic and mineralogical data indicates that some
ilmenite alteration occurred prior to deposition and that
dune reworking has blended the alteration products (Hugo
and Cornell, 1991).
Further studies (Merret, 1998) have indicated that
ilmenite alteration increases with increasing dune depth and
that several dune horizons within the orebody also affect
the ilmenite alteration throughout the orebody. Because
alteration types do not show statistically uniform
distributions within the deposit, it is believed that most of
the alteration occurs in situ. As the alteration process
proceeds to more advanced stages, Si, Al, Mg, Mn, Ca, K,
P and Na are found. These impurities are present in larger
quantities and have an effect on the separation process.
The effect of alteration
Numerous studies including MLA analysis (Temple, 1966;
Frost et al., 1986; Hugo and Cornell, 1991) have shown that
the magnetic susceptibility of ilmenite decreases with
increasing alteration, and findings indicate the following:
• The ilmenite-pseudorutile grains are slightly less
magnetic than unaltered ilmenite and will in most cases
report to the magnetic concentrate fraction
• Altered ilmenites containing leucoxene or TiO 2
pseudomorphs have a large range of magnetic
susceptibility and may report to either the magnetic
concentrate or the magnetic middlings fraction
• Leucoxenes have a large range of magnetic
susceptibility extending from that of ilmenite to nonmagnetic rutile, and that most leucoxenes report to the
magnetic middlings and the non-magnetic fractions.
Studies by Hugo (1988) have indicated that three groups
of leucoxene exist, namely:
• Magnetic leucoxenes (mags), which have a similar
magnetic susceptibility to ilmenite
• Magnetic middling leucoxene (mids), which have a
magnetic susceptibility between that of ilmenite and
rutile
• Non-magnetic leucoxenes (NM), which have a similar
magnetic susceptibility to rutile.
This last fraction is divided into the following
subdividions:
• NM leucoxenes having a specific gravity less than 3.6
g/cm3
• NM leucoxenes having a specific gravity between 3.6
and 4.2 g/cm3
• NM leucoxenes having a specific gravity greater than
4.2 g/cm3.
HEAVY MINERALS 2007
Sampling and analytical techniques
The study material consisted of a representative sample
taken from one of RBM’s product streams (rutile special
grade product stream consisting of mostly rutile). The
sample was split (based on mass) into four fractions using a
high intensity magnetic separator (HIMS/ IRM), resulting
in samples: L1 (very non-magnetic material, mostly
consisting of rutile), L2 (the remainder of the nonmagnetic material), L3 (middling materials associated with
this product) and L4 (the more magnetic material associated
with this product). The sample consisting of the very nonmagnetic material (L1) was not used for the purpose of this
investigation as the sample contained virtually no
leucoxene species. The main focus for this study was to
identify ilmenite alteration species (specifically leucoxenes)
associated with very non-magnetic product streams looking
specifically at sample fractions L2, L3 and L4. These
sample fractions were analysed by MLA.
The mineral liberation analyser (MLA) combines an
automated scanning electron microscope (SEM), multiple
energy dispersive X-ray detectors with state-of-the-art
automated quantitative mineralogy software developed by
JKMRC/ FEI. With the MLA ore particle sections can be
analysed to better understand, optimize and predict mineral
processing circuit performance. Information about the
liberation distribution of the minerals is vital in determining
whether inefficient separation is the result of the presence
of unliberated particles or as a result of poor mechanical or
separator performance. Geometallurgical and ore
characterization information such as mineral association
data and grain size distribution are essential to assist with
the optimization of plant feed quality by avoiding
metallurgically poor feed stock or by facilitating effective
ore blending. The quantitative chemical analysis of the
altered grains was performed using the MLA.
Discussion
The three study samples were analysed on the MLA and
their images quantified. Leucoxene, rutile and zircon were
the main mineral species identified. Leucoxene is
prominent in the non-magnetic fraction (13%), the
middlings fraction (14%) and the magnetic fraction (22%)
as shown in Figure 1. The full quantitative assessment of
the three sample fractions is shown in Table I.
The particle density distribution indicates the presence of
particles of various densities, as shown in Figure 2. Most of
the particles from the different samples fractions have a
density of around 4.3 g/cm3, i.e. 68%, 67.8% and 48.3%
respectively to the NM, mids and mags fractions. It appears
that most of particles (48.3%) in the Mags fraction reported
to a density of 4.3 g/cm 3, while 20.9% reported to the
3.8 g/cm3 and 10.3% to the 4.7 g/cm3 respectively. This
could be because 22% of the leucoxene species reported to
the mags fraction and resulted in the broader range of
densities of these particles. Leucoxene species have a much
broader spectrum of densities as the result of their chemical
composition cased by alteration.
The focus of the study was to investigate the mineral
species that constitute the leucoxene group and to
investigate these species individually focusing specifically
on modal mineralogy that reflects the chemistry of the
sample. Figure 3 shows the X-ray spectra for some
leucoxene species and confirms the presence of varying
proportions of aluminium and silicon in titanium-rich
leucoxenes.
At RBM the leucoxene mineral group consists of three
mineral sub-species based on the difference in chemical
composition (see Figure 4 and Table II).
Rutile leucoxene was the most abundant leucoxene
species present in the three study samples, i.e. with 12.25%
in the NM, 13.12% in the mids and 18.83% in the mags
Modal abundance
100%
80%
60%
40%
20%
0%
L2_NM
L3 _MIDS
Study material
L3_MAGS
Figure 1. Mineral abundance of study species
Table I
Mineral abundance of fractions used for the study
Mineral
L2—Nm
Ilmenite
Leucoxene
Rutile
Zircon
Titanite
Epidote
Quartz
Feldspar
Clays
Phos—Apatite
Phos—Xenotime
Phos—Monazite
FeOxide
Others
Total
0.01
12.94
76.72
7.30
1.13
0.00
0.90
0.02
0.34
0.00
0.00
0.00
0.30
0.32
100.00
L3—MIDS
0.01
13.85
75.34
7.73
1.04
0.00
0.90
0.03
0.36
0.02
0.00
0.00
0.36
0.35
100.00
L4—Mags
0.11
22.30
56.59
11.44
3.16
0.13
0.85
0.03
0.73
0.00
0.01
0.07
3.41
1.09
100.00
Percentage
Detailed microprobe analysis (Hugo and Cornell, 1991)
and MLA analysis have indicated a significant increase in
the SiO2 and Al2O3 content of the ilmenite alteration phases
(referred to as siliceous leucoxenes) and agrees with
findings by Frost et al. (1986). It is therefore evident that
altered ilmenite and leucoxenes will contribute to the silica
and aluminium impurity levels in the product streams.
Density (wt%)
Figure 2. Particle density distribution
LEUCOXENE STUDY: A MINERAL LIBERATION ANALYSIS (MLA) INVESTIGATION
169
D50
The BSE images (Figures 6–11) illustrate that a
significant amount of ilmenite alteration products
(especially leucoxene) are present within the RBM product
stream and that various ilmenite alteration stages are
present. These mineral species will have a significant effect
on product quality as well as on the overall mineral
processing as the behaviour of these mineral species is
difficult to determine.
Figure 3. ED spectra of a leucoxene grain indicating the presence
of Al, Si, K
Sample species
Percentage
Figure 5. Comparison of the mineral grain size distribution
among the different leucoxene species
Wt%
CK
18.91
OK
22.63
AlK 02.87
TaM 10.90
TiK
40.44
FeK 04.12
Study Samples
Figure 4. Leucoxene species abundance in the study samples
Table II
Comparative Assay analysis for the sample fractions
Element
Al
Ca
Fe
Mg
O
Si
Ti
Zr
L2—Nm
0.55
0.29
1.71
0.08
39.41
2.63
51.54
3.55
L3—Mids
0.59
0.28
1.83
0.09
39.38
2.73
51.07
3.76
L4—Mags
0.93
0.85
5.22
0.13
38.32
4.17
44.10
5.57
fraction (as illustrated in Figure 4). Pseudorutile leucoxene
reported mostly to the mags fraction, whereas the siliceous
leucoxene reported relatively evenly to all the sample
fractions.
It appears that the siliceous leucoxene in the NM and
mids fractions is much finer in comparison with the mags
fraction. This might suggest that they are possibly
inclusions rather than fully liberated particles, whereas all
the species reporting to the mags fraction appear to be fully
liberated because of their similar grain size distribution
pattern.
170
Figure 6. BSE image of a pseudorutile leucoxene grain from
sample L2 NM fraction illustrating the nature of complex
alteration
Elem.
CK
OK
AlK
SiK
TiK
FeK
Wt%
22.70
23.80
01.01
01.47
42.51
08.51
Figure 7. BSE image of a pseudorutile leucoxene grain inclusion
within a zircon grain from sample L2 NM fraction
HEAVY MINERALS 2007
Figure 8. MLA classified image of a highly altered grain from
sample L2 NM fraction
Figure 11. BSE image of a highly altered (siliceous) leucoxene
grain, porous leucoxene consisting of thin, prismatic
microcrystals resembling rutile, taken from sample
L4 mags fraction
Conclusion
Figure 9. BSE image of stages in the alteration process in a single
grain from sample L2 NM fraction
The main focus of the study was to identify ilmenite
alteration species, specifically leucoxenes, which are
associated with very non-magnetic product streams.
Leucoxene is prominent in the non-magnetic fraction
(13%), the middlings fraction (14%) and the magnetic
fraction (22%) of the non-magnetic product stream as the
magnetic susceptibility decreases with increasing alteration.
At RBM the leucoxene mineral group consists of three
mineral sub-species, namely: siliceous leucoxene, rutile
leucoxene and pseudorutile leucoxene. The mineral subdivisions are based on differences in chemical composition
caused by alteration processes. Rutile leucoxene is the most
abundant leucoxene species within the study material with
12.25% in the L2 NM, 13.12% in the L3 mids and 18.83%
in the L4 mags sample. The alteration process alters the
chemical composition of the mineral species, thereby
affecting the particles’ magnetic susceptibility as well as
the density. Most of the leucoxene sub-species in the
sample fractions have a density of between 3.6 and 4.3
g/cm3 as indicated by previous studies. Observations have
shown that as density of the minerals increases, a general
increase in TiO2 and a corresponding decrease in SiO2
occur. The mineral grain size also indicates that the sizing
of the individual particles influences the mineral separation.
The SEM analysis also revealed that SiO2 in the siliceous
leucoxenes occurs as coatings and mostly as infillings in
cavities or pores or as distinct highly siliceous areas in
inhomogeneous leucoxene grains.
References
BAILY, S.W., CAMERON, E.N., SPEDDEN, H.R., and
WEEGE, R.J. The alteration of ilmenite in beach
sands. Econ. Geol., vol. 51, 1956. pp. 263–279.
Figure 10. BSE image of a grain consisting almost entirely of
rutile leucoxene from sample L3 mids fraction
DEER, W.A., HOWIE, R.A., and ZUSSMAN, J. An
introduction to the rock-forming minerals. 2nd
Edition. Prentice Hall. Pearson Education Limited,
England. 1992.
LEUCOXENE STUDY: A MINERAL LIBERATION ANALYSIS (MLA) INVESTIGATION
171
FOCKEMA, P.D. The heavy mineral deposits north of
Richards Bay. Anhaeusser, C.R. and Maske, S. (eds.).
Mineral Deposits of South Africa. Geol. Soc. S. Afr.,
Johannesburg, 1986. pp. 2301–2307.
FROST, M.T., GREY, I.E., HARROWFIELD, I.R.,
MASON, K. and LI, C. Alteration profiles and
impurity element distribution in magnetic fractions of
weathered ilmenite. Amer. Mineral., vol. 71.
1986. pp. 167–175.
GREY, I.E. and REID, A.F. The structure of pseudorutile
and its role in the natural alteration of ilmenite. Amer.
Mineral., vol. 60. 1975. pp. 898–906.
HAMMERBECK, E.C.I. Titanium. Coetzee, C.B. (ed.),
Mineral Resources of the Republic of South Africa.
Geol. Surv. S. Afr., Handk., 7, 5th Edition, 1976.
pp. 221–226.
HUGO, V.E. A mineralogical and chemical study of
titanium losses at Richards Bay Minerals. M. Sc.
Thesis (unpubl.), Univ. Natal, Durban. 1988.
HUGO, V.E. The relative abundance and alteration of irontitanium oxides along the South African coastline.
Abstracts Geocongress ’90, Geol. Soc. S. Afr., Cape
Town, 1990. pp. 261–264.
HUGO, V.E. A study of titanium-bearing oxides in heavy
172
mineral deposits along the east coast of South Africa.
PhD thesis (unpubl.), Univ. Natal. Durban. 1993.
HUGO, V.E. and CORNELL, D.H. Altered ilmenites in
Holocene dunes from Zululand, South Africa:
petrographic evidence for multistage alteration. S.
Afr. J. Geol., vol. 94, nos. 5/6, 1991. pp. 365–378.
MÜCKE, A. and CHAUDHURI, J.N. The continuous
alteration of ilmenite through pseudorutile to
leucoxene. Ore Geol. Rev., vol. 6, 1991. pp. 25–44.
MERRETT, G. The alteration of ilmenite in the Zulti North
orebody at Richards Bay Minerals. B.Sc (Honours)
Thesis (unpubl.), Univ. Natal. Durban. 1998.
TEMPLE, A.K. Alteration of ilmenite. Econ. Geol. vol. 61.
1966. pp. 695–714.
WARE, C.I. Evolution of the northern KwaZulu-Natal
coastal dune cordon; evidence from the fine-grained
sediment fraction. PhD thesis (unpubl.). Univ. Natal.
Durban. 2003.
WHITMORE, G., ARMSTRONG, R., UKEN, R., SUDAN,
P., and WARE, C.I. Evolution of the KwaZulu-Natal
coastal dunes: evidence from zircon provenance. 16th
International Sedimentological Congress, Rand
Afrikaans University, South Africa, July 2002, 2002.
405 pp.
HEAVY MINERALS 2007