Limestone probably has the largest
number of commercial applications of all the industrial
minerals. These include construction (aggregate, rail ballast
and dimension stone), mineral fillers (in paper, paint,
plastic, rubber and pharmaceuticals), adhesives, abrasives,
fertilisers, food additives, environmental applications
(acidity neutralisation, flue gas desulphurisation, soil
conditioning and stabilisation), and production of cement, lime
and calcium chemicals.
‘High purity’ limestone
is defined as carbonate rock that contains greater than 97%
calcium carbonate (CaCO3, usually as calcite). It is
often referred to as high-calcium, highly-calcitic or
industrial limestone. Its suitability as a high purity
industrial mineral (sold as calcium carbonate) is defined by
the intended applications, as outlined in specification
agreements between producers and consumers.
These applications define the
required chemical properties (such as lime, silica, magnesia
and iron contents), the physical properties (such as particle
size distribution, colour and surface area) and the mechanical
properties (such as strength, abrasiveness and durability).
Detailed information is available in many industrial mineral
reference sources (Harben, 2002; Kogel et al, 2006;
and BGS, 2006). Limestone resource assessments carried out by
the British Geological Survey (BGS) are guided by these
industrial requirements.
The BGS conducts a reconnaissance assessment
at a high purity limestone deposit in the Middle East
British Geological Survey
Reconnaissance survey
Limestone usually occurs as extensive sedimentary deposits with
a generally consistent composition. Lateral variations reflect
differing depositional environments, with, for example,
coarser-grained, sandy limestone deposited closer to shore and
finer-grained, calcite-mudstone deposited in deeper water.
Mineral impurities that may occur
in limestone include dolomite and other carbonate minerals
(such as siderite), silica (as fine-grained quartz or chert),
clay minerals (such as kaolinite, illite or chlorite), organic
matter (often bituminous), pyrite and fluorite. Trace amounts
of ‘accessory’ minerals such as zircon, tourmaline,
feldspar, iron minerals (haematite, magnetite and limonite),
garnet and titanium minerals (ilmenite, rutile and leucoxene)
may also occur (Summerson et al 1957).
The first stage of a reconnaissance
survey is a review of the existing available geological
information in maps and reports. This information is usually
limited to descriptions of the stratigraphy, lithology and
palaeontology. Apart from the occasional petrographic
description and chemical analysis, there is usually little
information on the technical properties of the limestone.
As far as possible, limestone
resources are categorised into priorities for the
reconnaissance survey field work. The highest priority is given
to those formations with thick, uniform sequences that consist
largely of limestone. The lowest priorities are given to those
formations that do not contain significant amounts of
limestone; have large amounts of dolomite, chert and other
non-limestone mineral impurities; have thin, inconsistent
sequences; and that are interbedded with non-limestone rock
types such as sandstone, siltstone and mudstone. A GIS
(Geographical Information System) reconnaissance map is then
created to incorporate all the available information on geology
including limestone resources (ranked as potentially high,
medium or low purity), topography, satellite imagery and
infrastructure.
The next stage is the
reconnaissance survey field work. This is carried out with the
aim of collecting ‘representative’ samples of all the
limestone formations identified over a wide geographical area.
The location of each locality is recorded using a hand held GPS
(Global Positioning System) device using WGS84 (World Geodetic
System) coordinate system and UTM (Universal Transverse
Mercator) map projection system is used as a standard.
Limestone samples (typically 1.5kg)
are taken from rock outcrops avoiding overly weathered,
fractured and mineralised surfaces. Due to the nature of the
survey work these are surface samples and not taken from
boreholes or as channel samples. Other information recorded
includes descriptions of the lithology and rock mass (including
bedding, jointing, fractures and other discontinuities), name
of the area and accessibility.
Technical evaluation
The samples collected by reconnaissance surveys are analysed to
determine their chemical, mineralogical and physical properties
(Harrison, 1992). The technical evaluation of the data is
informed by the needs of industry and relies on the
availability of technical data and specifications.
The major element oxide chemical
composition is typically determined by X-ray fluorescence (XRF)
analysis. The BGS limestone purity classification (as devised
by Cox et al in 1977) is based on the calcium
carbonate content (Table 1). The BGS has used this
classification in all major limestone resource work carried out
in the UK and other parts of the world over the last 30 years
(Harrison, 1985).
Recent BGS reconnaissance surveys
have added magnesia (MgO), silica (SiO2) and iron
oxide (Fe2O3) as quality criteria, as
shown in Table 1. This was devised by the author based on the
data in industrial mineral reference sources (Harben, 2002 and
Kogel et al, 2006) and in commercial data sheets
(summarised in Table 2). Table 3 gives the chemical composition
of high purity limestone from past BGS reconnaissance
surveys.
The mineralogical composition is
determined by X-ray diffraction (XRD) analysis. This will
determine the presence of calcite and dolomite together with
common impurities such as quartz, feldspars, clay minerals,
pyrite and iron oxides. Thermogravimetric analysis (TGA) is
carried out to determine the carbonate mineral content to lower
levels than those achieved by XRD. Petrographic analysis is
carried out on thin sections using a binocular microscope in
order to provide information on the lithology, fabric, texture
and mineralogy. Samples can be impregnated with blue-dye resin
in order to facilitate identification and description of their
pore space characteristics. Thin sections can be stained using
a standard dual carbonate alizarin red-S and potassium
ferricyanide chemical stain to help differentiate non-ferroan
calcite, ferroan calcite, dolomite and ferroan dolomite.
Commercial data sheets also provide
information on many other properties including moisture, pH,
bulk density, specific gravity, oil absorption, surface area
and particle size (top size, mean size and proportion finer
than two microns). These can be determined for samples
collected in a reconnaissance survey. However, many of these
properties are directly influenced by the particle size of the
test material. This can vary according to the amount and type
of milling carried out. Therefore, these are not as reliable as
chemical properties to indicate the quality for a
reconnaissance survey.
One physical property that has been
included in BGS reconnaissance surveys is the whiteness (or
brightness). This property is also influenced by the particle
size of the test material but it is such an important
commercial parameter that it is included. Brightness is
measured (in accordance with ISO 2470:1977) using a reflectance
spectrophotometer, with the percentage of reflectance being
directly proportional to the ‘whiteness’ and, to some
extent, the purity of the sample. Brightness values of greater
than 80% are a minimum threshold for high purity limestone
samples. The brightness of commercial calcium carbonate
products is shown in Table 2.
BGS limestone resource assessments
The BGS undertook a comprehensive limestone resource survey of
England and Wales in the 1980s. This study produced a
geological map of the limestone resources (1:625,000 scale) and
collated chemical data (as shown in Table 3).
Limestones of high purity are shown
to be extensive in many parts of England and Wales,
particularly in the Carboniferous Limestone of the Peak
District, North and South Wales, North Pennines, Lake District
and Mendips, as well as in the Cretaceous Chalk. It was also
found that many of those areas containing pure limestone also
contain limestones of lower purity or are affected by
mineralisation. This reinforced the importance of a thorough
understanding of the regional geology in resource surveys
(Harrison et al, 1991).
Since the 1990s the BGS has been
involved in limestone resource studies in Thailand, Zambia and
the Middle East. In Thailand, the limestone resource survey was
carried out in collaboration with the Department of Mineral
Resources (Harrison et al, 1998). As part of the work
carried out cost effective field and laboratory procedures were
developed for the rapid assessment of limestone resources. High
purity limestone resources were identified in the Surat Thani
area of southern Thailand (Table 3). In Zambia, a
resource survey was carried out which sampled all of the main
limestone areas (Mitchell et al, 1997). This
identified high purity limestone in the Copperbelt Province
that was being worked by Ndola Lime Co. (Table 3).
Limestone resource work recently
carried out in the Middle East identified over 65 occurrences
of high purity limestone (this work is due to be published).
Some limestone resource work was also carried out by the BGS in
Kabul as part of the institutional strengthening work for the
Afghanistan Geological Survey in 2005 to 2007.
Often high purity dolomite is
identified as a consequence of carrying out a survey for high
purity limestone. In the recent limestone study in the Middle
East, six occurrences of high purity dolomite were identified.
Dolomite was the target for a second resource survey carried
out in Zambia. The aim was to identify suitable raw material
for the production of agricultural lime (Mitchell et
al, 2005). Occurrences of potentially high purity dolomite
were identified in the Central Province.
Conclusions
Limestone resources are widespread in many countries of the
world and are often well documented in geological reports and
maps. The identification of high purity limestone requires a
technical assessment that includes a reconnaissance survey,
field sampling and laboratory analysis. The evaluation of the
technical data relies on a detailed knowledge of the industrial
market needs, typically expressed as technical data sheets and
specifications. It is important that national geological
surveys maintain a detailed knowledge of the use of minerals,
current industrial practice and trends in raw material
usage.
Identification of high purity
limestone resources is just one component of the economic,
environmental and social equation that needs to be solved
before resources can be mined. The role of a national
geological survey is to provide impartial, accurate and
relevant geological and technical information on resources for
mining companies, planning authorities, government and the
public. Ensuring that the mineral resource information remains
in step with current industrial demands and is also available
in accessible formats, especially online, is an ongoing
challenge for all surveys. The BGS provides this information
via its website, www.mineralsuk.com.
Acknowledgements: The author acknowledges the guidance
of David Harrison in reviewing this paper and passing on his
knowledge of limestone resource assessment over the last 20
years. This paper is published by permission of the Director,
British Geological Survey (NERC).
Contributor: Clive Mitchell, Industrial Minerals
Specialist, British Geological Survey, Nottingham, UK. Mitchell
has 22 years of experience of industrial minerals and capacity
building projects in Africa, the Middle East, Thailand and
Afghanistan. He manages the BGS website www.mineralsuk.com and
is currently acting Head of Communications for BGS. cjmi@bgs.ac.uk
References: available on request.