Garnet is a group of rock-forming minerals which share
a common crystal structure and a similar yet diverse chemical composition (X3Y2(SiO4)3).
The garnet consists of SiO4 and is therefore, a silicate
mineral. The X in the above formula can be calcium (Ca), magnesium (Mg), Iron
(Fe2+) or manganese (Mn2+). The “Y” in the
chemical formula can be aluminum (Al), iron (Fe3+), manganese (Mn3+),
vanadium (V3+) or chromium (Cr3+) (King, n.d). The
specific gravity of garnets typically range between 3.4 to 4.3 and they have a
wide range of colors with red and blue being the most and least common,
respectively (Database, n.d; King, n.d; Myers and Anderson, 1925). Its crystal
structure is classified as isometric which means that the mineral has a four
three-fold rotation and closed crystal forms. A garnet’s hardness typically
ranges between 6.5 to 7.5 in Mohs scale but crystallized garnet generally has a
hardness of 7.5 which is harder than quartz but softer than topaz (Myers and
Anderson, 1925; Amfed.org, n.d).
The most common garnets are classified into three groups- aluminum
garnets, iron garnets and chromium garnets. The aluminum garnets consist of
almandine, pyrope, grossularite and spessartite. The iron garnet includes
andradite and the chromium garnet consists of uvarovite. Garnet can be found in
the United Sates, China, India, Germany, Russia and Tanzania; and in sedimentary,
metamorphic and igneous rocks (Wood n.d). Although garnet can be found in all
three rock types, it is principally found in metamorphic rock and more
specifically, gneisses, schists and marble (King, n.d; Olson n.d).
The method of extracting garnet varies depending on
the geological environments responsible for the host rock. Barton Mines in New York operate as an
open pit mine while in Idaho’s Emerald Creek Mines, garnet is extracted from
alluvial deposits. Garnet is more easily extracted from alluvial
deposits using backhoes or small draglines to cut slots into stream gravels
which expose the garnet. The garnet is collected and transported to the mill
for final processing and packaging (Database, n.d). Hard rock mining as
exemplified at the Barton Mines, utilizes drilling and explosive methods to
liberate the crystals from the parent rock without shattering them. The broken
ore deposits are loaded into cars and transported to the mill (Myers and
Anderson, 1925). In general, the processing of the garnet at the mill involves
the separation of the heavier garnet from the lighter garnet minerals and then
further separation of the minerals based on grain size. When the material
reaches the mill, the ores are passed through a jaw crusher and then a trommel.
The trommel separates the heavier and larger minerals from the smaller and
lighter minerals. Once the material has been separated by grain size, spiral
classifiers along with hydrosizer are used to wash the material by specific
gravity. Once washed, the concentrates are dried and sorted further using
magnetic and electrostatic separators. During magnetic separation, minerals are
separated by their differences in magnetic properties. Electrostatic separation
uses high voltage to separate particles of different electrical charge (ions)
(Encyclopedia Britannica, n.d). The purpose of separating the mineral is to
meet the needs of specific markets and the end use of the mineral (Myers and
Anderson, 1925; Database, n.d).
Historically, people are most familiar with garnet due
to its use as a gemstone, although only a low percentage of garnets are
actually pure enough to be used for jewelry. (Database, n.d; King, n.d; Olson,
n.d.). The physical properties of garnet which include angular fractures and
high hardness (6.5-7.5 on Mohs scale), and that it can be recycled, make it an attractive mineral for
industrial usage. The four main industrial uses of garnet are abrasive
blasting, filtration, abrasive powder and waterjet cutting. Waterjet cutting is
the largest industrial use of garnet in the United States. A combination
of water entrained with abrasive granules are blasted out of a waterjet cutting
machine and directed at metal, ceramic or stone. This process can cut with
little, to no, dust. Similarly to waterjet cutting, abrasive blasting (sand
blasting) is the process of propelling abrasive granules at a surface along
with highly pressurized air or water. Sandblasting is used to smooth, clean or
remove oxidation products from various materials including metal and stone. It
is much faster than manually sanding by hand or using a sanding machine. Due to
garnets hardness and angular pieces with sharp edges, it assists in the cutting
of material during the waterjet cutting and sandblasting processes. Garnet
minerals processed to a size of approximately 0.3 millimeters can be used to
filter out contaminants in water. An interesting use of garnet is that it can
be used as a geological indicator. Garnets can be formed in the mantle and
brought up to the surface during deep-source volcanic eruptions. During these
eruptions, xenoliths are formed which are a source of diamonds. There are many
more garnet minerals for every diamond mineral within an xenolith. The deep
source garnets formed during these volcanic eruptions are different than the garnets
formed at shallower depths and thus, these deeply formed garnets can be used as
indicator minerals for the presence of diamonds (King, n.d).
Halite otherwise known as salt is primarily a
sedimentary mineral that consists of sodium chloride (NaCl). A rock that
consists primarily of halite is known as rock salt (Database, nd; King, nd). Halite forms from
the evaporation of seawater in arid environments where the influx of saltwater
is low compared to evaporation. Although outcrops of halite are found in very
arid environments, subsurface halite deposits have been found during oil exploration.
Currently, halite forms along the coastlines of Great Salt Lake in Utah and the
salt pans in Death Valley located in west Texas. Outside of the United States,
halite forms in the Dead Sea, located between Jordan and Israel. Halite can be
recognized by a white streak and vitreous luster. When pure, halite appears
colorless or white but impurities can create a variety of colors which include
yellow, gray, black, brown and red. Halite is associated with other minerals
including calcite and sylvite. Although halite can be identified based on its
salty taste, it is advised that you should not directly lick the mineral as
sylvite is poisonous. Instead, moisten or lick your finger, place finger on the
mineral, and taste the residue left on your finger. Halite has a hardness of 2
to 2.5 on the Moh’s scale and has an isometric crystal structure, similar to garnet.
Halite feels rather light and has a specific gravity of between 2.1 to 2.6
(Esci.umn.edu, nd; Pellant, 2002 pp 70).
Halite can be extracted or mined through solution
mining, dry mining, seawater evaporation and inland evaporation. Solution
mining is the process in which freshwater or a solvent is injected into the
salt beds to dissolve the salt. The saturated brine is then pumped to the
surface and allowed to dry and evaporate. Dry mining involves mining the salt in
underground caverns, often using the room-and-pillar method (Database, nd;
Geo.msu.edu, nd). When the salt is extracted from the rock, it creates a “room”
and the surrounding material that is untouched creates the “pillar”. The first
step of dry mining is undercutting. Large machines cut a slot which is in
excess of 10 feet in depth across the bottom of a salt wall. Following
undercutting, holes are driven into the salt wall and loaded with explosives.
Salt is then recovered and sent through a crusher and a conveyor belt. The salt
is then transported to the surface where it is screened by size using
mechanically operated screens. Once separated, the salt is placed into bins of
varying sizes to await packaging and shipping (Morton Salt, nd). Seawater
evaporation can take between two to five years before any salt is ready for
harvest. Seawater contains a number of constituents other than halite including
Ca, Mg, and Fe compounds. The first step is to concentrate the brine and to
increase the salinity in order to allow the Ca, Mg and Fe compounds to
precipitate out of solution. The seawater circulates between the
interconnecting gravity fed pounds with salinity increasing with each transfer
which allows the impurities to precipitate out of solution. By the last pond,
the majority of the impurities have precipitated out of solution leaving a
relatively pure salt product (Database, nd; Madehow.com, nd). The salt is then
picked up by machines and washed with highly saturated saltwater. The salt
water is used to wash any residual impurities off the salt without dissolving
it. It is further washed with freshwater and stored in piles for two to three
months to drain (Madehow.com, nd; Morton Salt, nd). The fourth and final method
of salt extraction is inland solar evaporation. The processes involved in
seawater evaporation and inland solar evaporation are similar; however,
salinity in inland lakes is much higher than on the coast. Lakes are
topographically and potentiometrically lower than the surrounding area.
Subsequently, water flowing over the surface and beneath the ground flows
towards the lakes and dissolve minerals along the way and therefore, become
great sources of minerals. Inland solar evaporation provides a yearly harvest
as opposed to the seawater evaporation which provides a yield every two to five
years and thus, is more productive (Database, nd).
Today, the largest use of halite in North America is
in the chemical industry which uses it to produce chlorine, sodium hydroxide,
soda ash, hydrochloric acid and metallic sodium. Salt is also used as a de-icer
on roads. Salt has a lower freezing point than water and therefore, when
applied to snowy, icy roads melts the snow and ice. The chemical and
transportation industries account for over 75% of salt consumption (Database,
nd; esci.umn.edu, nd; King, nd). Human consumption of salt (table salt)
accounts for a marginal percentage of total consumption.
As halite is a soft rock, it deforms under pressure
creating Salt domes. The movement of the salt deposit upward due to pressure
results in the deformation of overlying rocks that can trap oil and gas. Salt
domes formed along the Gulf Coast of Texas and Louisiana have been good reservoirs for hydrocarbons and have
been exploited by the oil and gas industry (esci.umn.edu, nd; Keller, 2011 pp
is a sulfate (SO4) mineral that is found in igneous and sedimentary
rocks. Gypsum is found in gypsum rock which forms most commonly within layered
sedimentary rock and more specifically limestone. It is most commonly formed in
lagoons in which seawater high in Ca and SO4 evaporates and the
evaporated water can be readily replenished. Gypsum can be found in a variation
of colors including white, gray, brown, beige, orange, pink, yellow, light red
and green; it can also be colorless. Gypsum has a white streak, vitreous luster
and is soft (1.5 to 2.0 Mohs scale). Gypsum has a monoclinic crystal system
which means that it has a two-fold rotation (Database, nd; King, nd;
Minerals.net, nd;). Gypsum is found in over 85 countries; the United States,
Canada and Mexico having some of the largest reserves of high-quality gypsum.
Within the United States, Iowa, Texas, Utah and Mexico are the highest
producing States (Gypsum Association, nd).
extract natural gypsum rock deposits underground, a drill or a cutter breaks
away the face of the gypsum rock. Alternatively, when deposits are located
closer to the surface, the open-pit mining method is adopted. Benches are
drilled and blasted using ammonium nitrate. In both instances, once the mineral
is extracted, it is then crushed into smaller pieces and passed through a
homogenizer where the gypsum is graded and sorted. The gypsum is further
crushed prior to calcination. Gypsum is a hydrous calcium sulfate (CaSO4
2H2O) and thus has water in its structure. Calcination is the
process of removing the water from the gypsum and once finished produces a
product resembling plaster powder. The plaster powder is transported to the mill
where the particle size of the plaster is further modified to suit the type of
plaster being produced. The last stage in the process is the inclusion of
additives which give each mix of plaster its own specific properties. The final
stage is the delivery of the plaster to merchants and distributors (British-gypsum.com,
nd; Database, nd).
can also be produced as a by-product of flu-gas desulfurization (FGD),
otherwise known as FGD gypsum. Flu-gas produced in the fossil fuel power plants
can be captured from smoke stacks and purified into a hard material and manufactured
into gypsum. Natural gypsum and FGD gypsum have the same chemical composition;
however, FGD gypsum production encourages power plants to recycle and capture
its waste that would otherwise be disposed of in a landfill. The United States
Environmental Protection Agency (USEPA) and U.S. Green Building uses FGD gypsum
board in their offices due to the environmental benefits (Gypsum Association,
common use of gypsum is wallboard and plaster products. However, there are a
variety of other uses for gypsum including plaster of Paris, a hardening
retarder in Portland cement, glass manufacturing and ornamental purposes. It is
important to note that pure gypsum is required for glass manufacturing and that
gypsum is used in a limited capacity for ornamental purposes due to its low
hardness. The United States consumes the most amount of gypsum in the world which
has been estimated to be upwards of 30 billion square feet per year (Database,
nd; King, nd).
Amfed.org. (n.d). Mohs Scale of Hardness.
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https://mineralseducationcoalition.org/minerals-database/garnet/ Accessed 24
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https://mineralseducationcoalition.org/minerals-database/gypsum/ Accessed 29
Database, M. (n.d.). Salt/Halite
| Minerals Education Coalition. online Minerals Education Coalition.
Available at: https://mineralseducationcoalition.org/minerals-database/salthalite/
Accessed 28 Jan. 2018.
Encyclopedia Britannica. (n.d). Mineral processing
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https://www.esci.umn.edu/courses/1001/minerals/halite.shtml Accessed 28 Jan.
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mining: mining part. online Available at:
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https://www.gypsum.org/about/gypsum-101/what-is-gypsum/ Accessed 29 Jan.
(2011). Environmental Geology. 9th ed. Boston: Pearson Prentice Hall,
pp. 406 and 415.
King, H. (n.d). Garnet: a mineral, a gem, an
abrasive, a filter and more!. online Geology.com. Available at:
https://geology.com/minerals/garnet.shtml Accessed 24 Jan. 2018.
King, H. (n.d.). Gypsum Mineral | Uses and Properties.
online Geology.com. Available at: https://geology.com/minerals/gypsum.shtml
Accessed 29 Jan. 2018.
King, H. (n.d.). Halite
Mineral | Uses and Properties. online Geology.com. Available at:
https://geology.com/minerals/halite.shtml Accessed 28 Jan. 2018.
Madehow.com. (n.d.). How
salt is made – material, used, processing, procedure, industry, machine, Raw
Materials. online Available at: http://www.madehow.com/Volume-2/Salt.html
Accessed 29 Jan. 2018.
(nd). Gypsum, selenite: The mineral gypsum information and pictures.
online Available at: http://www.minerals.net/mineral/gypsum.aspx Accessed 29
Morton Salt. (n.d.). Salt
Production and Processing – Morton Salt. online Available at:
http://www.mortonsalt.com/salt-production-and-processing/ Accessed 29 Jan.
Myers, W. and Anderson, C. (1925). Garnet: Its Mining,
Milling and Utilization. online Department of Commerce, Bereau of Mines(256).
Available at: http://digicoll.manoa.hawaii.edu/techreports/PDF/USBM-256.pdf
Accessed 24 Jan. 2018.
Olson, D. (n.d.). Garnet, Industrial. USGS Mineral
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Accessed 26 Jan. 2018
Pellant, C. (2002). Rocks
and minerals. 2nd ed. New York: DK, pp.70, 133.
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Processed?. online Available at:
https://ourpastimes.com/about-6464026-garnet-mined-processed-.html Accessed 26