GarnetGarnet is a group of rock-forming minerals which sharea common crystal structure and a similar yet diverse chemical composition (X3Y2(SiO4)3).The garnet consists of SiO4 and is therefore, a silicatemineral. The X in the above formula can be calcium (Ca), magnesium (Mg), Iron(Fe2+) or manganese (Mn2+). The “Y” in thechemical formula can be aluminum (Al), iron (Fe3+), manganese (Mn3+),vanadium (V3+) or chromium (Cr3+) (King, n.d). Thespecific gravity of garnets typically range between 3.4 to 4.3 and they have awide 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 crystalstructure is classified as isometric which means that the mineral has a fourthree-fold rotation and closed crystal forms. A garnet’s hardness typicallyranges between 6.5 to 7.
5 in Mohs scale but crystallized garnet generally has ahardness of 7.5 which is harder than quartz but softer than topaz (Myers andAnderson, 1925; Amfed.org, n.d).The most common garnets are classified into three groups- aluminumgarnets, iron garnets and chromium garnets. The aluminum garnets consist ofalmandine, pyrope, grossularite and spessartite.
The iron garnet includesandradite and the chromium garnet consists of uvarovite. Garnet can be found inthe United Sates, China, India, Germany, Russia and Tanzania; and in sedimentary,metamorphic and igneous rocks (Wood n.d). Although garnet can be found in allthree rock types, it is principally found in metamorphic rock and morespecifically, gneisses, schists and marble (King, n.d; Olson n.d).The method of extracting garnet varies depending onthe geological environments responsible for the host rock.
Barton Mines in New York operate as anopen pit mine while in Idaho’s Emerald Creek Mines, garnet is extracted fromalluvial deposits. Garnet is more easily extracted from alluvialdeposits using backhoes or small draglines to cut slots into stream gravelswhich expose the garnet. The garnet is collected and transported to the millfor final processing and packaging (Database, n.d). Hard rock mining asexemplified at the Barton Mines, utilizes drilling and explosive methods toliberate the crystals from the parent rock without shattering them. The brokenore deposits are loaded into cars and transported to the mill (Myers andAnderson, 1925). In general, the processing of the garnet at the mill involvesthe separation of the heavier garnet from the lighter garnet minerals and thenfurther separation of the minerals based on grain size.
When the materialreaches 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 andlighter minerals. Once the material has been separated by grain size, spiralclassifiers along with hydrosizer are used to wash the material by specificgravity. Once washed, the concentrates are dried and sorted further usingmagnetic and electrostatic separators. During magnetic separation, minerals areseparated by their differences in magnetic properties. Electrostatic separationuses high voltage to separate particles of different electrical charge (ions)(Encyclopedia Britannica, n.d). The purpose of separating the mineral is tomeet the needs of specific markets and the end use of the mineral (Myers andAnderson, 1925; Database, n.
d).Historically, people are most familiar with garnet dueto its use as a gemstone, although only a low percentage of garnets areactually 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 andhigh hardness (6.
5-7.5 on Mohs scale), and that it can be recycled, make it an attractive mineral forindustrial usage. The four main industrial uses of garnet are abrasiveblasting, filtration, abrasive powder and waterjet cutting. Waterjet cutting isthe largest industrial use of garnet in the United States. A combinationof water entrained with abrasive granules are blasted out of a waterjet cuttingmachine and directed at metal, ceramic or stone. This process can cut withlittle, to no, dust. Similarly to waterjet cutting, abrasive blasting (sandblasting) is the process of propelling abrasive granules at a surface alongwith highly pressurized air or water.
Sandblasting is used to smooth, clean orremove oxidation products from various materials including metal and stone. Itis much faster than manually sanding by hand or using a sanding machine. Due togarnets hardness and angular pieces with sharp edges, it assists in the cuttingof material during the waterjet cutting and sandblasting processes. Garnetminerals processed to a size of approximately 0.
3 millimeters can be used tofilter out contaminants in water. An interesting use of garnet is that it canbe used as a geological indicator. Garnets can be formed in the mantle andbrought up to the surface during deep-source volcanic eruptions. During theseeruptions, xenoliths are formed which are a source of diamonds. There are manymore garnet minerals for every diamond mineral within an xenolith. The deepsource garnets formed during these volcanic eruptions are different than the garnetsformed at shallower depths and thus, these deeply formed garnets can be used asindicator minerals for the presence of diamonds (King, n.
d). HaliteHalite otherwise known as salt is primarily asedimentary mineral that consists of sodium chloride (NaCl). A rock thatconsists primarily of halite is known as rock salt (Database, nd; King, nd). Halite forms fromthe evaporation of seawater in arid environments where the influx of saltwateris low compared to evaporation. Although outcrops of halite are found in veryarid environments, subsurface halite deposits have been found during oil exploration.Currently, halite forms along the coastlines of Great Salt Lake in Utah and thesalt 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 berecognized by a white streak and vitreous luster. When pure, halite appearscolorless or white but impurities can create a variety of colors which includeyellow, gray, black, brown and red. Halite is associated with other mineralsincluding calcite and sylvite. Although halite can be identified based on itssalty taste, it is advised that you should not directly lick the mineral assylvite is poisonous. Instead, moisten or lick your finger, place finger on themineral, and taste the residue left on your finger. Halite has a hardness of 2to 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 solutionmining, dry mining, seawater evaporation and inland evaporation. Solutionmining is the process in which freshwater or a solvent is injected into thesalt beds to dissolve the salt. The saturated brine is then pumped to thesurface and allowed to dry and evaporate. Dry mining involves mining the salt inunderground 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 firststep of dry mining is undercutting. Large machines cut a slot which is inexcess of 10 feet in depth across the bottom of a salt wall. Followingundercutting, 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 saltis then transported to the surface where it is screened by size usingmechanically operated screens. Once separated, the salt is placed into bins ofvarying sizes to await packaging and shipping (Morton Salt, nd).
Seawaterevaporation can take between two to five years before any salt is ready forharvest. Seawater contains a number of constituents other than halite includingCa, Mg, and Fe compounds. The first step is to concentrate the brine and toincrease the salinity in order to allow the Ca, Mg and Fe compounds toprecipitate out of solution. The seawater circulates between theinterconnecting gravity fed pounds with salinity increasing with each transferwhich allows the impurities to precipitate out of solution. By the last pond,the majority of the impurities have precipitated out of solution leaving arelatively pure salt product (Database, nd; Madehow.
com, nd). The salt is thenpicked up by machines and washed with highly saturated saltwater. The saltwater is used to wash any residual impurities off the salt without dissolvingit.
It is further washed with freshwater and stored in piles for two to threemonths to drain (Madehow.com, nd; Morton Salt, nd). The fourth and final methodof salt extraction is inland solar evaporation. The processes involved inseawater evaporation and inland solar evaporation are similar; however,salinity in inland lakes is much higher than on the coast. Lakes aretopographically and potentiometrically lower than the surrounding area.Subsequently, water flowing over the surface and beneath the ground flowstowards the lakes and dissolve minerals along the way and therefore, becomegreat sources of minerals. Inland solar evaporation provides a yearly harvestas opposed to the seawater evaporation which provides a yield every two to fiveyears and thus, is more productive (Database, nd).
Today, the largest use of halite in North America isin 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-iceron roads. Salt has a lower freezing point than water and therefore, whenapplied to snowy, icy roads melts the snow and ice. The chemical andtransportation 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 pressurecreating Salt domes. The movement of the salt deposit upward due to pressureresults in the deformation of overlying rocks that can trap oil and gas. Saltdomes formed along the Gulf Coast of Texas and Louisiana have been good reservoirs for hydrocarbons and havebeen exploited by the oil and gas industry (esci.umn.edu, nd; Keller, 2011 pp415). Gypsum Gypsumis a sulfate (SO4) mineral that is found in igneous and sedimentaryrocks.
Gypsum is found in gypsum rock which forms most commonly within layeredsedimentary rock and more specifically limestone. It is most commonly formed inlagoons in which seawater high in Ca and SO4 evaporates and theevaporated water can be readily replenished. Gypsum can be found in a variationof colors including white, gray, brown, beige, orange, pink, yellow, light redand green; it can also be colorless. Gypsum has a white streak, vitreous lusterand is soft (1.5 to 2.0 Mohs scale).
Gypsum has a monoclinic crystal systemwhich 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 highestproducing States (Gypsum Association, nd).Toextract natural gypsum rock deposits underground, a drill or a cutter breaksaway the face of the gypsum rock. Alternatively, when deposits are locatedcloser to the surface, the open-pit mining method is adopted. Benches aredrilled and blasted using ammonium nitrate.
In both instances, once the mineralis extracted, it is then crushed into smaller pieces and passed through ahomogenizer where the gypsum is graded and sorted. The gypsum is furthercrushed prior to calcination. Gypsum is a hydrous calcium sulfate (CaSO42H2O) and thus has water in its structure.
Calcination is theprocess of removing the water from the gypsum and once finished produces aproduct resembling plaster powder. The plaster powder is transported to the millwhere the particle size of the plaster is further modified to suit the type ofplaster being produced. The last stage in the process is the inclusion ofadditives which give each mix of plaster its own specific properties. The finalstage is the delivery of the plaster to merchants and distributors (British-gypsum.com,nd; Database, nd). Gypsumcan 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 plantscan be captured from smoke stacks and purified into a hard material and manufacturedinto gypsum.
Natural gypsum and FGD gypsum have the same chemical composition;however, FGD gypsum production encourages power plants to recycle and captureits waste that would otherwise be disposed of in a landfill. The United StatesEnvironmental Protection Agency (USEPA) and U.S. Green Building uses FGD gypsumboard in their offices due to the environmental benefits (Gypsum Association,nd).The mostcommon use of gypsum is wallboard and plaster products.
However, there are avariety of other uses for gypsum including plaster of Paris, a hardeningretarder in Portland cement, glass manufacturing and ornamental purposes. It isimportant to note that pure gypsum is required for glass manufacturing and thatgypsum is used in a limited capacity for ornamental purposes due to its lowhardness. The United States consumes the most amount of gypsum in the world whichhas been estimated to be upwards of 30 billion square feet per year (Database,nd; King, nd).