Gypsum is a soft sulfate minerale composed of calcium sulfate dihydrate.
It is used as fertilizer and the main constituent for plaster, blackboard chalk (creta), drywall.
Massive variety of gypsum named alabaster has been used for sculpture in Ancient Egypt, Mesopotamia, Ancient Rome, Byzantine Empire and Nottingham alabasters of Medieval England.
Gypsum also crystallizes as translucent crystals of selenite.
Forms as an evaporite mineral and as a hydration product of anhydrite.
When gypsum is heated in air it loses water and converts first to calcium sulfate hemihydrate (bassanite or often called plaster), and if heater further, to anhydrous calcium sulfate (anhydrite).
Anhydrite solubility in water is strongly dependent on NaCl (table salt) concentration.
Gypsum crystals are found to contain anion water and hydrogen bonding.
In some places, gypsum can form flower-like forms called desert rose (trandafir de desert).
Gypsum is also formed as a by-product of sulfide oxidation, among other by pyrite oxidation when the sulfuric acid generated reacts with calcium carbonate.
Its presence indicates oxidizing conditions.
Orbital pictures from the Mars Reconnaissance Orbiter have indicated the existence of gypsum dunes in the northern polar region of Mars, which were later confirmed at ground level by the Mars Exploration Rover Opportunity.
There are big scale caves of gypsum crystals that forms very large crystals of meters long like the ones in the Cave of the Crystals in Mexico.
In rare cases, water gets trapped inside a crystal in a hollow channel while the crystal forms and by rotations, a water bubble moves around inside it, specimens called enhydros , very desirable to collectors.
Sometimes sandy areas can trap sand inside when forming causing a specimen to become brown or gray and opaque; these sometimes form hourglass formation in crystal.
Synthesis
Gypsum is recovered via flue-gas desulfurization at some coal-fired power plants. It can be used interchangeably with natural gypsum in some applications.
Uses:
In construction as drywall, wallboard, sheetrock or plasterboard, primarily used as finish for walls and ceilings.
Mortar in ancient mortar used in building construction and as gypsum blocks like concrete blocks in building.
Plaster, fertilizer, soil conditioner, sculpture (alabaster), wood substitute in ancient world.
In food creams, shampoos, hair products, traditional chinese medicine called shi gao; dentistry; mushroom cultivation.
Can be used to remove pollutants like lead or asenic from contaminated waters.
How does it forms:
Gypsum forms in lagoons where ocean waters high in calcium and sulfate content can slowly evaporate and be replenished with new sources of water. The result accumulate large beds of sedementary gypsum.
Gypsum is commonly associated with rock salt and sulfur deposits.
Characteristics:
Category sulfate minerals
Formula CaSO4 2H2O
Calcium 23,28%; Hydrogen 2,34%; Sulfur 18,62%; Oxygen 55,76%
Crystal system monoclinic
Hardness 1,5-2 Mohs
Color: colorless, white, yellow, blue, pink, brown, reddish
Habit massive, flat, elongated, prismatic, tabular crystals more commonly
Cleavage perfect
Fracture conchoidal, uneven
Tenacity flexible, inelastic
Luster vitreous to silky, pearly or waxy
Tenacity: sectile and slightly flexible
Streak white
Transparent to translucent
Specific gravity 2,31-2,33
Density: 2,3
Optical properties biaxial (+)
Water soluble at 25 Celsius; becomes less soluble at higher temperatures
Dilute in HCl
Gypsum crystals can slightly bend and are slim, with good flexibility for a crystal.
Rock type: igneous, sedimentary
Common minerals associations: Halite, Dolomite, Barite, Anhydrite, Sulfur
Luminescence: fluorescence bluish white; yellowish white; phosphorescence
Tests:
Low hardness 1,5-2 Mohs
Solubile in water and HCl
Slightly flexible, can be bend
Streak white
Distinguishing similar minerals:
Brucite - harder
Calcite - harder, not flexible
Barite - harder 3 - 3,5 ahd heavier
Luminescence of gypsum
Asociated luminescent varieties: alabaster, selenite; anhydrite, angelite (maybe)
Intensity medium; frequency often; color yellowish white in long wave; bluish white in short wave
can also be white, yellow, yellow-orange; gray-white; green; green dark; white pink
Phosphorescence strong; yellowish white in long wave and bluish white in short wave
Activator: most common organic impirities; other: UO2 Uranyl ion as impurities
Peak in Gypsum:
Organic: 440-640; 494; 510; 544; 601 nm
UO2: 469; 486; 507; 554 nm
Alabaster can have fluorescence white, bluish white, yellow, orange, green, blue and bluish white phosphorescence
Anhydrite can have fluorescence pink, white or blue in long wave and red, white or blue in short wave; can also present thermoluminescence; activators are Sm; S; Eu; Ce; Eu; Dy; Tb; Mn; O; Nd: Yb
Peak for Anhydrite:
Mn not replacing Ca: 600-630 nm S: 500-600 nm Eu 385 nm
O replacing SO4: 400-500 nm Ce: 320, 340 nm Tb 416, 438 nm
Dy: 475, 480, 546, 574, 577, 582 nm Nd: 838, 839, 899 nm Eu: 593, 618, 702 nm
Sm from 630 to 832 nm Yb: 525 nm
Varieties:
Satin spar - pearly, fibrous masses
Selenite - transparent and bladed crystals; forms distinct crystals
Alabaster - fine grained, slightly colored; often used for carving
Anhydrite - has the same chemical composition as gypsum but lacks water; thus anhydrite specimens absorb water very well, transforming into gypsum; some specimens show evidence of this, containing growths of clumpling layers that testify to their expansion from addition of water
Desert rose - rosette shaped barite or rosette shaped gypsum with sand inclusions
Angelite - nodular, polished form of anhydrite; without water like anhydrite; blue-ish white
Anhydrite
Is not a common mineral, has a similar composition to gypsum but without water
Transforms into gypsum by adding water.
Occurs in arid regions forming from dehydration of Gypsum.
It is used in the production of sulfuric acid and filler in paper.
Formula CaSO4
Calcium 29,44%; Sulfur 23,55%; Oxygen 47,01%
Calcium sulfate
Color: colorless, white, yellow, gray, blue, orange-red, red, pink, purple
Streak white to light gray
Hardness 3-3,5
crystal system orthorhombic
Forms tabular and prismatic crystals; are uncommon; often fibrous
Transparent to translucent
Specific gravity 2,9-3,0
Cleavage 2,3 forming cube
Fracture uneven to splintery
Tenacity brittle; not flexible
Special characteristics:
Usually fluorescent; specimens that are not fluorescent, may become after heating
Rock type igneous, sedimentary
Common mineral associations: gypsum, halite, calcite, brucite
Distinguishing similar minerals:
Calcite - effervescent in hydrochloric acid
Barite - much heavier
Varieties: Angelite - nodular, polished for mof Anhydrite; blue-ish white
Gypsum
Other names: trandafir de desert trandafirul de desert desert rose alabaster alabastru alabastrul selenit selenite selenitul gips gipsul jips jipsul gyps gypsul gypsum gypsu gipsum ghipsul ghips ghipsum gypsite angelite angelitul angelit anhydritul anhidritul anhidrit anhydrit anhidrite
Comments
Post a Comment