GEOL 333 #9 - Carbonates, Sulfates, Halides, Phosphates

I) Carbonates - contain triangular carbonate ion, CO32-. Various ways to classify carbonate minerals.

Rhombohedral Group (type of hexagonal symmetry; have rhombohedral cleavage, i.e., pushed over cube) - calcite* (CaCO3), dolomite* (CaMg(CO3)2, siderite (FeCO3), magnesite (MgCO3), rhodochrosite (MnCO3, photo #1, #2), and smithsonite (ZnCO3).

Orthorhombic - aragonite* (polymorph of calcite - CaCO3);

Monoclinic - Azurite (blue) and Malachite (bright green, gemstone); various amounts of Cu2+, CO32-, and OH-, form during hydrothermal alteration in copper ore deposits.

Calcite* (CaCO3) - wide range of crystal forms (>300); planes of Ca2+ and CO32-, characteristics = clear to white color, relatively soft (H = 3), reaction with dilute acid.

CaCO3 + 2 HCl --> Ca2+ + CO2 (gas) + H2O + 2Cl-

Environmental important reaction because calcite can neutralize acids generated by human activities, e.g.,



Acid Rain (link #2, #3) (pH < 5) - due to combustion of fossil fuel, S and N oxidize to sulfuric and nitric acid, which cause building and monument degradation, nutrient leaching in soil (and forest decline), and sometimes acidification of lakes and rivers (and stress or death of aquatic organisms); entire eastern USA receiving acid rain. Why only eastern USA?




Only certain areas in North America are susceptible to lake acidification. Why?




Calcite is very common (important) mineral in sedimentary rocks; major mineral in limestone, chalk, cave deposits, and marble (metamorphosed limestone); also occurs commonly as cement in sedimentary rock and as fracture-filling in all rock types. In limestone, calcite forms due to activity of calcite-secreting organisms, e.g., coral and clams in shallow, warm, marine waters extract Ca2+ and CO32- to form calcite shells (biochemical precipitation) as well as direct chemical precipitate from seawater.

Major use of calcite (limestone) = to make cement, involves heating calcite at 900C to drive off CO2 gas and produce lime (CaO):

CaCO3 + heat --> CaO + CO2

Lime is reacted with quartz sand and water to make cement (disordered Ca-Si-OH). Lime also used for fertilizer and processing of iron ore. Limestone and marble used as building stone and aggregate (gravel roads, base for railroad tracks, concrete filler).

Dolomite* (CaMg(CO3)2) - some (but not continuous) solid-solution between calcite and dolomite due to ~large difference in size of Ca and Mg. In dolomite, Ca and Mg occupy different planes (for ordered dolomite). Dolomite commonly looks similar to calcite (rhombohedral cleavage, low hardness), but reacts with dilute acid only if powdered. Why?

If dolomite contains Fe, weathers to rusty brown color. Major component in dolostone (dolomite rock); forms by alteration of limestone by Mg-rich waters during burial. Dolomite also used as building stone and aggregate.

Siderite (FeCO3) - light brown with rhombohedral crystals, reacts weakly with dilute acid; associated with iron formation (sedimentary deposits of Fe-rich minerals mined for iron ore), minor source of iron ore.

Aragonite* (polymorph of calcite - CaCO3) - fizzes with dilute acid, forms different shapes of crystals from calcite (long prisms topped by pyramids). Calcite is more stable (and more common) at Earth's surface and aragonite is more stable at higher pressures. Aragonite does commonly form at Earth's surface (metastable) from organisms that secrete aragonite shells (e.g., green algae, pearls, and others) and some hot springs and caves; used to make lime and cement.

II) Sulfates - contain sulfate anion, SO42-, S = S6+

Gypsum* (CaSO4 . 2H2O) = most common sulfate mineral, 2 water molecules in crystal structure (i.e., hydrous); clear to white, very soft (H = 2), and 2 perfect cleavage directions; occurs in sedimentary marine evaporites (evaporation of seawater and chemical precipitation of gypsum), commonly with halite, limestone, and dolomite. Used to make plaster and drywall. Plaster prepared by heating gypsum to drive off 75% of water, later mixed with water (gives off heat), and as it dries, it hardens. Check out these photos of massive gypsum crystals (up to 12 m long!!) in a cave in Naica, Mexico (photo #1, #2, #3).

Anhydrite - (CaSO4) anhydrous, more rare than gypsum, forms at higher temperatures but in similar geologic settings as gypsum.

Barite* (BaSO4) relatively high specific gravity, Why?
can form as cluster of radiating elongated crystals = "barite rose"; occurs as fracture fillings in certain hydrothermal ore deposits; used extensively in oil and gas industry as component in "drilling mud" (smectite clay minerals, barite and water, sketch of drilling rig with mud pits). Addition of barite > density of drilling mud, which < chance of "blowout", geyser caused by penetration of pocket of pressurized gas in subsurface.

III) Halides (contain halide element such as Cl-, Br-, I-, or F-) - Ionic bonds dominate, usually high (e.g., cubic) symmetry due to simple crystal structure.

Halite* (NaCl) forms cubic crystals and has cubic cleavage (3 directions at right angles); relatively soft and salty taste; occurs in marine evaporites (evaporation of seawater) and in certain lake evaporites (e.g., Great Salt Lake) with gypsum (or anhydrite) elemental S (in salt domes). Salt dome = large massive body of salt, common in U.S. Gulf Coast area (onshore and offshore). Salt domes begin as horizontal layers of halite (in sedimentary rock), but due to ~low density of halite (G = 2.1) compared to overlying silicate or carbonate rock (G = 2.5 - 2.6), halite rises like flowing finger of salt. Salt punctures through overlying rock and causes upwarping of surrounding rock layers, resulting in geologic structure called dome. Some salt domes/fingers rise up to 13 km above their source. Salt domes = economic deposits of salt and possible oil traps (sketch of salt dome formation, salt dome trap) also considered for storage of high-level radioactive waste; used in foods, deicing roads, and in manufacturing (production of Na-compounds such as lye, NaOH). Before refrigerators, salt was used primarily as food preservative.

Fluorite* (CaF2) = state mineral of Illinois, was mined extensively in southern IL. Fluorite forms cubic crystals but 4 directions of cleavage (octahedral cleavage); color varies, purple and green are most common. It is common in hydrothermal ore deposits of sedimentary origin (Mississippi Valley-type Pb/Zn deposits); used to make HF (hydrofluoric acid), powerful acid used to etch glass (can actually dissolve quartz); also used as flux (i.e., makes slag more liquid) in steel and Al industry.

IV) Phosphates contain PO43-- apatite^ (Ca5(PO4)3(OH, F, Cl) = principle component of human bones and teeth (and minor mineral in igneous rocks); 3 end-members: OH end-member = hydroxylapatite, F end-member = fluorapatite, Cl end-member = chlorapatite. Hydroxylapatite is weaker and more prone to cavities than fluorapatite, fluoride is used in municipal water and toothpaste to promote change to fluorapatite end-member; also used as P source in fertilizer.