Earth Materials J.D. Price Carbonates Earth Materials J.D. Price

Aragonite CaCO3 Calcite CaCO3 Magnesite MgCO3 Rhodochrosite MnCO3 Carbonates Aragonite CaCO3 Calcite CaCO3 Magnesite MgCO3 Rhodochrosite MnCO3 Siderite FeCO3 Dolomite CaMg (CO3)2 Ankerite CaFe(CO3)2 Malachite Cu2CO3(OH)2 Azurite Cu3(CO3)2

3Cu2 CO3 (OH)2 Mal + CO2 gas = 2Cu3 (CO3)2 (OH)2 Az + H2O liq GºTP - Gº = RT ln K = 2.303 RT log K At s.s. GºTP =0 with T=298K, log kº = -0.1750 Gº = 1/p CO2 Malachite + H2O Gºf Az = -1429.56 kJ/mol Gºf Mal = -900.41 kJ/mol Gºf H2O = -237.23 kJ/mol Gºf CO2 = -394.37 kJ/mol Gºrxn = -0.75 kJ/mol log p CO2 = -0.13 Tenorite + H2O Azurite + H2O Cuprite + H2O Copper + H2O

CaCO3 We discussed this previously. Aragonite and calcite are polymorphs. Aragonite is the higher pressure configuration of CaCO3.

Calcite Image from Klein and Hurlbut, 1985

Dolomite The substitution of Mg for Ca maintains the same structure. As a consequence, dolomite is also rhombohedral, has similar high birefringence and similar density. Not so much effervescence in dilute, cold HCl What’s the reaction? Humbolt College

Ca-Mg Solid solution is only permitted at high temperatures. At ambient conditions there is virtually no solid solution in the Ca-Mg carbonate system.

Ca-Mg-Fe There is much more (but still limited) solid solution in the Ca-Fe carbonate system. And extensive solid solution in both CaMg-CaFe and Mg-Fe space. Lines - coexisting compositions Image from Klein and Hurlbut, 1985

Details By now you know that minerals can be recorders of the processes that create and sustain them. Carbonates - some powerful information Trace element incorporation (divalent cations, such as Sr in calcite) Isotopes of C and O (13C and 18O) These are influenced by temperature (directly or indirectly) Because carbonate materials are incorporated into marine organisms, carbonate minerals may record ocean climate history.

18O is a stable isotope influenced by global ice content. H2O16 vs. H2O18 For a volume of water, more 16O water evaporates relative than water with 18O. Evaporated water from near the equator is eventually transported toward the poles through repeated evaporation and precipitation. The 18O/16O ratio will be lower in the snow that falls on a glacier than it is in the ocean from which the water evaporated. If global ice volume increase, the 18O values of seawater become larger as more 16O stored is locked away in ice. Standard for carbonates is a belemenite fossil from the K Peedee formation, SC 18O/16O = 0.0020672

Analyzing the 18O/16O ratio of a dated carbonate mineral from a fossil or marine precipitate gives you an estimate of global ice - a proxy for two parameters Global climate trends Sea level

Carbonate origins Carbonate is abundant at the earth’s surface, but may be produced deeper in the earth where the activity of CO2 is elevated. Igneous: Carbonatites - rare magmatic systems that produce carbonate materials Hydrothermal: Alteration of mantle rocks and dissolution in aqueous fluids Ground/surface water: Chemogenic precipitation, karst Metamorphic: Recrystallization and metasomatism Sedimentary: Biogenic and reworking of biogenic materials, clastic continental materials

Carbonatites Mt. Erebus, Antarctica - a very active carbonatite volcano MEVO

Biogenic Sediments Critters make minerals Aragonite and calcite In general the oceans are nearly saturated in CaCO3 Ca2+ + HCO3- = CaCO3 + H+ Temperature is important (latitude, depth)

Gastropoda and Pelecypoda,Cretaceous, New Jersey Trilobite Ameura , Upper Pennsylvanian, Kansas KGS Website

Corals Corals are great limestone builders. They put a lot of time and energy into building skeletons made from aragonite The coral polyp only inhabits the outer 1-2 cm of the structure. The rest is just a framework to increase reproduction and feeding

Coccoliths Tiny plankton with calcareous shells. Abundant enough to live and die to make massive limestone deposits like Santa Elena Canyon, TX

Carbonate producing organisms die, but their shells will only be preserved if they are deposited above 4 km depth

Vostok, Antarctica Ice Core 375 Current Atmosphere CO2 Ice cores trap gas and dust. Temperature from concentration of deuterium in ice.

Feely et al., 2001 Increased atmospheric CO2 means big changes in ocean chemistry. This could be detrimental to much of marine life.

Carbon isotopes show the location of industrial CO2 in the worlds largest oceans. As we know ocean mixing is a slow process - the most recent excess CO2 remains in the first 1 km of the ocean. Feely et al., 2001

Diagenesis In sedimentary rocks, the individual particles need to be cemented together. Carbonate minerals precipitate out of water to cement grains. Thin section under XP shows calcite overgrowth on micrite grains.

Limestone Fragmented or whole aquatic invertebrate hard parts Origin: Bodies of water with suitable environments 0.3mm ©Ryan Hanson

Fossiliferous limestone - pore space colored blue

Oolitic limestone Ooids form through growth and/or accumulation in dynamic carbonate environments.

Dolomite precipitation Dolomite dominates older rocks and has decreased over time. It only forms in special chemical environments. Thermodynamically it should be the dominant phase in sea water. The dolomite problem. Calcite Dolomite Brucite Magn

Speleothems

Carbonates easily dissolve into water (rain water is slightly ascidic), Many limestone will dissolve into meteoric waters; and perhaps precipitate out at another location Pit Cave - Gregg Eckhardt The Edwards Aquifer is hosted in within an initially low-porosity limestone. Water has worked its way into cracks over last 1 million years Fault in Wonder Cave

Pucek Property, near San Antonio, TX This is the largest water well in the world.  When tapped, it blew out rocks the size of basketballs 20 feet into the air. Water flowed out at 25000 gallons each minute. Not potable, but enough water to support about 250,000 people

So what’s the problem? It’s bad water. The problem is that the Pucek well was draining the aquifer at a much more rapid rate than water could be replaced. If allowed to persist, the well would eventually tap the sweet water and lower San Antonio’s supplies beyond use.

Reprecipitation of carbonates occurs, particularly if energy conditions, composition, or pH of the water changes. Travertine Tuffa

Marble Conditions: low to moderate P & T Minerals: calcite or dolomite (carbonates) Origin: biogenic (limestone or dolostone) Univ. of North Carolina Web atlas of metamorphic textures