Introduction • Dolomite mineral • CaMgCO3 • Dolomite used for mineral & rock • use dolomite (mineral) dolostone (rock)
Dolomite Precipitation • Precipitation requires elevated Mg/Ca ratio • Problem: How get elevated ratios Precipitation: Ca2+ (aq) + Mg2+ (aq) +2CO32- (aq) = CaMg(CO3)2 (solid)Dolomitization: 2CaCO3 (solid) + Mg2+ (aq) = CaMg(CO3)2 (solid) + Ca2+ (aq)
Most dolomite diagenetic www.ask.com/wiki/Unconformity • Early/late diagenetic replacement • May cut across depositional units • May be associated with unconformities • May destroy textural features • May occur as randomly distributed rhombs • Some dolomite precipitated from water 500-million-year-old dolomite nonconformably overlies 1.5-billion-year-old rhyolite.
Stromatolites in dolomite Precambrian stromatolites South Africa Modern stromatolite mats, Brazil vsites.unb.br/ig/sigep/sitio041/fig2.jpg
Classification • A. According to carbonate content • 1. limestone = 0-10% dolomite • 2. dolomitic limestone = 10-50% • 3. calcitic limestone = 50-90% • 4. dolomite = 90-100% • B. According to grain size • 1. dolorudite, dolarenite, dolosparite, dolomicrite • C. Can use Folk or Dunham modifiers: • 1. e.g. dolomitic biosparite or dolobiosparite
Dolomite increases back through time • Possibilities: • Change in seawater chemistry • More dolomite forming environments in past • Diagenetic replacement of older limestones
Dolomite forming environments • Bahamas, Arabian Gulf, Bonaire Island, and Florida • 1. hard crust on surface--supratidal to 1m depth • 2. dolomitized pellets, gastropods = early replacement • 3. In Arabian Gulf & Bonaire evaporites associated with dolomitic seds • 4. All above cases Mg/Ca ratio 3-4x seawater
Dolomite forming environments • B. San Andreas Island, Colombia • 1. Pleistocene l.s. dolomitized by downward moving brines • 2.brines formed through evaporation- supratidal zone
Dolomite forming environments • C. Coorong, S. Australia • 1. dolomite forms in coastal lagoons & lakes • 2. no evaporation associated • 3. groundwater modified through evaporation, enters lakes
Controversial Origin of Dolomite • Seepage-reflux model • Covers San Andreas Island • Flooding and capillary movement of seawater • Evaporation = pore waters w/high Mg/Ca ratio • Pore waters move downward causing dolomitization • Problem--this type of dolomitization not demonstrated on geologic scale
Controversial Origin of Dolomite • Evaporative pumping • Covers Arabian coast • Seawater moves laterally from lagoon to supratidal seds • Process driven by evaporation in sabkhas • Evaporation results in high Mg/Ca ratios; get dolomitization • Widespread dolomitization possible during regressions
Seepage-reflux & Evaporative pumping models • imply dolomitization = supratidal process • some dolomites not supratidal
Alternative Models • groundwater/seawater mixing--Andros IS • Seawater supersaturated w.r.t. to dolomite • Easier to precipitate aragonite and calcite • Hghly ordered dolomite structure difficult to produce • Mix meteoric water with seawater • produce undersaturation w.r.t. calcite but increased saturation of dolomite • Then contact with freshwater • Freshwater mixing with seawater in buried seds also possible • Problem: slow rate of dolomite precip=no large scale dolomite formation
Alternative Models • Precipitation of scattered dolomite • Mg leaching from high Mg calcite or from clays
Dedolomitization • Calcite replaced by dolomite • Rhomb occupied by calcite • Or calcite replaces edges http://www.lib.utexas.edu/geo/balcones_escarpment/pages101-114.htm The centers of dolomite rhombs have been leached and later refilled with single crystals of calcite. Some of the rhombs in this photograph are still hollow.