Dolomite

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

• 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.

www.ask.com/wiki/Unconformity

Paleozoic Dolomite, UK

Typical dolomite outcrop

Stromatolites in dolomite

Precambrian stromatolites South AfricaModern stromatolite mats, Brazil vsites.unb.br/ig/sigep/sitio041/fig2.jpg

Classification

• A. According to carbonate content– limestone = 0-10% dolomite– dolomitic limestone = 10-50%– calcitic limestone = 50-90%– dolomite = 90-100%

• B. According to grain size– dolorudite, dolarenite, dolosparite, dolomicrite

• C. Can use Folk or Dunham modifiers:– 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– hard crust on surface--supratidal to 1m depth– dolomitized pellets, gastropods = early

replacement– In Arabian Gulf & Bonaire evaporites

associated with dolomitic seds– All above cases Mg/Ca ratio 3-4x seawater

Dolomite forming environments

• B. San Andreas Island, Colombia– Pleistocene l.s. dolomitized by downward

moving brines– brines formed through evaporation- supratidal

zone

Dolomite forming environments

• C. Coorong, S. Australia– dolomite forms in coastal lagoons & lakes– no evaporation associated– 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

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.

http://www.lib.utexas.edu/geo/balcones_escarpment/pages101-114.htm

Paleozoic Dolomite, UK