W9L2Carbonates and Evaporites Flashcards
Calcium carbonate polymorphs CaCO3
calcite: sedimentary rocks, marbles, oyster shells, most common
aragonite: most shells, HP metamorphic, sediments
Vaterite: rare
and many more carbonate minerals
Igneous carbonate minerals
carbonatites
not as common but for ex carbonatite melts and carbonatite lavas
Biominerlisation
is carbonate as biominerals
Biomineralisation through
Saturation of the required elements is driven & controlled by . the secretions of plants and animals
Cell structure modifies crystal morphology
common biominerals
CaCO3, SiO2 & calcium hydroxylapatite (calcium phosphate)
shells, eggshells, bone, coral
Crystallisation in the sedimentary (surface) environment
growth of ‘authigenic’ minerals
- precipitation of carbonates
- evaporites
- crystallisation in hydrothermal regions
commonest in marine environments
limestones
freshwater limestones
travertines
textural name: micrite
precipitates of finely crystalline carbonate
fine grined
textural name: sparite
crystalline calcite cementing ooids , spiral shape
textural name: ooids
ooids or ooliths= oolitic limestone
Marble
Metamorphism of Carbonate Rock
Metasomatism
hydrothermal fluids can move materials around or change there composition
happens in igneeous rocks and metamorphic terrians
Skarns
Skarns - metasomatism of carbonates and/or evaporites by alkaline fluids; calc-silicate metamorphic rocks
Evaporites occur when
salts precipitate from brines
Brine dfinition
A brine is a low temperature, aqueous solution at the Earthʼs surface, saturated with dissolved salts
crystal growth evaporites
crystal growth is initiated when brines become supersaturated as a result of evaporation of H2O → Therefore brines can only dissolve minerals that are water
soluble at 1 atmosphere pressure → Evaporite minerals typically contain Na, Ca, K, Mg, Cl, SO4, CO3
common evaporite minerals
Halite NaCl, Aragonite CaCO3, Gypsum CaSO4.H2O, Anhydrite CaSO4
Evaporite enviromnets and deposits
Halokinesis, Salt flows upwards to form domes
Messinian Salinity Crisis
Typical of arid alkaline lacustrine environments & some tropical shorelines (ʻsabkhasʼ)
Fumarolie
a hole in a volcanic region from which hot gases and vapors issue
Fumarole Activity
Incrustations of sublimates & products of acid-rock reactions occur on the throats of orifices and nearby surfaces.
activity where materials precipitate
sulphates and sulphides
can also crystallize from gases
