19/20) Geochemical Kinetics 1&2

difference between kinetics and thermodynamics

thermo: study of energy transfer, tells what should happen, not when or how fast it will happen
excludes time

kinetics: introduces time, describes how fast reactions occur

water rock reactions are much __ than speciations reactions

slower

when are kinetics significant?

not very important when rate constants are fast: speciation and complexation reactions, dissolution of salts, high temperature

tend to be important
dissolution of aluminosilicate minerals (very), dissolution of other minerals, gas exchange reactions, redox

what do we want to achieve with kinetic theory

predict how fast a mineral will dissolve or precipitate as a function of geochemical parameters
pH, T, saturation state (go towards eqlm things go towards 0)

need a way of measuring rates in the lab so we can calculate them in the field

types of surface area

BET surface area (brunauer, emmett, teller): total surface area (crushed rock), quantified using physical adsorption of gas molecules to the mineral surface

geometric surface area: surface area of a simple geometry (sphere, cube) that approximates the general shape of the mineral grain

difference between transport limited and reaction limited

transport limited: solution equilibrates with the mineral quickly, so reaction rate is determined by new solution coming in

reaction limited: solution is slow to equilibrate with the mineral, the reaction rate is determined by the innate reaction rate (kinetics)

in limestones and sandstones, which is transport limited and which is reaction limited

limestones are transport limited

sandstones are reaction limited

what are different rate dependences?

pH, temperature

what are 2 kinetic experiments

batch: mineral + water in container sampled over time

flow-through: mineral + water, solution washing in and out, measuring change in concentration as fluid moves through the reactor
requires pumps, need a way of controlling flow rate and measure flow rate
control very precisely the solution that goes in

explain this

To start, there is basically no mineral in solution, so adding more mineral, the solution is still so undersaturated that the reaction rate is unchanging, it’s going as fast as it can at that particular pH, until it reaches a point where it's close to equilibrium but still undersaturated, and adding mineral begins to decrease the reaction rate as it moves closer to eqlm. Eventually eqlm must happen, so the reaction rate reaches zero

it’s easy to do experiments on dissolution plateau, because the rate is the same no matter what the saturated state is

K-feldspar dissolution rates ___ with increasing pH and the approach to equilbrium

slow down

why might mineral lifetimes be underestimates

reactive surface are is not equal to effective surface area

pH is generally more neutral

temperature is lower

if omega is less than one then what?

-rtln(omega) is negative

∆G is positive

dissolution

if omega is greater than one then what?

then -rtln(omega) is positive

∆G is negative

precipitation

what is transition state theory

explains chemical reaction rates by proposing that reactants form high-energy, unstable "activated complexes" at the peak of the energy barrier before becoming products, assuming a quasi-equilibrium between reactants and this state

mineral precipitation rates depend on both growth mechanism and saturation state

explain this

1) is reversible, the H+ are booting out a potassium, the charge balance is maintained. this happens all the time at mineral surfaces

2) is reversible, adding another H+ and now there is a charge on the surface, this is the activated complex

3) is irreversible, destruction of the kspar structure, the constituents are in solution

the overall rate of kspar destruction is controlled by reaction 3, the other reactions are instantaneous

what is the problem with affinity factors p and q

assuming p and q are 1 is inaccurate in many cases

details on batch expt

pros: cheap, easy, effective
fewer places to leak from, no pumps
easily repeatable and to vary parameters

cons: difficult to control fluid chemistry, change temperature
cannot prevent secondary minerals from precipitating

no control over solution chemistry once reaction has begun, once pH starts changing, it’s very quick

when would you need parr type reactors

elevated temperatures with gases

gas escape is low, maintaining constant pressure during sampling requires a constant pressure pump

maintaining constant pressure during CO2 injection requires fluid removal

details on gold

details on nucleation on glass

rough laser etching on the bottom of glass makes different nucleation patterns
nucleating CO2 bubbles out of supersaturated solution

explain this

critical saturation state must be reached before growth occurs

high threshold of nucleation, make some bubbles then rate goes up drastically

very low threshold of nucleation, only require a little bit of supersaturation, then can make big nuclei right away

types of growth on mineral surfaces

mononuclear

polynuclear

screw dislocation: some part of mineral is raised above the rest, it grows in a spiral

crystal cube showing edge and kink

why is it difficult to predict precipitation on calcite

impossible to know how many screw dislocation points there are on calcite

explain this

if K2 is really fast, then everything leaves solution and we are really far from eqlm