Weathering and mobility Flashcards
How do you balance weathering reactions?
a) Al goes into secondary minerals
b) Excess Si in H4SiO4
c) Alkali metals or alkaline earth metals in ion state
d) Balance O with H20
e) Balance H with H+
f) Must be balanced in terms of element
O2 may be present!
(2) Name three minerals that weather without any acid added.
Pyrite
Magnetite
Olivine
(3) Name the major elements in freshwater
Four major cations: Ca2+, Mg2+, Na+ and K+.
Three major anions: HCO3-, SO42-, Cl-
(5) Why is it common to find high dissolved concentrations of Fe and low dissolved concentrations of Al in groundwater?
Fe is soluble under reducing conditions which groundwater normally has. Al mobility depends on pH and at neutral pH which groundwater normally is Al is immobile
(8) The minerals fayalite (Fe2SiO4) and forsterite (Mg2SiO4) belong to the olivine group. (A)Which one is most easily weathered and why?
What is needed when weathering olivines?
a) Mg is more reactive than Fe, looking at the periodic table. Depends on how far we are from their building temperature. Both are easily weathered and weather congruently.
O2 needed for weathering
(9) Weathering reactions are usually acid-base reactions. How do you know when a weathering reaction is a redox reaction?
Acid-base reactions involve the transfer of hydrogen ions between reactants, free H+, fast reactions. Redox reactions involve a change in oxidation number for one or more reactant elements free e- do not exist.
Describe the concept of ionic potential that shows if an element will occur as a mobile cation, mobile anion, or if it will form immobile solids in waters. How is “Ionic potential” defined?
I = Z (ionic charge)/r (radius of ion)
Low I – mobile as cations (Na, K etc). Hydrated in solution.
Intermediate I – immobile, forms solid hydroxides (Fe, Mn, Si etc)
High I – mobile as oxyanions (As, P, S etc)
Describe the mobility of Zn and Cd
Zn, Cd only immobile in reducing conditions with hydrogen sulfide.
Describe the mobility of REE
REE mobile in acidic oxic conditions, somewhat mobile in oxic neutral conditions. Otherwise immobile
What is incongruent and congruent weathering and their formulas?
Congruent: Primary mineral + O2 + H+ + H20 -> dissolved ions
Incongruent: Primary mineral + O2 + H+ + H2O -> Secondary mineral + dissolved ions
Describe podzol profiles
O horizon: organic matter
A horizon: weathering and mobilization of metals. Eluviation. Low pH
B horizon: zone of precipitation, immobilization. Illuviation. Higher pH
C horizon: unweathered soil
R horizon: bedrock
What does organic acid complexes do?
Complexes with organic humic (HA) or fulvic acids (FA) increases mobility, elements don’t get reduced.
When is smectite formed and what is the formula?
In dry climate. Ca, Na, Al, Mg, Si in the secondary mineral.
(0.5Ca,Na)Al3MgSi8O20(OH)4
When is kaolinite formed and what is the formula?
In cold, humid climate. Al, Si in secondary mineral
Al2Si2O5(OH)4
When is gibbsite formed and what is the formula?
In tropical, wet climate. Al in secondary mineral.
Al(OH)3)
