Problem A melt or water solution that a

  • Slides: 13
Download presentation

Problem: • A melt or water solution that a mineral precipitates from contains ALL

Problem: • A melt or water solution that a mineral precipitates from contains ALL natural elements • Question: Do any of these ‘other’ ions get into a particular mineral?

Goldschmidt’s rules of Substitution 1. The ions of one element can extensively replace those

Goldschmidt’s rules of Substitution 1. The ions of one element can extensively replace those of another in ionic crystals if their radii differ by less than about 15% 2. Ions whose charges differ by one may substitute readily if electrical neutrality is maintained – if charge differs by more than one, substitution is minimal

Goldschmidt’s rules of Substitution 3. When 2 ions can occupy a particular position in

Goldschmidt’s rules of Substitution 3. When 2 ions can occupy a particular position in a lattice, the ion with the higher charge density forms a stronger bond with the anions surrounding the site 4. Substitution may be limited when the electronegativities of competing ions are different, forming bonds of different ionic character

 • What ions would substitute nicely into pyrite? ? • S- radius=219 pm

• What ions would substitute nicely into pyrite? ? • S- radius=219 pm • Fe 2+ radius=70 pm Fe. S 2

Chemical ‘fingerprints’ of minerals • Major, minor, and trace constituents in a mineral •

Chemical ‘fingerprints’ of minerals • Major, minor, and trace constituents in a mineral • Stable isotopic signatures • Radioactive isotope signatures

Major, minor, and trace constituents in a mineral • A handsample-size rock or mineral

Major, minor, and trace constituents in a mineral • A handsample-size rock or mineral has around 5*1024 atoms in it – theoretically almost every known element is somewhere in that rock, most in concentrations too small to measure… • Specific chemical composition of any mineral is a record of the melt or solution it precipitated from. Exact chemical composition of any mineral is a fingerprint, or a genetic record, much like your own DNA • This composition may be further affected by other processes • Can indicate provenance (origin), and from looking at changes in chemistry across adjacant/similar units - rate of precipitation/ crystallization, melt history, fluid history

Chemical heterogeneity • Matrix containing ions a mineral forms in contains many different ions/elements

Chemical heterogeneity • Matrix containing ions a mineral forms in contains many different ions/elements – sometimes they get into the mineral • Ease with which they do this: – Solid solution: ions which substitute easily form a series of minerals with varying compositions (olivine series how easily Mg (forsterite) and Fe (fayalite) swap…) – Impurity defect: ions of lower quantity or that have a harder time swapping get into the structure

Compositional diagrams Fe. O wustite Fe 3 O 4 magnetite Fe 2 O 3

Compositional diagrams Fe. O wustite Fe 3 O 4 magnetite Fe 2 O 3 hematite A Fe O A 1 B 1 C 1 A 1 B 2 C 3 x x B C

Si fayalite forsterite enstatite Fe Mg fayalite Fe ferrosilite forsterite Mg Pyroxene solid solution

Si fayalite forsterite enstatite Fe Mg fayalite Fe ferrosilite forsterite Mg Pyroxene solid solution Mg. Si. O 3 – Fe. Si. O 3 Olivine solid solution Mg 2 Si. O 4 – Fe 2 Si. O 4

Stable Isotopes • A number of elements have more than one naturally occuring stable

Stable Isotopes • A number of elements have more than one naturally occuring stable isotope. – Why atomic mass numbers are not whole they represent the relative fractions of naturally occurring stable isotopes • Any reaction involving one of these isotopes can have a fractionation – where one isotope is favored over another • Studying this fractionation yields information about the interaction of water and a mineral/rock, the origin of O in minerals, rates of weathering, climate history, and details of magma evolution, among other processes

Radioactive Isotopes • Many elements also have 1+ radioactive isotopes • A radioactive isotope

Radioactive Isotopes • Many elements also have 1+ radioactive isotopes • A radioactive isotope is inherently unstable and through radiactive decay, turns into other isotopes (a string of these reactions is a decay chain) • The rates of each decay are variable – some are extremely slow • If a system is closed (no elements escape) then the proportion of parent (original) and daughter (product of a radioactive decay reaction) can yield a date. • Radioactive isotopes are also used to study petrogenesis, weathering rates, water/rock interaction, among other processes