Week 11 Lecture

silicates

Pyroxenes

single chain silicates. Limited polymerization, include orthorhombic orthopyroxenes (enstatite) and monoclinic clinopyroxenes (augite). Often show exsolution; 1:3 ZO ratio

Orthopyroxene extinction

parallel

Clinopyroxene extinction

inclined

Pyroxene composition

area above quadrilateral is where exsolution occurs

Amphiboles

Double chain silicates; monoclinic clino-amphiboles (hornblende) 4:11 ZO ratio. Common in igneous rocks in subduction zones; have (OH) groups

Mica group

Sheet silicates (muscovite)

Dioctahedral muscovite (sheet silicates)

one octahedra vacant for every two octahedra that are filled with Al3+. Here, dioctahedral sheet is gibbsite: Al2(OH)6

Trioctahedral phlogopite (sheet silicates)

all octahedral occupied and filled with Mg2+. Here, trioctahedral sheet is brucite: Mg(OH)2

Olivine group

High relief with lack of cleavage. Independent tetrahedra (orthosilicates). Forsterite-fayalite (Mg,Fe)2SiO4. Common in gabbro, basalt, peridotite.

Rock with >90% olivine = dunite

Common accessory minerals in silicates

Sediment

Particles derived from the weathering process

Diagenesis

Occurs during and after lithification.

Compaction → cementation → recrystallization (changing of fine-grained rocks into coarser ones)

Dissolution and removal of minerals (leaching) and the formation of clay or other minerals are both common

Authigenic minerals

Products of diagenesis (Zeolites, clays, feldspar, pyrite, and quartz)

Mechanical weathering

Weathering by physical processes (erosion); produces detrital sediment. Minerals that survive the weathering process (quartz in sandstone)

Chemical weathering

Weathering by chemical reactions (acid rain). produces chemical sediment (clay minerals, oxides, hydroxides)

Precipitation

Minerals that form as a chemical precipitates (halides, carbonates, sulfates)

Why are some minerals more common as detrital minerals?

Because of their weathering resistance. Rocks formed further below the earth will be more unstable at the surface due to the high pressures/temps they form at and their unstable nature (olivine)

The last minerals to crystallize in Bowen’s reaction series are

the most stable at the surface due to low Gibbs energy. They are most resistant to weathering

Goldich weathering series

inversion of Bowen’s series to describe weathering

Chemical reactions can alter

both the mineralogical and chemical composition of a rock

Hydrolysis

Minerals that react with water, causing new minerals to form. Happens in subsurface ad surface

Oxidation

reaction with oxygen (rusting). Can affect any Fe-bearing mineral. Reactions can form hematite and limonite.

Acid mine drainage

An example of oxidation. The oxidation of pyrite and other sulfide minerals forms sulfuric acid, which acidifies surface water

Dissolution

dissolving of minerals in a solution

Clay minerals further decompose to…

Aluminum hydroxides (bauxite) and dissolved silica

Controls on weathering

• Resistance of primary minerals to chemical weathering

• Climate

  • availability of water for chemical weathering

  • temperature (affects reaction rates)

  • vegetation (increases availability of carbonic acid)

Goethite

FeO(OH); Iron ore. Formed by oxidation of Fe-rich
minerals. Typically massive, fibrous; except bog ore

Kaolinite

Al2Si2O5(OH)4. Authigenic (forms in place). Hydrolysis of Al-silicates, such as feldspars. Clay - needs XRD for ID. Ceramics, cosmetics, Kaopectate

Carbonates

Have (CO₃)^2- as anionic complex

Calcite group (carbonates)

Hexagonal. Effervescence. Common as precipitates or replacement minerals. Dolomite is part of this group but is not isostructural with calcite; forms through replacement after calcite

Aragonite group (carbonates)

Orthorhombic; Aragonite and calcite are polymorphs. No rhombohedral cleavage. Less stable at surface conditions (forms at low T, high P)

Halides

Cations bonded to Cl- or F-. Isometric, common in evaporites

Sulfates

Built with the (SO₄)^2- anionic complex. Gypsum and anhydrite are common in evaporates

Chert/agate

Microcrystalline to cryptocrystalline quartz.

Chert: found in marine deposits (beds, nodules); BIF

Agate: fills vugs and other voids