Chapter 4 Mineral Reactions, Stability, and Behavior
I. Crystallization
a. Crystals are formed from solutions, melts and vapors – these are disordered states with random distribution,
i. Change temperature,
ii. Pressure,
iii. And Concentration
iv. Solution
1. Sodium chloride with slow evaporation, crystals will grow, size depends on time
2. lowering temperature or pressure, hot water and/or high pressure dissolves more salt than cold water
v. Melt
1. Crystallization from fusion is ice crystals, water is fused ice?
2. Crystal growth in a cooling magma is the result of two competing tendencies
a. Thermal vibrations that tend to destroy the nuclei of potential minerals
b. Attractive forces that tend to aggregate atoms (and/or ions) into crystal structures.
c. As the temperature falls, the second tends to dominate
vi. Vapor
1. less common
2. Sulfur in volcanic fumaroles.
b. Crystal growth
i. First step is nucleation with a nucleus or seed
1. Not all becomes a crystal as the nucleus and dissolve or break down, due to a high surface area with a high degree of unsatisfied ions
2. It must grow rapidly to reduce its surface energy, reach a critical size
ii. After the critical size has been reached, then the surface energy shapes the crystal, edges have more energy in some ionic bonds producing a dendritic habit
c. Intergrowths of crystals
i. Most grow in random aggregates of anhedral grains
ii. Common intergrowths of well-formed crystals are not random in nature.
1. parallel growths of same substance
2. Twins of same minerals
3. epitaxis growths of different substance such as staurolite and kyanite
II. Mineral reactions
a. Reactions in an Igneous Regime
i. 95% of upper 10 miles of earth’s crust.
ii. Formed from magma about 900 to 1600C
iii. Composed of several elements see page 107
iv. Definite order of crystallization (if elements are available)
v. Bowen’s reaction series
1. Continuous reactions
a. Solid solution series where the composition of early formed crystals change continuously due to interactions with the surrounding melt
2. Discontinuous
a. In which early formed crystals react with the melt to give rise to new minerals with different crystal structures and chemical compositions
3. Magmatic differentiation
4. Involves Mg-Fe-rich igneous silicates
5. Crystals can settle out thus changing the composition of melt
6. lower of the series is the higher order of crystallization which affect weathering properties etc.
b. Reactions under metamorphic conditions
i. Preexisting rocks
ii. Changes caused by temperature, pressure, and shearing stress
iii. Weathering and chemical reactions are not involved and are listed in other categories
iv. Solid state reactions
v. Isochemical – bulk chemistry of a rock remains constant
1. Under high temperatures, some chemicals can be destroyed such as slate to shist
2. Dehydration can occur
3. Decarbonation
vi. Metasomatism – the addition of elements introduced into the rock by circulating fluids
vii. Contact
1. concentric zones (aureoles) around hot igneous intrusive bodies.
2. Lack schistosity
3. large temperature gradient producing zones that differe greatly in mineral assemblages
4. Sandstone to quartzites, shales to hornfels, etc.
viii. Regional
1. Increases in T or P or both on a regional scale (few hundred to thousands of miles in extent)
2. Response to mountain building or deep burial of rocks
ix. Both reflect an increases in temperature (progressive or prograde metamorphism)
1. Arrested prograde example is muscovite gives way to several products such as K feldspar, biotite, spinel and corundum
2. Arrested retrograde example is pyroxene to a complex mixture of hydrous silicates
x. However, when assemblages of high-temp origin (such as igneous rocks) fail to survive conditions of lower-temp metamorphism the process is referred to as retrograde to retrogressive metamorphism.
c. Reactions in a Weathering Environment
i. Stabilities relate to Bowen’s reactions series
ii. Three important high tempt minerals
1. layer silicates such as kaolinite and montmorillonite
a. ex. Orthoclase to muscovite to kaolinite
b. ex. Silimanite with quartz to kaolinite
2. Silica in solution as H4SiO4
3. Metal ions in solution
d. Ultrahigh-Pressure reactions
i. Mantle
1. change in density
2. change in temperature
ii. Makeup of the upper part of the mantle has been sampled directly in places where rocks have been brought to the earth’s surface by violent volcanic events as seen in kimberlite pipes
1. Most have their root zones in the upper mantle in a depth range of about 150 to 200 km. Some found in roots of mountains at about 100 km down that were later uplifted and exhumed
iii. Lab tests, using diamond anvil cell see page 113
iv. Minerals change with increasing pressure, see page 114
III. Mineral stability
a. Phase diagrams
i. The behavior of solids, liquids and gases under variable external conditions can be express in what is known as a phase diagram (phase meaning homogeneous substance with a well defined set of physical and chemical properties and can be used instead of the term mineral)
ii. See the diagram for water
1. first one showing polymorphs of ice
2. second one is simple with different parts
a. T = triple point
b. C = critical point – two phases are identical, beyond that is the supercritical aqueous fluid
c. A = line between two phases
b. Stability, Activation Energy, and Equilibrium
i. Stability is related to the energy of the system (see Gibbs free energy)
1. metastable – can easily change
2. some are conditionally stable
3. Others are completely stable
ii. Activation energy is the energy needed to produce a stable condition or for chemical reactions to occur
iii. Equilibrium – coexistence of two forms over time
1. No reaction rims
c. Components
i. Are the chemical entities necessary to define the compositions of all the phases in a system, end members can be components
d. Thermodynamics
i. All organizations of matter drive toward a minimal energy state or arrangement or the most stable state for their constituents
ii. 1 law of Thermodynamics – The internal energy (E) of an isolated system is constant
1. in a closed system, the mass remains the same but the energy can change dE = dQ - dW or the change in Energy is = change in heat – change in work, W = force x distance, force = pressure x surface area, W = pressure x volume
2. Therefore dE = dQ – PdV
iii. 2 law – Entropy (S), change in thermal energy of a system at a constant P and T to change in the degree of order, degree of disorder
1. dQ/T =dS
iv. 3 law – at absolute zero (0 Kelvin, or -273C) a crystalline structure approaches perfect order and S = 0.
v. Gibbs free energy G = E + PV – TS or Internal energy plus Pressure x Volume less Temperature x entropy.
1. Energy in excess of the internal energy or extra energy needed to drive a chemical reaction
2. dG = VdP – SdT
3. At equilibrium the Gibbs free energies of the reactant and product are equal
e. Examples of Phase diagrams
i. One component
1. Pressure and Temperature and one mineral with polymorphs
ii. Two component diagrams
1. Temperature composition with two end members
2. Top line is known as the liquidus diagram
3. Bottom line is the solidus diagram or curve
a. Pure end member A melts at Ta and pure end member B melts and Tb but intermediate compositions AB which consist of a single phase melt through a range of temperatures between Ta and Tb.
b. A melt of composition M at temp Tb will be entirely melt. When it cools to T1 it will start to crystallize out of a member of solid solution series AB with specific composition xA and yB.
c. These crystals are enriched in the B component with respect to the melt composition M and their growth will deplete the melt in component B.
d. The melt composition will as a result of this depletion, move along the liquidus curve toward a as indicated by the upper arrow.
e. As a result of the continual lowering of the temperature, the solid phase of original composition xA, yB will react with the melt in the direction of the lower arrow along the solidus.
f. As such, both the melt and the crystalline products will increase in content of A with decreasing T and the ratio of the solid to melt will increase.
g. Finally, at T2 the crystallized products have a composition which is tat of the original melt M and the amount of melt in equilibrium with the crystals will reach zero.
h. Only a solid phase remains with continued lowering of the temperature the composition of the crystalline product will remain constant at the bulk composition M of the original melt
iii. Type 2
1. Two component system for feldspars series is an example of a limited solid solution
2. No solid solution at low temp (due to differences in size of ion)
3. Point e is the eutectic point, where A + B able to coexist with the melt. Also, lowest temperature of the melt
4. perthetic intergrowths represent exsolution in a solid state
5.