Untitled Flashcard Set

Crystal growth: stability and energy

Mineral stability

• Availability of constituent atoms: aluminum garnet growth requires - Si, Al, O, and at a minimum one of ……………

• Lower energy states or configurations of a system [rock] are more stable than higher energy states

• By analogy consider a rock in the air, on a cliff ledge, and at the base of the cliff. In the air the rock is unstable, on the ledge it is metastable [requires activation energy to dislodge], and at the cliff base is 'stable'.

Gibbs free energy [G] is a measure [energy] for evaluating chemical stability. The units are calories or joules. G is evaluated by comparison to the energy needed to make a material from elements - free energy of formation - deltaGf. At a specified T and P, the most stable mineral is that with the lowest deltaGf.

Minerals may be metastable. For example, in spite of predicted stability of quartz over tridymite at STP, tridymite can be found at the surface.

----slide in between----

Nucleation

Homogenous nucleation: atomps move into locations to form a small lattice element - nucleus. Fluids, hydrothermal vents, etc.

Heterogeneous nucleation: atoms attach to a preexisting mineral in such a way that a new nucleus can form. Common in igneous rocks

Epitaxial nucleation

Chemistry

Chemical composition of the earth

Atomic Structure

The Bohr Model:

1. An atom is comprised of:

a. Nucleus = protons & neutrons; AMU=1.00728 & 1.00867,

respectively, &

b. Electrons; AMU=0.00055.

*1 AMU=1/12 mass 12C atom

2. Electrons ‘orbit’ in discrete energy levels

3. Each nucleus has a specific # of protons = atomic number (Z) -- 111 elements

Quantum Mechanics

Why is the Bohr model not good enough?:

1. We know that electrons are not always in spherical distributions

around the nucleus

2. Cannot precisely predict location at any given time [Heissenberg];

therefore, we need a tool for statistical prediction

Energy levels for electrons described by:

1. Principle quantum number [n]

-positive integer principle control on energy -- shells = K, L, M, N…

2. Angular momentum quantum number [l]

-0 to n-1 -- subshells and shapes = s, p, d, f

3. Magnetic quantum number [ml]

- -l to l differently oriented orbitals in subshell (2 l + 1 orbitals in subshell)

4. Spin quantum number [ms]

- +/-1/2

Wave Model of the Atom

deBroglie hypothesized wave nature of the atom:

E = hv = mc2 = hc/delta and

delta = h/mc

Energy (Joule) J- kgm/s2

Frequency (Hertz) v = 1/s

Planck’s constant h - Js, 6.62 x 10-34

Wavelength delta - m

Speed of light c - m/s, 2.99 x 108

Mass m - kg

m = Js/(kg/s) = (kgm/s2 s)/(kg/s)

Schrodinger Wave Equation

Statistical prediction for electron position. This avoids the Heissenberg

Uncertainty Principle - namely the inherent impossibility of

measuring position and velocity of small particles.

One-dimensional case:

Consider a standing wave in a string fixed at both ends - sine wave

Points of no displacement are nodes - zero e- density

Different numbers of nodes are produced as frequency changes

Quantum number n gives the number of half wavelengths

Amplitude (psi ) at any distance = A sin 2(pi)r / delta

A = max amplitude

Square of amplitude is probability of occurrence at r

Three-dimensional case:

psi= wave function in 3d, psi 2(x,y,z) = probability density of finding an electron

Bonding

End member bond types:

1. Ionic- can be considered as attraction between + and - ions or e-exchange between atoms. Occurs between atoms of greatly different e-affinity.

2. Covalent- can be considered as short-range e- sharing between atoms [overlap between valence orbitals]. Occurs between atoms of similar e-affinity.

3. Metallic- can be considered as long-range e- sharing in a conduction band [throughout a lattice]. Occurs between atoms of similar e- affinity.

4. Hydrogen- polar water molecules: ice

5. Van der Waals/Molecular - can be considered as weak attraction between polarized molecules

Ionic Bonds

Forces are electrostatic w/ attraction between positive and negative ions

The negatively-charged electron clouds of Na and Cl repel each other when the shells overlap -

FR a -n/d 1+n

where n is an integer depending on the number of e- shells filled

The net opposite charges attract (Coulomb’s law) -

FA a (q+)(q-)/d2

Equilibrium and chemical stability is attained when repulsive and attractive forces balance; therefore, there is an equilibrium distance that is prescribed by:

FN = FA + FR = 0

Molecular Properties:

Strong bonds result in hard minerals with high melting T

Localized electrons allow these minerals to dissolve and produce ions in solution

Generally low electrical and thermal conductivity - electrons are localized

Bonds are strongly symmetric so close packing of ions is common

Halite (Ionic Bonds):

Covalent Bonds

Covalent bonds are formed when e- sharing occurs between adjacent atoms and valence shell

orbitals overlap. Strength of bonds is proportional to the amount of orbital overlap. Sharing

occurs for enough e- to approximate each atom having a ‘noble gas’ configuration.

Diamond:

C ground = 1s2, 2s2, 2p2

C promoted = 2s1, 2p3

C hybrid = sp3

Properties:

Strong bonds result in hard minerals with high melting T

Generally low electrical and thermal conductivity - electrons are localized

Bonds are strongly directional so close packing is uncommon

Metallic Bonds

Metallic bonds are similar to covalent bonds except that the e- are

delocalized and form a conduction band. These bonds are

favored by weakly held e- and numerous e- in the valence shell

Bonds between two atoms of the same transition element or two

similar transition elements are most likely to have metallic bonds

Properties:

Relatively weak bonds result in soft minerals with

low melting T

Conduction band allows for high electrical and

thermal conductivity - electrons are not localized - they are free to move about

Bonds are symmetrical about near-spherical atoms so close packing is common

Molecular/Van der Waals Bonds

Discrete molecules bonded by attraction

between polarized molecules. Internal

bonds within molecules may vary.

Ex: graphite, sheet silicates

Properties:

Weak bonds result in soft minerals with moderate melting T

-- bonds tend to be weak in one or more orientations - they often have

pronounced cleavage

Low electrical and thermal conductivity - electrons are localized

Bonds are heterodesmic and asymmetrical - no closest packing

Oxide & Silicate Bonds

Most oxide and silicate bonds can be considered as partly ionic and partly

covalent. % ionic character can be determined from the difference in

electronegativity between cation and ion:

% ionic = (1 - e -0.25(xa - xc)2 ) x 100

Where xa = electronegativity of the anion, and

xc = electronegativity of the cation

Complex silicate minerals contain more than one bond type = heterodesmic [as opposed to homodesmic]

Crystal Structure

Ionic-covalent mixed bonds-

Ex: silicates

O-Si is ca. ?? ionic

O- Al, Fe, Mg, Ca, Na, & K are more ionic

Ionic radii

Atoms are not perfect spheres and their shape is controlled by outer orbital

shape and adjacent atoms (bonds). However, we generally can learn

from approximating each atom as a sphere.

Effective radius = size based on the distance between the centers of adjacent

atoms.

Atomic & Ionic Size

Atomic size depends strongly on e- configuration

Neutral atoms decrease in size from left to right across periodic table with

increased attraction between nucleus and electrons. However,

neutral atoms increase in size downward in the periodic table with

addition of orbital shells.

Ionic size is strongly dependent on oxidation state and on the number of

surrounding atoms. Ions decrease in size as charge increases.

Effective Radii

Anions are generally larger than cations so substances can be considered

as a framework of anions with cations in the gaps between anions.

Metallic bonds: L = 2 Rm

Ionic bonds: L = Ra + Rc

where L = length between centers of adjacent atoms

Ionic radius depends on coordination number

(e.g., Shannon, 1976):

For Al3+

CN = IV, r = 0.53 Å

CN = VI, r = 0.68 Å

Ionic radius (see appendix in text) depends on coordination number (e.g.,

Shannon, 1976):

For the REE element Nd3+

CN = VI, r = 1.12 Å

CN = VIII, r = 1.25 Å

CN = XII, r = 1.41 Å

Crystal Structure

The first order control on crystal structure is the nature of chemical bonds:

Metallic- highly ordered arrangements with dense packing (little void space)

Closest Packing of Spheres

- Hexagonal closest packing HCP

or

- Cubic closest packing CCP

Coordination number (CN) = number of nearest

neighbors

Radius ratio: RR = Rc/Ra

Pauling’s Rules

Rule 1: Interatomic distances

A coordination polyhedral of anions is

formed around each cation and the cation-

anion distance is determined by the radius

sun and the CN by radius ratio (Rc/Ra)

Rule 3: Sharing of polyhedral elements 1

Sharing polyhedral elements brings

cations closer and this destabilizes the

structure due to repulsion of positive

charges

The most stable configuration is with cations

furthest apart -share one corner only.

Rule 4: Sharing of polyhedral elements 2

High valency low CN cations are least likely to share polyhedral elements

High valency small cations with low CN are least likely to share polyhedral elements. When they do share, polyhedra are distorted and cations displaced away from each other

Rule 5: Parsimony

Minerals with complex compositions have a limited number of sites and ions of similar size will substitute for one another - solid solution. For example, in amphibole the M1-M4 octahedral sites may contain Ca, Mg, Fe, and other elements.

Types of Crystal Structures

Isostructural: two or more minerals with identical structures

Example: halite [Na and Cl] and galena [Pb and S]

Identical symmetry (holohedral isometric), cleavage (3 at right

angles), and habit (cubic)

Octahedral coordination of anions and

cations

Polymorphism: the ability of one chemical compound to crystallize with more than one structure

Example: Silica Group = SiO2

• a-quartz

• b-quartz

• Tridymite

• Cristobalite

• Stishovite

• Coesite

Example: Aluminum Silicate Group = Al2SiO5

• Andalusite

• Kyanite

• Sillimanite

Reconstructive

Example: SiO2

high cristobalite

high tridymite

Displacive

Example: SiO2

a-quartz

b-quartz

Order-Disorder

Example: KAlSi3O8 - Feldspar

microcline (ordered)

high sanidine (disordered)

Polytypes

Variations in the way that sheets or layers are stacked

Examples: graphite, mica, and clay minerals

Consider stacking octahedral sheets which have O or OH at ‘corners’ of octahedra

Compositional Variation in Minerals

• In general, minerals are not pure, but may be part of a solid solution or contain trace impurities that may be important [e.g., color and some other properties are controlled by trace chemistry]. Therefore, mineral analysis is important.

• The amount of solid solution, trace elements, and the analytical accuracy/precision are facilitated by calculation of chemical formulas from analyses.

• Mineral compositions may be important indicators of natural resources and/or

conditions of rock formation (e.g., pressure, temperature, and chemical

environment).

• Pure phases need only be analyzed if we are interested in trace components – a few to hundreds of parts per million.

• However, solid solution minerals may vary substantially in composition and

we often want to know the exact composition for a specific sample.

Mineral grains may be analyzed in bulk or in situ.

1) Bulk analysis can be undertaken by crushing a rock or mineral grain and then using either the dissolved or powdered material. Typically X-ray fluorescence (XRF) or inductively coupled plasma techniques are used for this type of analysis.

2) In situ analysis is typically done on a thin section, which has been evaluated petrographically, or on a polished rock surface.

Typically electron probe micro analysis is used for major element concentrations. Micro XRF, laser, or ion beam techniques can be used for obtaining trace element concentrations and the latter 2 techniques can also be used for isotopic analysis.

Solid solution: different cations can occupy the same structural sites [e.g., octahedral, tetrahedral, etc..] amongst the anions.

Types of solid solution

• Substitution simple one for one vs. coupled

One for one-example: olivine (Fe,Mg)2SiO4

Fe - Mg

Coupled-example: plagioclase (Na,Ca)Al(Al,Si)3O8

Si4+Na+ - Al3+ Ca2+

Omission: vacant site(s)

• Example: pyrrhotite Fe2+(1-3x) Fe3+2x❑xS

Interstitial: between sites

• Example: beryl Al3+Na+ - Si4+ ❑

Crystal Structure: a slight return

Radius ratio: RR = Rc/Ra

Solid Solution in Minerals

Solid solution: property describing the variation of minerals within a definite range of composition [between end members] imparting a range of properties. Different cations can occupy the same structural sites [e.g., octahedral, tetrahedral, etc.. amongst the anions.

We generally think about minerals varying between limits - these limits are called mineral end members.

• Example - olivine = (Mg,Fe)2SiO4

• Fayalite = Fe2SiO4

• Forsterite = Mg2SiO4

The Bohr Model:

1. An atom is comprised of a nucleus & electrons

2. Electrons ‘orbit’ in discrete energy levels, but can be excited by incoming

energy

Types of Chemical Analyses

• ”Wet chemical"/gravimetric analysis

• ICP-OES & ICP-MS [Inductively-coupled plasma -- Optical emission or mass spectrometry]

• X-ray fluorescence [XRF] [pressed pellet or fused sample]

• In situ techniques

  • Electron probe microanalysis [EPMA]

  • Laser ablation ICP-MS

  • Micro XRF

Wet chemical techniques allow determination of the oxidation state for elements like Fe and determination of H2O content. Most techniques do not yield this information.

Electron Probe Microanalyzer

The EPMA has several component parts:

• an electron source (gun),

• sample chamber (vacuum),

• wavelength dispersive spectrometers with crystals and detectors,

• energy dispersive spectrometer,

• back-scattered electron detector,

• scanning electron detector, and

• computer control.

Operation

• electron beam excites atoms in sample that emit background & characteristic X-rays

• electron beam can be moved (rastered) across sample by lenses and reflected/backscattered electrons collected to produce surface map OR characteristic X-rays collected for a chemical map

Uses

• quantitative chemical analysis [from characteristic X-rays] of microscopic areas (ca. 1 micron) of solids

• imaging of surfaces (scanning electron mode) and chemical variation (backscattered electron or X-ray map modes)

Results = chemical composition

Wt. % elements or oxides

No direct mineral identification

Mineral Formulas

Principles & Procedures:

Balanced charge for chemical stability

For many analytical technique the number of anions is assumed in

order to balance the charge

Cations that will occupy one structural site are grouped

Cations in different sites are listed in order of decreasing CN

Calculation of Mineral Formulas from Chemical

Analyses

Anhydrous silicate minerals - example pyroxene

1. divide oxide weight % by mole weight to get mole proportions -MP

2. multiply MP by the # of cations in oxide to obtain the # of cations

3. calculate the # of oxygen by multiplying MP by the # of O in the oxide

formula, then sum to determine the total # of O

4. calculate O correction factor by dividing the total # of O desired (X) by

the actual total of O - multiply the # of each cation by the correction

factor; this yields the cations on the basis of X O [X=6 for pyroxene]

5. assign cations to mineral structural sites on basis of radius and charge

6. Repeat above for each oxide

Calculation of Mineral Formulas from Chemical Analyses

Anhydrous silicate mineral: pyroxene - Fe, Mn, Mg, Ca, Ti, Na, K, Al, Si, & O

Calculate the cations on the basis of 6 oxygen

Assign cations to specific sites

Fill tetrahedral site with Si then Al Total = 2.0

Assign inter. size cations to M1 (Al, Ti, Fe3, Fe2, Mn, & Mg

Assign remaining cations to the larger M2 site (Ca, Na, K)

The analysis contains Ca, Mg and Fe; therefore, the analysis can be cast in terms of

wollastonite, enstatite and ferrosilite end members. Simply sum the cations of these end

members and calculate the % of each.

activity models for mineral components

Graphical Representation of Mineral Compositions

Two components - linear

Three components - ternary

Four components - tetrahedron [must use projection -

graphical or mathematical]

Exsolution in Minerals

Solid solutions can become unstable as temperature or pressure changes. T is generally

the most important variable. As T decreases bonds shrink and instability of large ions

may ensue.

This can result in physical separation [exsolution] of one mineral composition or phase

from another.

Example - pyroxene

CaMg0.5Fe0.5 Si2O6 -

Mg1Fe1Si2O6

X-ray Analysis of Minerals

Application of X-rays to the study of crystals was one of the greatest advances

in crystallography. Prior to 1912, all information about crystal structure was

deduced from cleavage, optical properties, and crystal morphology. X-rays

provided the first means for crystallographers to test their hypotheses

about crystal structure.

X-rays were discovered by Roentgen in 1895, who accidentally produced them

by bombarding glass with electrons.

Electromagnetic radiation:

propagates in a straight line ca. 300,000 km/s (vacuum) or 2.998 x 108m/s

reflects [angles of incidence & reflection are equal]

refracts according to Snell’s law

diffracts at edges and around slits and gratings

E = hv = hc/delta

where E=energy; h=Planck's constant; v=frequency; c=velocity of light; delta=wavelength

X-rays are electromagnetic radiation with delta = 10 Å to 0.1 Å. X-rays used in crystallography - delta = ca. 1 Å (10-10 m or 0.1 nm)

X-ray Production

X-rays are produced in a vacuum tube which consists of an electron source [filament] and a

target of pure material [e.g., Cu, Mo etc…].

Electrons are accelerated into the target where X-rays are produced and exit the tube through

a thin Be window

Filters and slits collimate the X-rays and filter out most of the continuous spectrum

Continuous/background: this spectrum results from energy loss of electrons hitting the target -

all possible amounts of energy may be lost up to the initial energy = max energy.

Characteristic: Intense peaks caused by the dislodgement of electrons in the target and

transition of electrons into the vacated orbitals

delta = c h /∆E = 12.4/∆E

c = speed of light, h = Plank’s constant

X-ray Diffraction

X-rays will be in phase only if pqr = n l

pqr = 2 pq, and

pq = d sin theta

Therefore: n delta = 2 d sin theta      Bragg's Law

n is an integer, d is the distance between successive planes of atoms,

and

theta is the angle of incidence and reflection

X-ray Diffraction: The powder method

A finely-powdered sample is hit by monochromatic X-rays

assumption: the powdered sample will have randomly oriented grains

The sample is rotated through a range of 2 theta while the detector is moved in order to satisfy the Bragg law for the monochromatic X-rays

Powder X-ray Diffractometer

All ‘reflections’ with relative intensities are compared with the mineral

database in the

International Tables for X-Ray Crystallography

Result: mineral identification

No direct mineral composition data

What to know:

Definition of a mineral

Solid solution

X-rays and characteristic X-rays

EPMA & XRD principles and deliverables

Polymerization and cation coordination

silicate polymers – different types

Mineral classification

Mineral Classification

Anion or Anionic Group

Why?

Subclassification by lattice/structure or cation

Anionic Groups

Polymerization

Anionic Groups

Many minerals are best considered as lattices that are comprised of

strongly-bonded anionic groups that are linked by cations

The elemental groups have a net negative charge

Examples:

Carbonates (CO3)2- ; sulfates (SO4)2- ; phosphates (PO4)3-

Polymerization

Polymerization. Linking of cations by sharing adjacent anions. In

silicate structures, Si share O and this requires mesodesmic bonds.

Mesodesmic molecules can polymerize or link together with

identical molecules into chains, sheets, and/or 3d frameworks

Example: silicate minerals

The Si-O bonds satisfy 1/2 of the O -2 charge therefore the O atom

can link with another silicon atom

Silicate Minerals

The silica tetrahedron CN = 4

Polymerization

Framework Silicates

4 shared O

Silica SiO2

Polymorphs - P and T dependent

Feldspar Group -- 4 shared O

Albite [Na]-Anorthite [Ca]-Potassium Feldspar [K]

Albite -NaAlSi3O8(triclinic); variety: cleavelandite

Sheet silicates

Sheet silicate minerals include a large number of

important mineral groups: clay, serpentine, talc,

mica, & chlorite

Gibbsite to Pyrophyllite -- three shared O

Muscovite -- three shared O

Chain silicates

Single or double chains of silica tetrahedron

Single – 2 shared O

Double – alternating tetrahedra 2 or 3 shared O

Pyroxene -- two shared O -- single chain: most are Ca-Mg-Fe

ternary (Ca,Mg,Fe)2Si2O6

Diopside

Amphibole -- two/three shared O -- double chain: complex Ca,Mg,Fe,Al silicate w/ OH

W0-1X[M4]2Y[M1,2,3]5T8O22(OH)2

Ring silicates

Tourmaline -- 2 shared O: complex Aluminosilicate w/ OH

Disilicates

Epidote -- 1 shared O: Fe,Ca,Al silicate w/ OH

Orthosilicates

Olivine -- no shared O: common = (Fe,Mg)2SiO4

Forsterite = Mg2SiO4

Garnet -- no shared O: aluminum garnet =

(Ca,Fe,Mg,Mn)3Al2Si3O12

Crystal Growth: stability & energy

Mineral stability

Availability of constituent atoms: aluminum garnet growth requires - Si, Al,

O, and at a minimum one of Fe, Mg, Mn, Ca

However, many minerals contain these elements. So, we must consider

stability and energy

Mineral stability

Lower energy states or configurations of a

system [rock] are more stable than higher

energy states.

By analogy consider a rock in the air, on a

cliff ledge and at the base of the cliff. In

the air the rock is unstable, on the ledge it

is metastable [requires activation energy

to dislodge], and at the cliff base ‘stable’.

Gibbs free energy [G] is a measure[energy] for evaluating chemical stability.

The units are calories or joules. G is evaluated by comparison to the

energy needed to make a material from elements - free energy of

formation - ∆Gf. At a specified T and P, the most stable mineral is that with

the lowest ∆Gf.

Minerals may be metastable. For example: in spite of predicted stability of

quartz over tridymite at STP, tridymite can be found at the surface.

Mineral Reactions

Balanced chemical reactions can be evaluated to determine the stable minerals -

reactants OR products:

∆Greaction = ∑∆Gf-products - ∑∆Gf-reactants

If ∆Greaction < 0, then the products are more stable and the reaction should proceed

forward. If ∆Greaction > 0, then the reactants are more stable and the reaction should

proceed backward. If ∆Greaction = 0 then equilibrium has been achieved and no net

reaction occurs.

Mineral Reactions-example

muscovite + quartz = orthoclase + sillimanite + water

KAl2(Al,Si3)O10(OH)2 + SiO2 = K(Al,Si3)O8 + Al2SiO5 + H2O

∆Greaction = ∑∆Gf-products - ∑∆Gf-reactants

= (C+D+E) - (A+B)

Crystal Growth: nucleation

Homogeneous nucleation: atoms move into locations to form a small lattice

element -nucleus

Heterogeneous nucleation: atoms attach to a pre-existing mineral in such a

way that a new nucleus can form

Epitaxial nucleation

Homogeneous nucleation [crystallize from melt]:

G of transition from melt to crystal:

∆Gv = [∆Gf-xstal - ∆Gf-melt]v

where v = volume

G of surface [surface free energy]:

∆Gs = ga

where g is the surface energy per area & a is the crystal surface area

For a hexahedron with sides of length c:

∆G = ∆Gv - ∆Gs

∆Gv = [∆Gf-xstal - ∆Gf-melt]c3 + g6c2

Heterogeneous nucleation:

Epitaxial nucleation -kyanite & staurolite

Crystal Growth: Changing forms

Garnet

Crystal forms evidenced by

variation in chemical

composition which varies

spatially and temporally

Structural Defects in Minerals

Real crystals always contain significant departures from the ideal

of perfectly regular and repeating patterns of atoms.

Defects:

Reduce the strength of minerals

Increase the Gibbs free energy – decrease stability

Types of Defects

Point:

– Schottky: vacancies. In ionic crystals, charge is in some way balanced.

– Frenkel: mislocations. Ion is located in a normally empty site.

– Impurity: elements not in the chemical formula. These may be interstitial or substitutional.

Line defects: edge of a propagating slip surface where chemical bonds are broken

These can be defined with a mismatch vector or Buergers vector.

– Edge: a line formed by a half plane of atoms that terminates in the edge-

Buergers vector or dislocation direction is at right angles to dislocation

line

– Screw: Buergers vector is parallel to line

Line defects result in planes that are weaker than ‘perfect’

lattice and accommodate strain or change in shape.

– Dislocation slip & climb

– Dislocation pinning

– Diffusion

Planar defects

– Grain boundaries: In a monomineralic material, angular mismatch of

lattice across a boundary

– Stacking faults: disruption in the expected pattern of atomic layers

– Antiphase boundaries: boundary between crystal segment with a

translational relation to remainder of the lattice

Twinning

Symmetrical intergrowth of two or more crystal segments of the same mineral. Segments

are joined along a surface; therefore, it can be considered a variety of planar defect.

– Twin operations: symmetry operations relating the crystal segments.

– Twin laws: symmetry and orient. of crystal plane or axis

– Composition surface: surface separating the crystal segments

Twin types

– Penetration: twins with irregular composition surface

– Simple: two twin segments

– Multiple: three or more twin segments

– Polysynthetic: multiple twins with parallel composition planes

– Cyclic: multiple twins related by rotation axis

Twinning:

• Growth-growth mistakes?

•  Transformation-cooling

• Deformation-change in shape from stress

Mineral Classification

Minerals are classified based on anion or anionic group

Minerals with the same anion (or group) have similar

crystallography and chemical properties, and occur in similar

chemical (geological) environments.

So, chemical (anion) primary and lattice structure secondary

Tectosilicate Lattices

Tecto-/Framework silicates - silica

group, feldspars, zeolites

Define

1. polymerization

2. Tectosilicate

3. Phyllosilicate

• Continuous framework of Si tetrahedra all sharing 4 O with adjacent Si cations

• Si/O = ½

• Mesodesmic Each Si satisfies ½ O2- charge

• Silica Group: Polymorphs of SiO2

• Very limited substitution of impurities

Quartz -SiO2

– Hexagonal [trigonal]

– 3-fold symmetry is characteristic of quartz

– Occurs in many igneous, metamorphic, & sedimentary rocks, and is the

most common mineral in hydrothermal and metal-bearing bearing veins

– Uses: gemstone and ornamental, sand, mortar, concrete, manufacture of

glass and brick, powdered form in porcelain, paints, sandpaper, and

soaps, lenses for optical instruments, radio oscillators to permit

transmission and reception, digital watches

-Polymorphs of SiO2: tridymite, cristobalite, coesite, stishovite

– Some “gem” crystalline forms of SiO2: smoky quartz, amethyst, & rose quartz

– Microcrystalline SiO2: chalcedony (pale fibrous variety), Agate (layered

alternating bands), Carnelian (red-orange chalcedony), and Onyx (straight

alternating layers)

– Granular varieties include: Flint, Chert, & Jasper

– Hydrated SiO2 = opal, SiO2 nH2O

Feldspars

– The most common mineral group in Earth’s crust; accounts for ca. 60% of the continental crust

-Alkali Feldspar 1 to 1 solid solution

Na1+ - K1+

-Plagioclase coupled solid solution –

charge balance

Si 4+Na1+ - Ca2+Al3+

-Ternary solid solution and immiscibility

Plagioclase Feldspars

– Triclinic – bar1

– Complete solid solution [high T]: albite to anorthite

– What type of substitution allows this solid solution?

– Distinguished from other feldspars by the common presence of albite

twin striations-polysynthetic

– Used commercially in ceramics

• Albite NaAlSi3O8

  • Commonly found in granites, rhyolites, and pegmatites

  • Sometimes shows opalescence, known as moonstone

• Anorthite CaAl2Si2O8

  • Found in low Si and high Fe+Mg volcanic and contact metamorphic deposits

• Example - Labradorite An50-60

  • Common in gabbro and basalt; major constituent of the rock -anorthosite

  • Often shows a fine iridescent play of colors

Alkali Feldspars

• Three Polymorphs: sanidine, orthoclase, & microcline

• Complete solid solution - high T: Potassium feldspar to

albite

• Orthoclase KAlSi3O8

  • Monoclinic 2/m

  • Short prismatic crystals; commonly cleavable to granular masses

  • Common Carlsbad penetration twins, characterized by right-angle cleavage and lack of twin striations on the best cleavage surfaces

  • Major constituent of granite, granodiorite, and syenite

• Sanidine KAlSi3O8

  • Monoclinic 2/m

  • Short prismatic crystals

  • Carlsbad penetration twins, right-angle cleavage and lack of twin striations on the best cleavage surfaces

  • Major constituent silica rich volcanic rocks – often a phenocryst

• Microcline KAlSi3O8

  • Triclinic

  • Habit, crystal forms, and twinning same as orthoclase

  • Distinguished from orthoclase by the presence of “tartan” twinning, not seen in orthoclase

  • Prominent constituent of igneous rocks

  • Used in the manufacture of porcelain

Feldspathoids

–minerals containing about 2/3 the amount of

silica crystallize from a silica deficient magma;

tend to form from melts rich in alkalis

• Leucite KAlSi2O6

  • Tetragonal [pseudoisometric]

  • Usually in trapezohedral crystals

  • Abundant in alkali rich silica poor lavas

• Sodalite Na8(AlSiO4)6Cll2

  • Isometric

  • Commonly massive; usually dodecahedron crystals

  • Identified by blue color

• Zeolites

  • Hydrous framework aluminosilicates, containing Na, Ca, and or K

  • Chain [natrolite] or sieve structures [chabazite]

  • Commonly fibrous or massive

  • Hexagonal, orthorhombic or monoclinic

  • Soft and low SG

  • Occurrence:

• Cavities and veins in mafic igneous rocks

• Also altered volcanic tuff and glass

• Properties

• Name from Greek –boil and stone, fuse and swell on heating

• Readily dehydrated and rehydrated

• Uses

• Dessicants –remove water from hydrocarbons

• Molecular sieve –absorb some hydrocarbons

• The zeolite mineral mordenite (MOR): SiO4 tetrahedra are yellow; AlO4 tetrahedra are aqua colored.