Crystal Structures- Crystallography 3

MIDTERM 3 GEOL 232

crystal structure gives clues to what properties?

• Cleavage (or lack)

• hardness (bonds)

• density

• melting point

• refractive index

• x-ray patterns

• solid solutions (chemistry)

define isostructuralism

minerals with same structure/crystallographic blueprint

provide example of isostructuralism

stishovite (SiO2) and rutile (TiO2), called the “stishovite structure” or “rutile structure”

define octahedral coordination

one cation surrounded by 6 oxygens

what is the expanded version of the tetragonal crystal system with space group P42/mnm

P42/m 2/n 2/m

describe the general chemistry of a structural group

one type of anion with various types of cations

define polymorphism

a specific chemistry represented by more than one species having different structures

etymology of polymorph

poly = many ; morphs = forms

provide the 4 examples of polymorphism and their crystal systems given in the slides

• Diamond (Iso) & graphite (Hex): C

• Pyrite (Iso) & Marcasite (Orth): FeS2

• Calcite (Hex) & Aragonite (Orth): CaCO3

• Kyanite (Tri) & Sillmanite (Orth) & Andalusite (Orth): Al2SiO5

how many polymorphs of ice have been found on Earth?

3 (& know graph has I - VII w/ increasing pressure

1Gpa = ?km

35 km

how many polymorphs does SiO2 have? give examples of SiO2 polymorphism

more than 8; alpha and Beta quartz (low-mod T), coesite & shishovite (high P), tridymite & cristobalite (high T)

label the silica polymorphs diagram

-

provide an example of a displacive polymorphic transition

low cristobalite inverts from high (beta) cristobalite (isometric) to low (alpha) cristobalite (which is tetragonal), skipping intermediate polymorphs, at about 270 degrees Celsius

at what temperature is alpha tridymite stable before it convert to Beta tridymite?

870-1470 degrees Celsius

describe moganite’s crystal class, geologic occurrence, and water content

• monoclinic

• found in hot pyroclastic deposits and in small amounts within chalcedony quartzite and flint

• contains up to 3% water

polymorphism

a specific chemistry represented by more than one species having different structures

etymology of polymorphism

poly = many; morph = forms

describe recontructive polymorphism

• extensive rearrangement

• breaking of atomic bonds

• reassembly of structural units

• takes a LOT of energy

• not easily reversible

• kinetically sluggish

example of reconstructive polymorphism

tridymite —> low quartz

so, minerals can survive metastably for billions of years

ex:

• C, diamond/graphite

• FeS2, pyrite/marcasite

• CaCO3, aragonite/calcite

describe displacive polymorphism

• small internal adjustment; little energy required

• generally no bonds broken

• slight adjustment in atom positions and bond angles (“kinking”)

• occurs instantaneously and is easily reversible

• ex. alpha and beta quartz

order disorder polymorphism

• variable “perfection” re. the presence of particular elements in particular sites

• common in metal alloys, but also in minerals

• theoretically, “perfect order” @ 0K

• slow cooling allows ordering, but no clean cut-offs

• disorder increases with T

• symmetry can change, but appearance probably won’t

give example of K-feldspars (Si & Al ordering)

• Sanidine, high T, disordered

• Orthoclase, medium T, moderately disordered/ordered

• Microcline, low T, ordered

(these structures will be different b/c of ordering; note ordering can also occur in minerals without changing the structure)

sketch perfect order, partial disorder/order, and total disorder

-

how are symmetry and order related?

higher symmetry = disordered

lower symmetry = ordered

describe polytypism

• have identical 2D sheets that are differently stacked

• between polytypes, unit cell dimension is parallel to sheets (usually a & b directions) constant, but diimnesion parallel to c varies

gives examples of polytypes

• CLAYS AND MICAS and würzite and sphalerite

poly type names: 1M, 2O, 2m1, 3T

can a mineral species have varying polytypes?

yes

describe metamict minerals

• crystal structure is destroyed (or partly destroyed) due to self-irradiation

• lowers a mineral’s RI, hardness, and density

• Minerals containing U or Th permit alpha particles (and fission products)

• = f(t,T, concentration, mineral species)

• fission tracks created: a dating technique

• minerals can ‘heal’ themselves above some T

• e.g. zircon heals poorly, but apatite can heal down to ca. 100 degrees Celsius

examples of metamict minerals

• zircon (ZrSiO4)

• Thorite (Th, U) SiO4

• Allanite (epidote group)

• Ce, La, and Y species of allanite

• Titanite (CaTiSiO5)

if a mineral is orderly, is it more or less metamict? what does x-ray diffraction pattern look like when it is most metamict?

less metamict, peak disappears

what does pyrognomic mineral? what is the draper point?

all solid or liquid substances incandesce when heating

draper point is 977 degrees F

pseudomorphism

one or more minerals takes on the external shape of another. original mineral is ideally euhedral

examples of psuedomorphism

pyrite —> limonite

azurite —> malachite

(some of original mineral may remain, or it may be completely removed)

list, describe, and provide examples of the three main types of psuedomorphism

• substitution: original material removed, replaced by new elements; no chemical rxn

  • ex: Fluorite (CaF2) —> quartz (SiO2)

  • Wood —> quartz (SiO2)

• encrustation: one mineral deposited on the surface of another, encrusting it; internal mineral may have been removed

  • ex in slides: chalcedony over aragonite crystals

  • drusy quartz after fluorite

• alteration: there IS a chemical reaction; may be incomplete

  • ex: azurite —> malachite

  • Forsterite —> serpentine or 3Mg2SiO4 + SiO2 +2H2O —> 2Mg3Si2O5(OH)4

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