Mineralogy: Crystallography Three

What clues does crystal structure gives clues about?

Cleavage, hardness, density, melting point, refractive index, solid solutions

Isostructuralism

When minerals have the same structure, or crystallographic blueprint

What is an example of isostructural minerals?

Stishouvite and rutile

How does charge affect size of atoms?

The larger the positive charge, the smaller the atom, because it has less electrons.

Structural Groups

One type of anion, with various types of cations

Polymorphism

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

What does polymorph mean?

Many forms

What are some polymorph examples?

Diamond and graphite, pyrite and marcasite, calcite and aragonite

Can you have polymorphs in the same crystal system, or do they need to be different?

You can, as long as they are different space groups

What are the conditions under which polymorphs kyanite, sillimanite, and andalusite will form?

Andalusite is low pressure, kyanite is low temperature high pressure, and sillimanite is high pressure high temperature.

Why are kyanite, sillimanite, and andalusite so important?

They tell you about conditions during metamorphism due to the fact that they form under different conditions.

How many kinds of ice are there, and how many of these were found on earth?

Seven kinds, three on earth

Polymorphs of quartz on the chart

Alpha quartz, coesite, stishouvite, tridymite, high-quartz, cristobalite

Metastable crystallization

When kinesthetic factors cause a crystal that should not be stable at certain temperatures/pressures to form there anyways.

How are tridymite and cristobalite further subdevided?

There are also alpha and beta versions of each

Moganite

A monoclinic quartz polymorph that contains up to 3% water

Where is moganite found?

Hot volcanic pyroclastic deposits, and within chalcedony quatzine, and flint in small amounts

Why do we have different polymorphs?

It takes a certain amount of energy to make a certain structure, so different conditions are capable of different structures

How do higher temps affect structure?

They are capable of making more difficult structures

How do higher pressures affect structure?

They favor more dense structures

Three main types of psuedomorph

Substitution, replacement, alteration

Reconstructive polymorphism

Extensive rearrangement that involves breaking of atomic bonds

Is reconstructive polymorphism easily reversable?

No, it takes a lot of energy and is kinetically sluggish

Why can you still find unstable minerals at the surface for a while?

Because reconstructive polymorphism is required to change them to something more stable

Displasive polymorphism

A small internal adjustment, resulting in a change in position of atoms and bonds

Is displasive polymorphism common and easily reversible?

Yes, it requires little energy as no bonds need to be broken

Example of displasive polymorphism, and where it can occur

Alpha quartz to beta quartz, which can be done on a hot-plate

What can cause displasive polymorphism

changes in temperature or pressure

Pressure buffering

The need to stay at the same pressure until a change is done occurring

Order disorder polymorphism

Variable perfection referencing the presence of particular elements in particular sites

Where should perfect order occur, theoretically?

At 0° K

What happens to disorder as temperature increases

Disorder increases

Is appearance affected by order-disorder?

Probably not

Is symmetry affected by order-disorder?

It may be

What is the order-disorder series with k-spars, from disorder to order?

Sanidine, orthoclase, microcline

Does order-disorder polymorphism always change structure?

No, not if atoms are close in size

What is an example of order-disorder minerals where this doesn’t mess up structure

Pyroxene and amphibole, because Mg and Fe are very close in size.

How quickly does order-disorder polymorphism occur and why?

Very slowly, because atoms have to move through a crystalline structure

Polytypism

A more subtle kind of polymorphism, caused by different stacking patterns of identical 2D sheets.

How does unit cell vary in polytypism?

Unit cell dimension parallel to sheets is common, but the dimension parallel to c varies.

What are the three polytypes possible for the OH group in micas?

1m polytype, 2m polytype, and 3m polytype

What do the 1 2 and 3m in polytypes represent?

The number of alternating directions of stacking

Metamic Minerals

Minerals where crystal structure is partially or fully destroyeddue to self-irradiation

How does self-irradiation affect a mineral’s physical properties?

It lowers hardness, refractive index, and density

What two processies cause metamic minerals?

Particle fission and particle emission of gamma rays

What two elements may cause metamict minerals?

U or Th

Controls on metamict minerals

Time, temperature, concentration of U/Th, mineral species

Why does temperature matter with metamict minerals?

It relates to the ability of a mineral to heal itself

Can you tell time by looking at fission trakcs?

Yes, but not super accurately

Example of a mineral that may be metamict

Thorite

What is the destroyed version of thorite called?

Thorogum

Pyrognomic

A special property that metamict minerals have, where they may luminesce at temperatures much lower than usual

Draper point

The temperature at which nearly all solid or liquid substances begin to glow

What is the Draper point (numerical value)

about 525°C

Pseudomorphism

When one or more minerals take on the extermal shape of another

Three types of polymorphism

Reconstructive, order-disorder, displacement

What does pseudomorphism mean?

false form

Substitution psuedomorphism

When the original material is replaced by new elements, and no chemical reaction takes place

Encrustation

One mineral deposited on the surface of another, encrusting it. No reaction takes place, and the original mineral may or may not remain

Alteration pseudomprhism

There is a chemical reaction caused by the addition of another element or molecule, which changes the original mineral into something else