Framework silicates chapter 12+feldsparsand alkali feldspars

Alpha vs Beta

________: The low and high varieties where each represents a polymorph based on the conditions the mineral is found in (temperature and pressure) Alpha is high energy B is low energy.

Reconstructive polymorphs

________: a crystal formation where the structure of the previous mineral is broken (chemical bonds) to create the new mineral.

reconstructive polymorphs

a crystal formation where the structure of the previous mineral is broken (chemical bonds) to create the new mineral

reconstructive polymorphs: a crystal formation where the structure of the previous mineral is broken (chemical bonds) to create the new mineral

a crystal formation where the structure of the previous mineral is broken (chemical bonds) to create the new mineral

Alpha vs Beta:

The low and high varieties where each represents a polymorph based on the conditions the mineral is found in (temperature and pressure) Alpha is high energy B is low energy

Primary and secondary polymorphs:

With repeated heating and cooling the minerals can essentially switch what they are, through becoming a reconstructive polymorph

Composition of quartz:

Mainly pure SiO2, if trace elements are present it is dependent on the surrounding temperature.

what Does anhedral mean?

having no plane faces

When cooling, what happens to B quartz?

It inverts into A quartz but retains the external habit of B quartz. This creates the undulatory extinction

What is the main composition of a feldspar?

Always (X Al Si3O8)

Albite and anorthite:

Albite is at one end of the series and anorthite is at the other. Because of a lack of K+ the composition is described by amount of albite and anorthite present

What is the structure of a feldspar?

a Framework silicate and si and Al tetrahedra

What are the three polymorphs of K-feldspar?

Sanidine (Mode 1) orthoclase (mode 2) and microcline(mode 3)

What is the difference between mode 1, 2 ,3:

Mode one has complete disorder, created at temperatures greatern than 1000C. This creates a monoclinic structure Mode 3 Ordering is the most ordered Si/Al distribution and creates a microcline structure- Because of distortion this is a triclinic system. Mode two is just 50/50

What twinning is observed in feldspars?

On the 010 plane, albite twinning is observed

What twinning is observed on the B axis in feldspars:

pericline twinning

what kind of twinning will a microcline feldspar exhibit?

a combo of polysynthetic albeite and pericline twinning

What is high albite?

A mode 2 ordering mechanism(very notordered) with low temperature and creates a triclinic Plagioclase This form is preserved in volcanic rocks

What is low albite?

A mode 3 ordering that is highly ordered

What is the driving force of exsolution?

Na+ and K+ have very different ionic radius. This creates a perithic texture driven by the amounts of K and Na present in the composition

What is Albite twinning?

all surfaces are parallel to one another, (dark in the same direction) and all relatively the same witdh

What is pericline twinning?

only seen in triclinic feldspars looks the same as albite twinning but a different orienttation

What is carlsbad twinning?

seen in monoclinic/ triclinic feldspars forms while the mineral is growing, with a single twin plane that splits the crystal in half.