Framework silicates chapter 12+feldsparsand alkali feldspars

Quartz, plagioclase and alkali feldspars are the most abundant silicate

always found in a TO4 setting, where T is Si, or Al

reconstructive polymorphs: 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.