Tectosilicates

Question 1

Quartz

Answer 1

SiO2

Question 2

Opal

Answer 2

SiO2*H2O

Question 3

Sanidine

Answer 3

KAlSi3O8

Question 4

Orthoclase

Answer 4

KAlSi3O8

Question 5

Microcline

Answer 5

KAlSi3O8

Question 6

Albite

Answer 6

NaAlSi3O8

Question 7

Anorthite

Answer 7

CaAl2Si2O8

Question 8

Leucite

Answer 8

KAlSi2O6

Question 9

Name minerals in quartz group.

Answer 9

Quartz, Opal

Question 10

Name minerals in the potassium-feldspars.

Answer 10

sanidine, orthoclase, microcline

Question 11

Name minerals in plagioclase group

Answer 11

alibte, labradorite, anorthite

Question 12

Name a feldspathoid

Answer 12

leucite

Question 13

Si:O ratio in tectosilicates

Answer 13

1 Si:2 O

Question 14

Si:O (with Al) ratio in feldspars

Answer 14

4(3Al,1Si):2O for alkali

4(2Al,2Si):2O for plagioclase

Question 15

Difference between alpha and beta quartz.

Answer 15

alpha quartz forms at lower temp and pressure and is stable at room temp and pressure. In addition, it has a 3-fold symmetry and is trigonal.
beta quartz froms at higher temp and lower pressures, but is not stable at room temp and pressure, so it is not found in hand sample. In addition, it has a hexagonal symmetry and a 6-fold axis.

Question 16

Where is alpha and beta quartz found?

Answer 16

They are both found at lower pressures, but beta quartz is only found between around 574°C to 800°C.

Question 17

What is the highest pressure polymorph of quartz?

Answer 17

Stishovite

Question 18

Where can the highest pressure polymorph of quartz be found?

Answer 18

Stishovite forms at pressures greater than 7.5 GPa. Found in meteorites and meteorite impacts.

Question 19

What is the highest temperature polymorph of quartz?

Answer 19

Cristobalite

Question 20

What temperature does the highest temp polymorph of quartz found? Where is it found?

Answer 20

Cristobalite forms at temps greater than 1470°C.
Terrestrial magmas aren’t hot enough to form this, but it can still be found because it forms metastabily in volcanic rocks.

Question 21

What is coupled substitution?

Answer 21

Based on charge.. fix this card later

Question 22

Sketch a composition and temperature graph for feldspars

Answer 22

See photo

Question 23

What is exosolution, why does it happen?

Answer 23

Exosolution is the process where an initially homogenous solid solution separates into two or more distinct crystalline materials to of from the system. (oil and water).

It happens when an originally homogenous high-temp mineral containing ions of considerably different size cools, thermal vibrations decrease and the original structure becomes unstable, resulting in a structural reorganization.

Question 24

Define miscibility gap

Answer 24

a region of immiscibility in temp-composition space.

Question 25

Define perthite and how/when it forms.

Answer 25

coarse grained intergrowths consisting of Na-rich feldspar lamellae exsolved from a k-rich host.

It forms in the alkali feldspar system, where at lower temperatures (<600°C), Na-K has limited solution in the feldspar structure, resulting in exoslution forming this texture.

Question 26

Draw the alkali feldspar triangle diagram.

Answer 26

See notes

Question 27

Do you ever find quartz and feldspathoids together in a rock?

Answer 27

No, feldspathoids only form in melts low in SiO2. If more quartz present, it would form a feldspar.

Question 28

What are the names for the two main solid solutions in feldspars and what are the names of the end members?

Answer 28

Solid solutions: alkali feldspars and palgioclase feldspars

End Members: orthoclase (KAlSi3O8), Albite (NaAlSi3O8), Anorthite (CalAlSi3O8)