L7 - Clays and Micas; Exsolution in MInerals

Question 1

Nanocrystals

Answer 1

formed by clays

< 1 mm in size

Smaller than the wavelength of light so you cannot see them in thin section

Appear as turbidity or alteration of other minerals

Question 2

Mica

Answer 2

Micas are effectively illite with a particularly well formed and crystalline interlayer site

Question 3

Two types of Mics

Answer 3

TRIOCTAHEDRAL MICAS (with Mg=Fe2+) are called BIOTITE.

DIOCTAHEDRAL MICAS (with Al and a vacancy) are called MUSCOVITE.

Question 4

break down of the formula :

K(Mg,Fe2+,Al,[vac])3AlSi3O10(OH)2

Answer 4

AlSi3O10(OH)2 = Sheets of Al,Si tetrahedral

(Mg,Fe2+,Al,[vac])3 - octahedral layers

K - interlayer site

Question 5

what is unique about serpentine

Answer 5

the layers in a sheet silicate don’t quite match up.
The layers buckle and warp slightly, meaning that the whole thing curves.
This happens in SERPENTINE.

Question 6

Some Feldspar features

Answer 6

60% of Earths Crust

HAs three end member compositions

many lie between two familes which lie between tese compositions (solid solutions)

Question 7

3 Feldspar end members are:

Answer 7

CaAl2Si2O8 ANORTHITE
NaAlSi3O8 ALBITE
KAlSi3O8 ORTHOCLASE

Question 8

Solid solutions are:

Answer 8

Anorthite-albite = PLAGIOCLASE
FELDSPAR

Albite-orthoclase = ALKALI FELDSPAR

Mostly Albite but some anorthite and orthoclase = TERNARY FELDSPAR

Question 9

complete vs incomplete solid solutions

Answer 9

a complete solid solution has all possible compositions between the end members are possible. e.g between Albite and Orthoclase.

incomplete means there is a gap where certain compsitions between end members are apparent. E.G between Orthoclase and Anorthite

Question 10

describe feldspar structure

Answer 10

• Low symmetry solids, either monoclinic or triclinic symmetry:
• Framework of TO4 tetrahedra with all the apices joined at each corner, making the AlSi3O8 of the formula.
• Metals site in pair in large holes in the structure, touching both sides of the hole

Question 11

at low temperatures Alkali feldspar…

Answer 11

breaks down (exsolves) into a mixture of two feldspars (albite and orthoclase). The intergrowth is called perthite.

Question 12

at high temperatures, alkali feldspar…

Answer 12

in dry melts, Alkali Feldspar is stable.

Melts crystallise a single crystal of alkali feldspar solid solution

Question 13

high temperature K feldspar is called:

Answer 13

sanadine

Question 14

on cooling, k feldspar forms

Answer 14

Microcline- has tartan twinning

Question 15

if you qunch lava in water you get:

Answer 15

Sanadine as it it preserved, otherwise in geological time it will invert to microcline