Transcript Chapter 12 - FIU Faculty Websites

CHAPTER 12
Framework Silicates
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Predominance of Silicate Minerals
in the Earth’s Crust
27% of all known minerals are silicates
40% of common minerals are silicates
>90% minerals in the earth’s crust are silicates
Framework silicates
• More than 2/3rd of earth’s crust is made up of framework silicates
• Quartz, Feldspars (alkali and plagioclase), Felspathoids
(Nepheline, Sodalite and Leucite), Zeolites, Scapolites
• TO4 structure; T = Si4+ or Al 3+
• Every Oxygen shared between two strongly charged cations – the
mutual repulsion between the cations ensures open structure
• Physical consequence: Lower density e.g., Quartz (SiO2)= 2.65,
Olivine (Mg2SiO4) = 3.27
• Compositional consequence: open framewok silicates can
accommodate large ions like K+, Na+, Ca2+ octahedral sites
between the tetrahedra
• Charge balance is maintained by replacing Si4+ with Al3+ in
tetrahedral sites
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
• Silica Group: SiO2: Many polymorphs: Quartz, Tridymite and Cristobalite are
common, Coesite and Stishovite are high pressure polymorphs – found in
mantle rocks or rocks shocked by meteorite or nuclear bomb impacts.
• Quartz, Tridymite and Cristobalite are reconstructive polymorphs. And each has
α (low) and β (high) varieties – and these are displacive polymorphs. β (high)
varieties have higher symmetry and stable at higher temperature
• Quartz can grow either has α or β
variety
• Tridymite grows only as β variety but
converts to lower energy α variety by
displacive polymorphism which
survives because transformation to
lower energy quartz requires
reconstructive polymorphism which is
slow.
• β quartz, typical of felsic volcanics, has
a stubby prismatic face topped by
pyramidal phase – this shape is
retained even when it converts to α
variety.
Introduction to Mineralogy,
Second edition
William D. Nesse
Figure 12.1 Stability fields of the silica minerals. Adapted
from Griffen (1992).
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.2 Quartz structure viewed down the c axis.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Microcrystalline varieties:
Chert, Chalcedony (chert +
moganite) Moganite
Clear
Rock Crystal
Violet
Amethyst
Pink
Rose Quartz
Yellow
Citrine
Milky
Milky Quartz
Smoky brown or
black
Smoky Quartz
Red,
microcrystalline
Jasper
Black
microcrystalline
Flint
Agate
Figure 12.3
mineraloid
Opal
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.4 Approximate variation of nω of chalcedony with density, which is a function of pore space between the fibers. After Frondel (1982).
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
• Feldspars are the most abundant mineral in the Earth’s crust
• Two solid solution series between three end members
• K-feldspar( KAlSi3O8) – Albite (NaAlSi3O8): Alkali Feldspar series
• K+ and Na+ has same charge but size differs. Complete solid solution at
high temperatures but the phases exsolve at lower temperature forming
perthite (or anti-perthite)
• Albite (NaAlSi3O8)– CaAl2Si2O8( Anorthite): Plagioclase series
• Ca2+ and Na+ has same size but
charges differ -- charge balance is
maintained by replacing Si4+ with Al3+
• Since Ca2+ and K+ has very dissimilar
size, hence no composition
intermediate between Alkali feldspar
and Anorthite
Figure 12.5 Composition range of common feldspars.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Structure:
• 4 member rings – 2 pointing up and two pointing down. One of each is T1 and
the other is T2 site
• The rings are joined with other rings by sharing oxygen forming chains
• Chains joined with each other laterally by sharing other oxygens
• Large space between chains that can accommodate large ions like Ca2+, Na+,
K+ which are coordinated with 9 oxygen ions
Figure 12.6 Idealized feldspar structure: T1
and T2 refer to different tetrahedral sites.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Al/Si Order-Disorder
Al: Si ratio = 1:3 – present in Alkali feldspars
• Mode I (high Sanidine) : Al and Si randomly
distributed in the tetrahedral sites. 25% of sites
occupied by Al. Forms only above 1000 C. Plane of
symmetry || (010). Must have quenched rapidly.
• Mode II ( Orthoclase): All Al are only in T1 sites,
none in T2 sites. Real Orthoclase is a mixture of
Mode II and Mode III. 2V indicates degree of
ordering.
• Mode III (Low Microcline) : All Al are only in one of
the T1 sites: highest degree of ordering. Seen in
slowly cooled igneous or metamorphic rocks. Mirror
symmetry is detroyed – so, Triclinic system.
Microcline adapts to Monoclinic to triclinic
transformation at about 450ºC by developing twins
in both with {010} twin plane – Albite twins and by
rotation along b axis (pericline twins) – this
developing polysynthetic twins
Al:Si ration 2:2 (Anorthite):
Mode IV: Al and Si occupy alternate sites.
Figure 12.7 Ordering of Al and Si in
tetrahedral sites in the feldspars.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Microcline – “Cross hatched” or grid twinning. Twin lamellae are spindle shaped
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
• Plagioclase:
• Na rich Plags show the same order/disorder of Modes I, II and III as Kfeldspars
• On colling below 980 C, high temp form Monalbite (monoclinic) coverts
polymorphically to triclinic structure because of a. potential ordering of Al and
b. collapse of oxygen ions around smaller Na.
• Triclinic Na-Plag can be high albite or low albite (highly ordered)– conversion
occurs naturally and easily. High Albite found only in volcanic rocks.
• With Mode IV ordering in Anorthite, the collapse of Oxygen ions around Ca2+
causes triclinic symmtery.
• Aluminum Avoidance Principle: Al does not occupy two adjacent tetrahedra.
Introduction to Mineralogy,
Second edition
Figure 12.8 Arrangement of oxygen anions about
K+ in sanidine (left) and the distorted or
collapsed arrangement around Na+ in albite
(right).
Exsolution:
In Alkali Feldspar:
Ionic radius of Na+ = 1.32 A, of K+ = 1.65 A. Na rich and
K rich domains exsolve on cooling; Albite exsolved in K
feldspar host is perthite. K feldspar in Albite host is
antiperthite.
Slow cooling helps exsolution hence Microcline more
likely to develop perthite compared to orthoclase or
sanidine.
In Plagioclase: less common (why?)
Intergrowth:
Myrmekite: worm-like quartz in sodic plagioclase.
Forms at the contact of K-Feldspar and Plagioclase in
granitic rocks
Graphic : cuneiform quartz in perthitic K-feldspar. Forms
when the two crystallize simultaneously.
Rapakivi: sodic plagioclase mantles K- Feldspar.
Antirapakivi – the opposite.
Figure 12.9 Perthitic exsolution.
Introduction to Mineralogy,
Second edition
Figure 12.10 Quartz–feldspar intergrowths.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Polysynthetic twins (only in Triclinic Feldspars)
Albite twins: formed by reflection on {010} and composition plane is parallel
to {010}
Pericline twin is produced by 2 fold rotation on b axis. Composition plane is
close to (001)
Both form by growth, deformation, and order/disorder conversion from
monoclinic to triclinic
Single twins
Carsbad, Baveno and manebach are single twins:
Carsbad twin is most common and forms by 2 fold rotation on {001}
Figure 12.11 Common twins in the feldspars.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.12 Plagioclase in thin section.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
https://www.youtube.com/watch?v=1OdYmVq9r2g
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.13 Optical properties of low (plutonic) and high (volcanic) plagioclase. Adapted from Smith (1958) and Burri and others (1967).
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.14 Index of refraction of the fast ray (nα’) for plagioclase fragments lying on cleavages. The diagram is constructed for
fragments on (001) cleavage surfaces but also may be used for fragments on (010) cleavage surfaces. After Morse (1968).
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.15 Michel-Lévy method.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.16 Diagram for use with the Michel-Lévy method.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.17 Carlsbad–albite method.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.18 Diagrams for use with the Carlsbad–albite method.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.19 Variation of 2Vx and indices of refraction for common K-feldspar as a function of the degree of Si–Al order.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.20 K-feldspar.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Feldspathoids (Si-poor feldspars)
Common in alkaline (Si-undersaturated) igneous rocks
Leucite – KAlSiO4
Nepheline – (Na,K)AlSiO4
Sodalite – Na8(AlSiO4)6Cl2
Hydrous Tectosilicates
• Analcime (Scapolite Gp)
NaAlSi2O6·H2O
• Natrolite (Zeolite Gp)
Na2Al2Si3O10·2H2O
• Heulandite (Zeolite Gp)
CaAl2Si7O18·6H2O
• Stilbite (Zeolite Gp)
NaCa2Al5Si13O36·14H2O
Figure 12.21 Photomicrograph of nepheline (N) in thin section with plagioclase (P) and microcline (M) in nepheline syenite. Crossed polarizers.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.22 Typical zeolite structure. Heulandite viewed down the c axis. The open channels provide space for water molecules and
mono- and divalent cations. Structural data from Gunter and others (1994).
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 12.23 Optical properties and specific gravity of scapolite. After Shaw (1960), Ulbricht (1973), and Graziani and Lucchesi (1982).
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.