Lecture 17Systematic Description of Minerals

Part 4: Silicates II:

Cyclosilicates, Inosilicates, Phyllosilicates and Tectosilicates

Silicate Mineral Classification(based on arrangement of SiO4 tetrahedra)

Cyclosilicates (Rings) x(SiO3) Unit Composition Hexagonal and Orthorhombic (pseudohexagonal) symmetries

Forms silicate minerals with: Moderate density(2.6-3.2) and hardness (7-8)Prismatic habitsPoor cleavage

BerylBeryl

BerylBeryl

Common Cyclosilicates

Beryl Be3Al2(Si6O18)Common accessory mineral in granite pegmatite

Gem varieties – Aquamarine, Emerald, Rose Beryl, Golden Beryl

Cordierite (Mg,Fe)2Al4Si5O18·nH20

Common mineral in contact metamorphosed argillaceous rocks

Resembles quartz in appearance and hardness (7-7.5)

Tourmaline Hexagonal, pleochroic, parallel extinction.

(Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3(Si6O18)(OH)4

Common accessory mineral in granite pegmatite

Characteristic striated prisms with trigonal outline

watermelonwatermelontourmalinetourmaline

Inosilicates (Chain) XY(Si2O6) in Pyroxenes, WX2Y5Si8O22(OH,F)2 in Amphiboles Single and double silicon tetrahedra chains respectively

Typically monoclinic and orthorhombic symmetry Single chains (Pyroxenes) develop ~90° cleavage Double chains (Amphiboles) develop 120 ° cleavage

Amphibole Amphibole StructureStructure

Pyroxene Pyroxene StructureStructure

O-coordination and Bond Strength of Other Common Cations in Silicate Minerals

ElectostaticElectostaticValence w/ OValence w/ O-2-2

1/8 - 1/121/8 - 1/121/6 - 1/81/6 - 1/81/3 – 1/41/3 – 1/42/6 = 1/3 2/6 = 1/3 2/6 = 1/32/6 = 1/32/6 = 1/32/6 = 1/33/6 = 1/23/6 = 1/24/6 = 2/34/6 = 2/33/6 = 1/23/6 = 1/23/43/44/4 = 14/4 = 1

WeakWeak

StrongStrong

big

medium

small

Pyroxenes (XYZ2O6 )X (M2) – Na+, Ca++, Mn++, Fe+2, Mg++, Li+ [8] CubicY (M1) – Mn++, Fe+2, Mg++, Fe+3, Cr+3 , Ti+4 [6] Octahedral

Z (Tetrahedral site) - Al+3, Si+4 [4]

Single Si2O6 chains (the tetrahedral sites) that run

parallelto the c-axis

Orthorhombic Pyroxenes (Orthopyroxenes - Opx)These consist of a range of compositions between Enstatite - MgSiO3 and Ferrosilite - FeSiO3

Monoclinic Pyroxenes (Clinopyroxenes - Cpx)The Diopside- Hedenbergite series - Diopside (CaMgSi2O6) - Ferrohedenbergite (CaFeSi2O6) 

Augite - (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 is closely related to the Diopside - Hedenbergite series with addition of Al and minor Na substitution

 There is complete Mg-Fe solid solution between Diopside and (Ferro)Hedenbergite

There is also a complete Mg-Fe substitution and small amounts of Ca substitution into the Orthopyroxene solid solution series. Old name Hypersthene

Pigeonite is a high Temperature clinopyroxene

Solid immiscibility

Pigeonite is only found in hot volcanic and shallow intrusive igneous rocks, or as exsolution lamellae

Pigeonite crystallizes in the monoclinic system, as does Augite, and a miscibility gap exists between the two minerals.

Cpx

At lower temperatures, Pigeonite is unstable relative to Augite plus Orthopyroxene.

Pigeonite => Augite + Opx

Pigeonite (Ca,Mg,Fe)(Mg,Fe)Si2O6

Augite (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6

Opx

Exsolution in Pyroxene

Common Pyroxene Species

Augite (Cpx)

Diopside (Cpx)

Enstatite (Opx)

Cleavage in Pyroxenes

Look at this drawing. What axis are we looking down?

Look at these two drawings.

Is M1 = larger or smaller than M2? RecallX (M2) – Na+, Ca++, Mn++, Fe+2, Mg++, Li+ [8] CubicY (M1) – Mn++, Fe+2, Mg++, Fe+3, Cr+3 , Ti+4 [6] Octahedral

Cleavage in Pyroxenes

Amphiboles (A0-1X2Y5Z8O22 (OH,F))A-site – Na+, K+ loose coordination 10-12 OxygensX (M4) – Na+, Ca++, Mn++, Fe+2, Mg++, Li+ 8-fold

Y (M1-3) – Mn++, Fe+2, Mg++, Fe+3, Cr+3 , Ti+4 6-fold octohedral

Z (Tetrahedral T-site) - Al+3, Si+4

double-chainbackbone

Common Types of Amphiboles

Double chains in Amphiboles

TOT stacks separated by big Na+ and K+ “A” cations

TOT – A assemblies staggered

Cleavage ~120 – 60o

Distinguishing Amphiboles with the Petrographic Microscope

Inosilicates - Pyroxenoids• Wollastonite CaSiO3 is a common pyroxenoid

pyroxenoid.

Wollastonite CaSiO3: Connection of silicate chains through [CaO6] octahedra in direction of [100] and [001], repeats every third tetr.Rhodonite MnSiO3 repeats structure every fifth tetrahedron

Note pattern:Up up down up up down

Is Wollastonite stable at the surface?

• CaSiO3(s) + CO2 (g) => CaCO3(s) + SiO2 (s)

-370.313 - 94.257 => -269.908 -204.65 Reactants Products

Grxn = Gproducts – Greactants

Grxn = -474.554 + 464.57

Grxn = -9.984 Kcal/gfw negative, so the reaction will go to the right, as written. Wollastonite will break down if exposed to CO2 at STP.

Data source: Robie and Waldbaum (1968) Thermodynamic Properties of Minerals….

GfKcal/gfw

Kcal/gfw

Pyroxenoids: Wollastonite and Rhodonite

Wollastonite Rhodonite

Phyllosilicate Structures

Alternating Tetrahedral and Octahedral layers bound by large cations or weak electrostatic

bonds

Common Phyllosilicates

AntigoriteAntigorite

ChrysotileChrysotileKaoliniteKaolinite

TalcTalc

PyrophyllitePyrophyllite

MuscoviteMuscovite

LepidoliteLepidolite

BiotiteBiotiteChloriteChlorite

Prehnite

Alternating Tetrahedral and Octahedral layers bound

by large cations or weak Van Der Waals bonds Infinite sheets of silicon tetrahedra Charge balancing metals in [6] (octahedral) Strong single cleavage parallel to silicon sheets

Pyrophyllite (clay)Pyrophyllite (clay) Muscovite (mica)Muscovite (mica)

Mica Structures

Alternating Si Tetrahedral and Octahedral layers (TOTs)bound by large cations Phlogopite is the Magnesium end-member of the Biotite solid solutionseries, with the chemical formula KMg3AlSi3O10(F,OH)2.

KAl2(AlSi3O10)(F,OH)2

Tectosilicates (Framework)

3-D framework of linked silicon tetrahedra

Variable physical properties and symmetries depending on linkage of framework groupings

SiO2 Group

Quartz SiOQuartz SiO22

Feldspar Group Most abundant minerals, by mass or volume, in the crust

Compositionsfor Feldspars are commonly described in terms of mole percents of the end membercomponents (e.g. Or85Ab15, An54Ab39)

Microcline

Anorthite

XAl(Al,Si)3O8

Notice that Albite is an end member of both the Plagioclase and K-Spar (Alkali Feldspar) groups

Feldspars are Tectosilicates with every oxygen atom shared by adjacent silicon or aluminum tetrahedra. The tetrahedra are arranged in four-member rings that are stacked to form “crankshafts” parallel to the a-axis of the monoclinic or triclinic structure. The crankshafts are joined together in an open structure with large voids to hold the alkali metals K+ or Na+, or the alkaline earth ion Ca++ .

Alkali Feldspars

Perthite (albite exsolution in microcline)Perthite (albite exsolution in microcline)

Triclinic K-spar “Microcline”

Sanidine

Albite

At low temperatures solid solution (ss) is unstable, ss exsolves to Albite + Microcline. We say the two phases are immiscible

Low Sanidine and Orthoclaseare more ordered. For these minerals to be monoclinic, the center of symmetry in each ring must be preserved.

At still lower temperatures, the Al+3 will be completely ordered: always on the two t1 tetrahedra. This ordering will destroy the center of symmetry and the mineral will become triclinic Microcline.

High Sanidine is fully disordered with a statistically random Al-Si distribution: each tetrahedron has, on averaging over a reasonable volume, 0.25 Al atoms and 0.75 Si atoms. The Al+3 can be anywhere.

Looking down the a-axis

K-spars

diagonal b -c

“TOT”

Close O=

Attracted toK+

Looking down c-axis

Plagioclase Feldspars

Albite TwinningAlbite Twinning

Compositional Compositional ZoningZoning(Oscillatory)(Oscillatory)

Plagioclase Composition from Albite Twins

Albite twins in Plagioclase reveal solid solution composition.

Feldspathoids (Si-poor)

Common in Alkaline (Si-undersaturated) igneous rocks

Leucite – KAlSiO4

Nepheline – (Na,K)AlSiO4

Sodalite – Na8(AlSiO4)6Cl2

We will see Alkaline plugs on the Petrology field trips next semester

Hydrous Tectosilicates

Analcime (Scapolite Gp) NaAlSi2O6·H2O

Natrolite (Zeolite Gp) Na2Al2Si3O10·2H2O

Heulandite (Zeolite Gp) CaAl2Si7O18·6H2O

Stilbite (Zeolite Gp) NaCa2Al5Si13O36·14H2O

Scapolite