Lecture 17Systematic Description of Minerals
Part 4: Silicates II:
Cyclosilicates, Inosilicates, Phyllosilicates and Tectosilicates
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
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
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
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
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
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
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