UNIVERSITY OF SOUTH ALABAMA

GY 302: Crystallography & Mineralogy

Lecture 23: Phyllosilicates 1 Mica Minerals etc. Last Time

Class VIII Minerals: Inosilicates (part 2) 1. Amphiboles Inosilicates (double chain)

Two major sub groups of amphiboles are distinguished on the basis of crystal structure:

1) Orthorhombic Amphiboles 2) Monoclinic Amphiboles

http://www.uwgb.edu/dutchs/GRAPHIC0/ROCKMIN/ATOM-STRUCT/amphibchain.gif Inosilicates (double chain)

General formula: XY((Al,Si)4O11)2(OH)2

X[larger ions] = Ca2+, Na+ Y[smaller ions] = Mg2+, Al3+, Fe2+/3+, Mn2+/3+)

http://www.tulane.edu/~sanelson/images/amphibole_cleavage.gif Inosilicates (double chain)

We recognize several amphibole series (solid solution)

1) Anthophyllite Series (Orthorhombic) 2) Cummingtonite Series (Monoclinic) 3) Actinolite Series (Monoclinic) 4) Hornblende Series (Monoclinic) 5) Glaucophane Series (Monoclinic) 6) Riebeckite Series (Monoclinic) 7) Arfvedsonite-Eckermannite Series (Mono) http://www.tulane.edu/~sanelson/images/amphibole-compositions.gif

“Hornblende”: (Ca,Na,K)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2 “Glaucophane”: Na2Mg3Al2Si8O22(OH)2 2+ 3+ “Riebeckite”: Na2Fe 3Fe 2Si8O22(OH)2 2+ “Arvedsonite”: Na3(Mg,Fe )4AlSi8O22(OH)2 Asbestos Minerals

Several minerals with a fibrous habit that have useful economic properties (flexible, high melting points, tensile strength, heat resistance)

Amphibole Asbestos minerals: Anthophyllite, Tremolite, Amosite, Actinolite, Crocidolite Serpentine Asbestos minerals: Clinochrysotile, Orthochrysotile

Crocidolite (“blue asbestos”) is part of the Riebeckite series and is the worst of the asbestos minerals (at least according to legal websites) http://www.earthsci.unimelb.edu.au/Thomas/lteng/engeimg/enge0247a.JPG Today’s Agenda

Phyllosilicates Part 1 1. General chemistry/structure 2. Phyllosilicate classification 3. Mica, chlorite and talc groups

Phyllosilicates Si:O ratio = 2:5 (3 shared oxygens)

Phyllosilicates Si:O ratio = 2:5 (3 shared oxygens)

-4 -8 Anion component: Si4O10 or Si8O20

-5 -6 With Al substitution: AlSi3O10 or Al2Si2O10

Phyllosilicates Si:O ratio = 2:5 (3 shared oxygens)

-4 -8 Anion component: Si4O10 or Si8O20

-5 -6 With Al substitution: AlSi3O10 or Al2Si2O10

General Formula: XYAlSi3O10(OH)2 X = K+, Na+, Li+, Ca2+ Y = Fe2+/3+, Mg2+, Al3+, rarely Cr3+, Ni3+, Mn3+

Example (Muscovite): KAl2 AlSi3O10(OH)2

Phyllosilicate Structure

Inosilicate Structure (Amphibole) 6- Si4O11

Phyllosilicate Structure

Inosilicate Structure (Amphibole) 6- Si4O11

Phyllosilicate Structure (Mica) 4- Si4O10

Phyllosilicate Structure

Phyllosilicate Structure (Mica) 4- Si4O10

cross sectional view Phyllosilicate Structure

Tetrahedral layer Octahedral layer Tetrahedral layer Phyllosilicate Structure Tetrahedral layers 4- (mostly SiO4 )

Cation layers (K+, Mg2+, Na+ etc.)

also H2O (clays)

Octahedral layers (variable; Al/Mg/OH) Phyllosilicate Structure C-axis

Perfect Basal Cleavage Phyllosilicate Structure

The octahedral layers are given specific names according to the nature of the “filling”.

Need 6 x OH for octahedral coordination Phyllosilicate Structure

The octahedral layers are given specific names according to the nature of the “filling”.

Need 6 x OH for octahedral coordination

Al2(OH)6 = Gibbsite Layer (Al(OH)3) 2 x Al = dioctahedral coordination

Mg3(OH)6 = Brucite Layer (Mg(OH)2) 3 x Mg = trioctahedral coordination

Phyllosilicate Classification

Is rather nasty, but most are either monoclinic or triclinic. Classification is further done of the basis of:

Phyllosilicate Classification

Is rather nasty, but most are either monoclinic or triclinic. Classification is further done of the basis of:

1. Composition of octahedral layer

Phyllosilicate Classification

Is rather nasty, but most are either monoclinic or triclinic. Classification is further done of the basis of:

1. Composition of octahedral layer 2. Repeat distance (distance between T-O-T “sandwiches”)

Phyllosilicate Classification

Is rather nasty, but most are either monoclinic or triclinic. Classification is further done of the basis of:

1. Composition of octahedral layer 2. Repeat distance (distance between T-O-T “sandwiches”) 3. Whether structure is expandable (clays)

Phyllosilicate Classification

Is rather nasty, but most are either monoclinic or triclinic. Classification is further done of the basis of:

1. Composition of octahedral layer 2. Repeat distance (distance between T-O-T “sandwiches”) 3. Whether structure is expandable (clays) 4. Number of layers in the “sandwiches” (2 versus 3)

Phyllosilicate Classification

2:1 Structure (Most micas and clays) Phyllosilicate Classification

2:1 Structure (Most micas and clays)

1:1 Structure (kaolinite, serpentine, halloysite, Chrysotile, allophane) Phyllosilicate Classification

2:1 Structure (Most micas and clays)

1:1 & 2:1 Interlayered (Chlorite Group)

1:1 Structure (kaolinite, serpentine, halloysite, Chrysotile, allophane) Phyllosilicate Minerals (excludes clays, serpentine groups, and others) Phyllosilicate Minerals (micas, 2:1 layers) Biotite

[K(Fe,Mg)3AlSi3O10(OH)2 ]

Crystal: Monoclinic Pt. Group: 2/m Habit: platy (flexible) SG: 2.8-3.4; H: 2.5-3 L: vitreous to pearly; Str: grey Col: brown (reddish to greenish) Clev: perfect [001] Optics: Biaxial (-); bir=0.030-0.070 nα=1.522; nβ=1.548, nγ=1.549 Occurrence: Intermediate Ig; Named after French physicist, Jean Baptiste Biot (1774 - med. grade metamorphic rocks 1862), who studied the optical properties of the micas. Phyllosilicate Minerals (micas, 2:1 layers) Muscovite

[KAl2 AlSi3O10(OH)2]

Crystal: Monoclinic Pt. Group: 2/m Habit: platy (flexible) SG: 2.82; H: 2 to 2.5 L: vitreous to pearly; Str: white Col: white (gold, silver, green) Clev: Perfect [001] Optics: Biaxial (-); bir=0.036-0.049 nα=1.552; nβ=1.582, nγ=1.587 From Muscovy glass, alluding to the Russian province Occurrence: pegmatitic Ig., med of Muscovy. grade Meta, sericite in Sed. rocks Phyllosilicate Minerals (micas, 2:1 layers)

Fuchsite (v. muscovite)

[K(Al,Cr)2 AlSi3O10(OH)2]

Crystal: Monoclinic Pt. Group: 2/m Habit: platy (flexible) SG: 2.82; H: 2 to 2.5 L: vitreous to pearly; Str: white http://www.dakotamatrix.com/images/products/fuchsite15805a.jpg Col: emerald green Clev: Perfect [001] The Cr-variety of muscovite. The Ba-rich variety is called Binnite Optics: Biaxial (-); bir=0.032 nα=1.569; nβ=1.598, nγ=1.601

Occurrence: metamorphic rocks Phyllosilicate Minerals (micas, 2:1 layers)

Phlogopite (biotiote series)

[KMg3AlSi3O10(OH)2 ]

Crystal: Monoclinic Pt. Group: 2/m Habit: platy SG: 2.8; H: 2 to 2.5 L: vitreous to pearly; Str: white Col: brown, green, reddish-brown Clev: perfect [001] Optics: Biaxial (-); bir=0.028-0.045 http://cache.eb.com/eb/image?id=9648&rendTypeId=4 Name Derivation: From the Greek flogopos - "resembling fire." nα=1.53; nβ=1.557, nγ=1.558 Occurrence: Ultramafic Ig; Meta carbonates Phyllosilicate Minerals (micas, 2:1 layers)

Lepidolite (biotite series)

[K(Li,Al)2-3AlSi3O10(OH)2 ]

Crystal: Monoclinic Pt. Group: 2/m Habit: platy SG: 2.84; H: 2.5-3 L: vitreous to pearly; Str: white

Col: lilac (yellowish) to colourless http://webmineral.com/specimens/picshow.php?id=671 Clev: perfect [001] Optics: Biaxial (-); bir=0.029-0.038 Name Derivation: From the Greek lepidion - "scale" nα=1.525; nβ=1.551, nγ=1.554 Occurrence: Li-bearing pegmatites Phyllosilicate Minerals (micas, 2:1 layers) Margarite

[CaAl2Al2Si2O10(OH)2 ]

Crystal: Monoclinic Pt. Group: 2/m Habit: scaly, massive SG: 3.03; H: 4 L: pearly; Str: white

Col: white, grey (pinkish) http://webmineral.com/specimens/picshow.php?id=671 Clev: good [001]

Optics: Biaxial (-); bir=0.012-0.032 From the Greek margaritos - "pearl." nα=1.595; nβ=1.625, nγ=1.627 Occurrence: alteration product associated with corundum Phyllosilicate Minerals (interlayered)

Clinochlore (Chlorite Group) (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6

Crystal: Monoclinic Pt. Group: 2/m Habit: scaly, massive SG: 2.65; H: 2 to 2.5 L: vitreous to pearly; Str: white Col: green to black http://itc.gsw.edu/faculty/tweiland/chlorit.jpg

Clev: perfect [001] From the Greek klino - "oblique" and chloros - " green. Optics: Biaxial (+); bir=0.005-0.011 nα=1.571; nβ=1.571; nγ=1.576

Occurrence: Low grade metamorphosed mafic rocks, burial-grade sedimentary diagenesis Phyllosilicate Minerals (2:1 layers)

Talc (Talc Group)

[Mg3Si4O10(OH)2]

Crystal: Monoclinic Pt. Group: 2/m Habit: scaly, massive SG: 2.75; H: 1 L: vitreous to pearly; Str: white Col: any colour, including smurf

Clev: perfect [001] http://www.soes.soton.ac.uk/resources/collection/minerals/minerals Optics: Biaxial (-); bir=0.037-0.050 From the Arabian “talcia” meaning keep baby’s ass dry nα=1.538; nβ=1.575, nγ=1.575 Occurrence: hydrothermal metamorphism

Phyllosilicate Minerals (talc group; 2:1 layers) Pyrophyllite

Al2Si4O10(OH)2

Crystal: Triclinic Pt. Group: 1 Habit: massive, acicular SG: 2.75; H: 1.5 to 2 L: pearly; Str: white Col: brown, grey, green, white Clev: perfect [001] Optics: Biaxial (-); bir=0.045-0.062 http://www.mindat.org/photo-60228.html From the Greek pyr - "fire" and phyllon – “leaf” nα=1.534; nβ=1.586, nγ=1.596

Occurrence: hydrothermal metamorphic veins GY 302: Crystallography and Mineralogy

Lecture 23: Silicates 6: Micas

Instructor: Dr. Doug Haywick [email protected]

This is a free open access lecture, but not for commercial purposes. For personal use only.