Mineralogy 250 Phyllosilicate Laboratory – 6 Name _________
Refer to the Chapter on Rock Forming Silicates ed-22 Ch 11 in Klein p462-475 or ed-23 Ch 19 in
Klein and Dutrow: p 534-549. Phyllosilicates comprise about 10% of all silicate minerals, which is
to say most crustal rocks. They are even more abundant in soils, sediments and sedimentary
rocks. In soils as clays, they hold water and exchangeable cations allowing plant life to exist on
land despite droughts. In the upper mantle they are the most likely mineral reservoir for K and
H2O present to some extent in all mantle derived magmas (phlogopite). Phyllosilicates are sheet
silicates based on the silica tetrahedron sharing corner oxygens with other tetrahedral in a 2Dimensional sheet, stacked up with other sheets of tetrahedral or octahedral ions and sometimes
with even larger coordination cations in interlayer positions. The most common rock-forming
examples of micas are: biotite, muscovite serpentine, talc, biotite (phlogopite) and chlorite in
metamorphic rocks and biotite (Mg-phlogopite to Fe-annite) and muscovite in igneous rocks.
There are other sheet silicate minerals in weathering settings (soils, sediments), hydrothermal
veins, and in rarer chemical systems like skarns and unusual pegmatites for igneous rocks. The
common soil clays are: kaolinite, illite, chlorite and smectites (swelling mixed layer clays like
montmorillonite). Where Na >>K paragonite forms in alkaline igneous rocks and metasomatized
rocks with salty metamorphic fluids instead of muscovite. Where late granitic melts are rich in
Lithium, it can also form its own micas like polylithionite and lepidolite. Where other divalent
cations abound instead of Mg+2 or Fe+2 such as: Ca+2 (Margarite), Ni (Ni-Laterites analogs of
serpentine), Cu+2, Mn+2, Co+2, Zn+2 etc., analogs of the more familiar micas can form and even be
useful as ore minerals for uncommon elements. Micas and clays can stack right side up or upside
down with respect to the symmetry of the sheets (do the apices of the tetrahedral sheet point up or
down, is this open faced sandwiches or clubhouses?). Because of this clays and micas have many
polytypes, are somewhat disordered and more variable in composition than other types of
silicates.
Clays and micas are built from tetrahedral silicate sheets [Si2O5] and octahedral sheets [X (OH)6] of
metal cations such as Al+3 , Mg+2 , Fe+2 etc. There are also allowable substitutions of OH for O,
divalent for trivalent cations and vacancies allowed in the octahedral sheets in addition to larger
higher fold coordinated interlayer cations to make up for charge balance. The simplest octahedral
sheet minerals are Brucite Mg(OH)2 and Gibbsite Al(OH)3 and these play roles as building blocks
in larger clays and micas. The octahedral are like closest packed sets of diamonds laying on one of
their octahedral faces so that there are effectively 2 sheets of hydroxyl ions with metal ions
sandwiched between. The sheets are all densely packed in hexagonal closest packing. Look at the
2 formulae for these hydroxyl sheet minerals. It is clear that there are different numbers of cations
per anions. If we rewrite these 2 formulae as per 6 hydroxyls, Brucite becomes Mg3(OH)6 while
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Gibbsite becomes Al2(OH)6 considering the metals occuply the 6 fold interstices, Brucite has all of
these sites full while Gibbsite only fills 2 out of every 3 possible sites down any row in the 3
possible directions within the octahedral sheet. While an octahedron is a strong unit, the extended
sheets (infinite) are even stronger and cause cleavages to occur parallel to their surfaces.
Micromount photos from Mindat, Brucite on left and botryoidal Gibbsite on the right.
I. a. Fill out this table for the 2 octahedral sheet minerals. Keep in mind they form from stacks of
sheets to form the mineral crystals.
Mineral
Brucite
(14)
Formula(6) Tri/di? Basal spacing Spec.Grav. hardness Crys.system symmetry
Gibbsite
b. Describe at least 1 geological occurrence and at least one associated mineral that has been found
to form with each of these minerals.
Mineral
Brucite
Formula Associated mineral
(6)
Geological Setting or Environment
Gibbsite
2
c. Name at least one other mineral that is iso-structural with each of these but is based on a
different metal cation. Give the formula and mineral name of that other mineral for both Brucite
and Gibbsite.
Mineral
Brucite
Formula
(4)
Isostructural mineral
Gibbsite
b. Which if these 2 octahedral sheet – hydroxyl minerals is more symmetric and why? (2)
Compare and contrast the properties for Kaolinite, Lizardite and Serpentine Group (Asbestos)
minerals below on tables provided.
Lizardite 0.2 mm (200 µm) Pompaneo, Italy (left) and SEM of authigenic Kaolinite on Quartz grain.
Antigorite 4.3 x 2.8 cm Tasmania (right) & Chrysotile 5.5 cm Cassiar B.C. (Left)
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Amesite stars 1.3 mm f.o.v. Cerro Sapo Bolivia (sector twins creat star shaped prismatic columns
(Right) and Purple 5.39 mm group from Saranovskaya, Middle Urals (Left)
II. Fill out table for the 2 simplest Tet/Oct minerals sharing a common plane of OH or O ions. (21)
Mineral
Kaolinite
Formula Tri/di? Basal spacing Spec.Grav. hardness Crys.system symmetry
Lizardite
Chrysotile
Talc (5mm), Trimons, Pyrenees, France (Left) and Pyrophyllite Polar Urals, W. Siberia (2x2.5 cm)
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III. Fill out the table for the 2 simplest Tet/Oct/Tet minerals sharing 2 common planes of bounding
hydroxyls for the central hydroxyl sheet. Minnasotaite is an Fe rich talc in meta-Iron formations 14
Mineral
Talc
Formula Tri/di? Basal spacing Spec.Grav. hardness Crys.system symmetry
Pyrophyllite
IV. Fill out the table for the 2 simplest Tet/Oct/Tet-Interlayer minerals sharing 2 common planes of
bounding hydroxyls for the central hydroxyl sheet and higher coordination number large and low
charge cations.
Mineral
(42)
Formula(10) Tri/di? Basal
spacing
Spec.Grav. hardness Crys.system symmetry
Muscovite
Illite
Paragonite
Margarite
Phlogopite
Mg-Biotite
Annite
Fe-Biotite
For classification purposes and understanding real mineral structures, natural variation is limited.
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Muscovite Linopolis, Minas Gerais Brazil (5.1 x 5.5 cm), Illite on Albite Petersberg Germany, Illite
is low temperature, K deficient Muscovite having H3O+ in some interlayer sites.
Paragonite, 1 cm, Oya-cho Yabu City, Japan & Margarite: Mt. Isa Australia (f.o.v. 4mm)
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Lepidolite, 2.2 x 3.0 cm Minas Gerias Brazil & Phlogopite on Marble 8.7 x 6 cm Afghanistan
Annite 3 cm from Granite Quarry, Tuften, Vestfold, Norway (Left) and Tetraferriannite 1 cm
sample Hastings County Ontario (Ferric analog of annite) (Right)
Eastonite 7 mm Chestnut Hill Penn (left) & Siderophyllite 40 mm f.o.v. Ferriera das aves, Portugal.
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Clinochlore, Badahkshan Afghanistan (2 cm f.o.v.), & (right) Cookeite: Paris, Maine, U.S.A. (1mm)
Chamosite with Epidote & Quartz 7 mm across Ashford Quarry Mass.
Chlorite Group (Clinochlore, Cookeite, Chamosite)
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Vermiculite 5.5 x 3.5 cm Phalabora Complex, Limpopo South Africa (left) & Montmorillonite in
variolitic cavity 12 mm across Kaisersthul, Baden Germany
V. a. Other Sheet Silicates (Metamorphic and Rare Igneous Types)
Mineral
Formula(10)
Tri/
di?
Basal
spacing
Spec.G
hardness
(35)
Crys.system
Sym.
Clinochlore
Cookeite
Chamosite
Vermiculite
Montmorillonite
b. What’s different about the layer make up & stacking sequences for these last phyllosilicates? (4)
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VI. Palagonite – Altered Basaltic (sideromelane) glass, Is mostly iron rich mixed-layer clays along
with other low temperature minerals such as feldspar, zeolite and calcite. It is often found in
pillow basalts, altered basaltic glass under water and resulting from magma injection into cold
glaciers or melt water. Common component of aquagene tuffs, tuyas and altered basalt flows
(glass rinds, amygdules, fractures).
Compare the thin section photo here and on the website to the hand specimen of palagonite tuff.
a. How can you tell this is a low temperature alteration mineral?
(2)
b. How could you tell this material contains iron and water?
(2)
VII Review:
Sketch the stacking geometry (layers & orientations) for: Kaolinite, Illite & Montmorillonite. Next
to each diagram write the geological environment where substantial amounts of each mineral
occur and list one industrial use for each of these common clays.
a. Kaolinite
b. Illite &
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(12)
c. Montmorillonite