30 Questions
Different sheet combinations make up these types of ______ of layers:
phyllosilicates
Main groups within the ______ are: 1) serpentine group 2) clay minerals 3) true micas 4) brittle micas 5) chlorite group
phyllosilicates
In antigorite, the T-O layers are curved but they reverse orientation ______.
regularly
The result is a “corrugation” (waviness) which prevents the layers from ______ easily over each other.
slipping
In chrysotile, the T-O layers curve and roll up like a ______.
carpet
The bad name of “asbestos” comes from ______!
amphiboles
Over long periods of exposure, crocidolite's needle-like crystals are less soluble and more damaging to ______ tissues than chrysotile.
lung
Serpentine and clay are not only the name of groups of ______.
minerals
Mg3Si2O5(OH)4 is the composition of ______ group minerals.
serpentine
The neutral T-O layers are held by weak Van der Waals forces and H..O ______ bonds.
hydrogen
What is the main reason for the change in crystallographic system and/or Bravais type of unit cell from one polytype to another?
Change in the stacking pattern of the layers
What is the result of the substitution Al3+ for Si4+ in tetrahedra in muscovite?
The formation of margarite
What type of mineral is chlorite?
A low-grade metamorphic mineral
What is the main difference between true micas and brittle micas?
Their hardness and flexibility
What type of bond holds the neutral T-O layers together in phyllosilicates?
Weak Van der Waals forces and H..O bonds
What is the characteristic of the 'corrugation' in antigorite?
It prevents the layers from sliding easily over each other
What is the result of the substitution of Al3+ for Si4+ in tetrahedra in muscovite, in terms of charge balance?
iv 3+ Al + Ca2+ = ivSi4+ + K+
What is the characteristic of the dioctahedral phyllosilicates?
They have a dioctahedral sheet structure
What is the characteristic of the trioctahedral phyllosilicates?
They have a trioctahedral sheet structure
What is the most likely composition of the garnet that was replaced by chlorite in a pseudomorph?
Ca3Al2(SiO4)3
The substitution of Al3+ for Si4+ in tetrahedra in muscovite requires a charge imbalance.
False
Octahedral sheets are characteristic of dioctahedral phyllosilicates.
False
Mesodesmic bonds are responsible for holding the neutral T-O layers together in phyllosilicates.
False
Trioctahedral phyllosilicates have a characteristic T-O-T layer structure.
True
Dioctahedral phyllosilicates have a characteristic corrugation in their layers.
False
The substitution of Al3+ for Si4+ in tetrahedra in muscovite results in a change in the crystallographic system of the mineral.
False
Polytypes are characterized by a change in the chemical composition of the mineral.
False
Chlorite is a high-grade metamorphic mineral.
False
Brittle micas are found in silica-rich rocks.
False
The result of the substitution of Al3+ for Si4+ in tetrahedra in muscovite is a change in the octahedral sheet structure.
False
Study Notes
Phyllosilicates
- Phyllosilicates are layered structures, consisting of two types of sheets: tetrahedral and octahedral sheets.
- Tetrahedral sheet: SiO4 units share their three basal oxygen to form infinite sheets, with a T:O ratio of 2:5 or 4:10.
- Octahedral sheet: MeO6 octahedra share edges to form infinite sheets, and may be referred to as "brucite" or "gibbsite" sheets depending on composition.
Subdivision of Phyllosilicates
- Dioctahedral phyllosilicates: valence of metallic cations filling the octahedral sheet is 3+ (Al3+).
- Trioctahedral phyllosilicates: valence of metallic cations filling the octahedral sheet is 2+ (Mg2+ or Fe2+).
Weathering and Formation
- Illite forms by weathering of K-feldspar through a hydrolysis reaction.
- Kaolinite forms by weathering of feldspars at the Earth's surface, with crystals remaining small.
- Talc forms at a low grade of metamorphism, with crystals growing larger.
Phyllosilicate Groups
- Serpentine group: three minerals of composition Mg3Si2O5(OH)4, with different sheet structures.
- Clay minerals: can absorb water molecules between the sheets, causing expansion.
- True micas: include phlogopite, biotite, and muscovite.
- Brittle micas: harder and less flexible than true micas, found in silica-poor rocks.
- Chlorite group: low-grade metamorphic mineral, with an extra octahedral sheet.
Smectite Clays
- Smectite clays: have a high cation exchange capacity due to an increased net negative charge.
- Can absorb water molecules between the sheets, causing swelling.
- Used in drilling mud, dam plugs, and to mop up heavy metals.
Illite
- Illite is a general term for mica-like clay minerals with a T-O-T layer.
- Forms from smectite through a temperature-driven reaction, with incorporation of K+ ions between layers.
- No longer swells when moistened due to stronger K-O bonds.
Polytypes
- Polytypes: structural variants found in minerals with definite sheet structures.
- Different stacking patterns of layers result in different crystallographic systems and/or Bravais types of unit cells.
- Most micas are monoclinic rather than hexagonal due to the inclined c axis relative to the sheets.
This quiz covers the properties and structure of phyllosilicates, a type of mineral commonly used in everyday products. It explores their characteristics, such as cleavage and chemical inertness, and their layered structure.
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