2. Phyllosilicates are a very important group of rock forming minerals. a.) Usin

Question

2. Phyllosilicates are a very important group of rock forming minerals. a.) Using a systematic approach based upon octahedral and tetrahedral sheets, show how each type of phyllosilicate structure may be derived. Use sketches to illustrate important structural types. b.) What is the distinction between trioctahedral and dioctahedral minerals? c.) Give the names of the 1:1,2:1, and 2:11 trioctahedral and dioctahedral phyllosilicates. d.) Describe, in general terms, the geologic occurrence of the phyllosilicate minerals. 3. Describe the crystal structures of the amphibole minerals (use sketches). Discuss how calcium fits into the amphibole structure and comment on the geologic occurrence of these minerals.

Explanation / Answer

2.a ) The structure of phyllosilicates consists of two types of sheets - O or Octahedral and T or Tetrahedral. These two sheets join together to form layers. The layers in turn stacked one upon another and bonded together to form the structural unit of the mineral. Octahedral sheets consist of two planes of OH- anionic groups. Between the anionic groups there are either divalent cations such as Fe2+, Mg2+ otr trivalent cations Fe3+ and Al3+. Trioctahedral sheets consist of tetrahedrally coordinated cations whose composition is T2O5.

The combination of one tetrahedra and an octahedral sheet is called TO or 1:1 structure. The combination of two T and an O sheets form TOT or 2:1 structure, formed by joining a tetrahedral sheet to both sides of an octahedral sheet.

b) If the octahedral sites consist of divalent cations, three out of three octahedral sites are filled and form a trioctahedral sheet. This structure is similar to brucite in which trioctahedral sheets are bonded together with van dar waals bond.

If the cations in the octahedral sheet are trivalent, charge balance requires that only two out of three sites is occupied, leaving one vacant and form a dioctahedral sheet. This is the same as mineral gibbsite.

c) 1:1(TO) group dioctahedral mineral is kaolinite and trioctahedral mineral is serpentine.

2:1(TOT) group dioctahedral mineral is pyrophyllite and trioctahedral mineral is talc.

2:1 +1 (TOT+ c) group dicotahedral mineral is muscovite and trioctahedral is phlogopite.

d) Geological occurrence : Phyllosilicate minerals are very common in igneous, metamorphic and sedimentary rocks. It is a common constituent os pegmatite, granite, granodiorite , aplite and related rocks ; commonly found within clastic sedimentary roks such as sandstone and related siliciclastioc rocks; and variety of metamorphic rocks including phyllite, slate, schist, gneiss have a constituent of these types of minerals.

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