1.  3-D Framework (Tecto-) Silicates
        -  SiO₄ tetrahedra are completely polymerized; all four oxygens are shared, Si:O = 1:2  [SiO₂]⁰
        -  Si tetrahedra are covalently bonded to each other.  This leads to a stable, strongly bonded, 3-D structure.
        -  Approximately ~64% of the Earth's crust if composed of minerals of this type..

• SiO₂ Group
• Chemistry:   SiO₂
• The SiO2 framework is electrically neutral and has 9 possible ways that it can be built which correspond to the 9 polymorphs of quartz (one of which is synthetic; see Table 13.4 in KH).
• Opal is a related substance which is essentially amorphous and contains variable amounts of H₂O  (SiO₂lnH₂O).
 
• Geologic Occurrence:
• Quartz is common and abundant in a great variety of rocks types and geological environments.  It is a common igneous and metamorphic mineral and is a major constituent of granites and granite pegmatites.  It is also common as a vein mineral in hydrothermal ore deposits.  Because of its mechanical and chemical stability it is commonly concentrated as sand that eventually lithifies to produce sandstone.
 
• Structure:
• Each of the 9 polymorphs has unique crystallographic properties (unit cell size, symmetry, etc.).  The stability of each polymorph is largely controlled by lattice energy. The high T forms have greater lattice energy and thus have more open, expanded structures which leads to lower specific gravity and refractive index.
• The most common SiO₂ polymorphs fall into three principle structural categories;  (1) quartz, with the lowest symmetry and most compact structure, (2) tridymite, with higher symmetry and a more open structure, (3) cristobalite, with the highest symmetry and most expanded structure.  Related by a reconstructive transformation process which involves the breaking of bonds between Si tetrahedron and then 'reconstruction' of the framework into a different structure.  This process requires considerable amount of energy and is usually quite 'sluggish' which allows one type to exist metastably for long periods of time.  Stability of these polymorphs is largely controlled by T, but P does play a role (see Fig. 13.113 in KH).  Each one of these structural types has a high-low T inversion (or high- or low-T form), for example high and low quartz, and high and low tridymite, and high and low cristobalite.  These are displacive transformations that take place quickly and at a specific T that do not involve the breaking of bonds, but just a small shift in the geometry of the structure.  
• Stishovite and coesite are the most dense silica polymorphs and are high P forms that are relatively rare in nature.  Usually found in mantle rocks or rocks associated with meteorite impact craters.
 

• Feldspar Group
• Chemistry:
• Common feldspars are expressed on a triangular diagram (ternary diagram) in the system KAlSi₃O₈ (orthoclase; Or)-NaAlSi₃O₈ (albite; Ab)- CaAl₂Si₂O₈ (anorthite; An).  There is complete solid solution between the members of KAlSi₃O₈ and NaAlSi₃O₈ series and are called alkali feldspars.  There is also complete solid solution between the members of NaAlSi₃O₈ and CaAl₂Si₂O₈, called plagioclase feldspars.  Feldspars are given different names depending on composition and/or structural state.
• Complete solid solution for alkali and plagioclase feldspars only exists at high temperatures.  At lower T, a single homogeneous feldspar exsolves ('breaks up') into two separate, roughly end-member compsition phases as heterogeneous intergrowths.  For example, upon slow cooling a crystal of alkali feldspar of composition An₅₀Or₅₀ will spontaneously segregate into Na⁺ and K⁺ rich feldspar domains.  The most common type of exsolution is called perthite in which thin layers of albite form in a host crystal of K-feldspar.
 
 
• Geologic Occurrence:
• Feldspar minerals are the most common and abundant silicate mineral in the Earth's crust.  They are common constituent of all types of igneous rocks, K-feldpars are more common in felsic igneous rocks.  They are a common high-grade metamorphic mineral in gneisses.  Less commonly. felspars occur as detrital grains that make up sedimentary rocks, mostly in akosic sandstones and conglomerates.
 
 
• Structure:
• Consists of a infinite, 3-D network of SiO₄ and AlO₄ tetrahedra.  Basically similar to polymorphs of SiO₂, but with Al substituting for Si in the tetrahedral network and Na⁺, K⁺, or Ca²⁺ in the voids between tetrehedra.  When only one Si⁴⁺ (per feldspar formula unit) is substituted by Al³⁺, the structure maintains charge balance with monovalent Na+ or K+.  When two Si⁴⁺ are substituted for by Al³⁺, charge balance is maintained by divalent Ca²⁺.
• Si-Al tetrahedron consist of 4-membered rings that are linked into chains parallel to the a-axis.  The 'chains' are systematically offset such that they look like a 'double crankshaft' (see Figs. 13.118 and 13.119 in KH).  The large voids between the crankshafts are the sites for Na⁺, K⁺, or Ca²⁺.  The square blocky outline of the crankshaft chains are expressed nicely by the ~90 cleavage and blocky shape of feldspar crystals.
 
• The Si-Al distribution is also a controlling factor on structure, particularly for the K-feldspars (sanidine, orthoclase, microcline).

Sanidine -  High-temperature polymorph of KAlSi₃O₈.  Si and Al are 100% randomly distributed between to distinct tetrahedral sites T1 and T2 (complete disorder).  The K+ ions occupy special positions on mirror planes perpendicular to the b-axis, and thus, sanidine has monoclinic symmetry.

Microcline -  Low-temperature polymorph of KAlSi₃O₈. Si and Al are completely ordered and K+ ions are not found at special sites, thus microcline is triclinic.

 Orthoclase -  Intermediate-temperature polymorph of KAlSi₃O₈. Si-Al distribution is between total randomness found in sanidine and complete order found in microcline.

Plagioclase Feldspars generally have a completely ordered Si-Al distribution, like microcline.

• Zeolite Group
• Chemistry:
• Hydrous aluminosilicates with Na, K, and Ca, and highly variable amounts of H₂O in large voids in their framework strucure.
• Common naturally occurring zeolites:
natrolite - (Na₂Al₂Si₃O₁₀l2H₂O)            stilbite - (NaCa₂Al₅Si₁₃O₃₆l14H₂O)
chabazite - (Ca₂Al₂Si₄O₁₂l6H₂O)            heulandite - (CaAl₂Si₇O₁₈l6H₂O)
 
• Geologic Occurrence:
• Commonly found in cavities and veins in mafic igneous rocks, particularly in basaltic lavas as secondary minerals from the precipitation of fluids flowing through these rocks. (e.g. Orange Mtn Basalt that the MSU campus is built on!; Paterson, NJ was one of the classic localities for collecting natural zeolites).
• They also form from the alteration of silicic volcanic glass and tuffs in lacustrine (lake) deposits, particularly lakes that were highly saline or alkaline.
• The occurrence of zeolites in rocks are used to define the lowest grade of metamorphism ('zeolite facies').
 
• Structure
• Open fameworks of AlO₄ and SiO₄ tetrahedra, with large interconnecting spaces or channels that house the Na, K, Ca, and H₂O.  This open strcuture is reflected in their relatively low hardness (3.5 to 5.5) and specific gravity (2.0 to 2.4) compared to quartz and feldspars.
• Na, K, Ca, and H₂O are loosely bounded in the structure.
• Large channels and intraframe cavities give zeolites desirable economic qualities such as:
 
(1) Dessication -  H₂O in the channels is easily driven of at low T, but leaves structure intact.  Can be rehydrated and thus can be used as a dessicant (e.g. something that removes H₂O).

(2) Molecular Sieve -  Dehydrated zeolites can absorb other molecules of the appropriate size as long as they fit into the channels (on the order of a few angstroms).  Molecules that are too big are excluded while smaller are incorporated into the structure.  Commonly used to separate large and small hydrocarbons (e.g. catalytic cracking in oil refining).

(3) Cation Exchange - Na, K, and Ca, (and H₂O) are only loosely bounded in the zeolite structure and are easily 'exchanged' by fluids pasing through them.  Used in commercial water softeners.  Ca in 'hard' tap water is exchanged with a filter containing Na-rich zeolites.  The Na is then released into the tap water, producing 'soft' water and Ca the is trapped in the zeolite filter.