1.  Chain (Ino-) Silicates
        -  SiO₄ tetrahedra linked by sharing O^2- in infinite single chains (pyroxenes) with Si:O = 1:3 and  [Si₂O₆]^4- as the basic building block and double chains
            (amphiboles) with Si:O = 4:11 and  [Si₄O₁₁]^6- where 1/2 the Si tetrahedra share two oxygens and 1/2 share three oxygens.
        -  Similarities between single (pyroxenes) and double chains (amphiboles) include:
                (1) crystallographic - most are monoclinic, with a few orthorhombic members; c-dimension is ~5.2A.
                (2) physical - similar color, luster, hardness, etc.
                (3) chemical - same cation substitutions.

        -  Differences includes:

• Pyroxene Group
• Chemistry:
• general formula  XYZ₂O₆
• X (M2 site) - is the largest site with Na⁺, Li⁺ Ca²⁺, Mn²⁺, Fe²⁺, Mg²⁺ in either cubic or octahedral coordination.
• Y (M1 site) - smaller than M2 with Mn²⁺, Fe²⁺, Mg²⁺, Fe³⁺, Al³⁺, Cr³⁺, Ti⁴⁺ in octahedral coordination.
• Z is the tetrahedral site with Si⁴⁺, Al³⁺.
• Most common pyroxenes are depicted on a triangular (ternary) diagram with Ca-, Mg-, and Fe- end member compositions at the apices of the diagram.  End members are wollastonite (CaSiO₃); enstatite (MgSiO₃); and ferrosilite (FeSiO₃).
Orthopyroxene Series - (orthorhombic symmetry); complete solid solution of Mg and Fe between enstatite (MgSiO₃) up to ~90% FeSiO₃.

Clinopyroxene Series - (monoclinic symmetry); complete solid solution of Mg and Fe between diopside (CaMgSi₂O₆) to hedenbergite (CaFeSi₂O₆).  Mg2+ and Fe2+ substitute in smaller Y site (M1).  Augite is the 'garbage can' pyroxene with Na substituting for Ca; Al for Mg, Fe; and Al for Si in the tetrahedral sites yielding the general formula  (Ca, Na)(Mg,Fe, Al)(Si,Al)₂O₆.

• Other pyroxenes include jadeite (NaAlSi₂O₆); aegirine (NaFe³⁺Si₂O₆); and spodumene (LiAlSi₂O₆).
 
• Geologic Occurrence
• Orthopyroxenes
Mg-rich varieties are commonly found in (1) Mg-rich mafic and ultramafic igneous rocks (e.g. basalts, gabbros, peridotites); (2) high P-T metamorphic rocks (granulite facies).  Fe-rich varieties mostly occur in metamorphosed sedimentary Fe-formations.
• Clinopyroxenes
augite - very common in mafic igneous rocks such as basalts, gabbros, less common in intermediate igneous rocks like andesite and diorite.
diopside - metamorphosed siliceous , Mg-rich limestones and dolomites (e.g. 'dirty limestomes').  Can write simple reaction expressing this
        as:    CaMg(CO₃)₂ + 2SiO₂ >>>> CaMgSi₂O₆ + 2CO₂
                   dolomite        quartz            diopside
hedenbergite - more Fe-rich metamorphic rocks.
 
• Structure
• Based on single tetrahedral chains that run parallel to c-axis and are bonded by double chains of octahedra (M1 and M2 sites) that also run parallel to c-axis (see Fig. 13.48 in KH).  M2 is cubic coordinated in clinopyroxenes and octahedrally coordinated in orthopyroxenes.
• Looking down c-axis (in a-b plane; see Figs. 13.48 and 13.49 in KH), M1 octahedral chains are coordinated by two opposing 'points' or apices of Si tetrahedral chains and thus produce tetrahedral-octahedral-tetrahedral ("t-o-t") strips.
• The t-o-t strips are cross-linked by the larger chains of M2 octahedra.
• The space between M2 sites in the a-b plane leads to the ~90 degree cleavage in pyroxenes.
 

• Pyroxenoid Group
• Structure
• have single chain structure (Si:O 1:3), but geometry of Si tetrahedra are twisted such that the chain is not a 'straight' chain.
• Main differences:  (1) c-dimension of unit cell is >5.2A; (2) lower symmetry (triclinic); and (3) splintery, sometimes fibrous habit.
 
• Chemistry and Geologic Occurrence::
• Wollastonite (CaSiO₃);  contact metamorphism of limestones;  CaCO₃ + SiO₂ >> CaSiO₃ + CO₂.
• Rhodonite (MnSiO₃); metamorphosed Mn-rich sediments (e.g. Franklin, NJ).
• Pectolite (Ca₂NaH(SiO₃)₃;  common secondary mineral filling cavities in basaltic lavas (e.g. the Watchung Basalts of NJ).
 

• Amphibole Group
• Chemistry
• general formula  WX₂Y₅Z₈O₂₂(OH,F)₂
• W (A site) - largest site with Na⁺ and K⁺ in 10-12 fold coordination; complete solid solution
• X (M4 site) - slightly smaller than A site with Ca²⁺, Na⁺, Mn²⁺, Fe²⁺, Mg²⁺, Li⁺, in either cubic or octahedral coordination.; complete solid solution between Mn²⁺, Fe²⁺, Mg²⁺.
• Y (M1, M2, M3 sites) - smaller than M2 with Mn²⁺, Fe²⁺, Mg²⁺, Fe³⁺, Al³⁺, Ti⁴⁺ in octahedral coordination. Limited solid solution.
• Z is the tetrahedral site with Si⁴⁺, Al³⁺.
• Can be depicted on a "quadrilateral" diagram (similar to pyroxenes) with Ca-, Mg-, and Fe- endmember compositions.
Anthophyllite-Cummingtonite-Grunerite Series - Endmembers range from an orthorhombic variety called anthophyllite (Mg₇Si₈O₂₂(OH)₂) to monoclinic grunerite (Fe₇Si₈O₂₂(OH)₂; amosite or "white asbestos"); cummingtonite is a common monoclinic intermediate composition (Fe₂Mg₅Si₈O₂₂(OH)₂).

Tremolite-Ferroactinolite Series - Ca-rich monoclinic amphibole endmembers include tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂) to ferroactinolite (Ca₂Fe₅Si₈O₂₂(OH)₂).  Actinolite (Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂) is the common intermediate variety. Hornblende is the 'garbage can' amphibole with Na⁺ housed in the A and M4 sites; Mn²⁺, Fe³⁺, Al³⁺, Ti⁴⁺ going into the M1, M2, and M3 sites; Al for Si in the tetrahedral sites along with the Ca, Fe, and Mg in the appropriate sites.

• Other amphiboles include glaucophane (Na₂Mg₃Al₂Si₈O₂₂(OH)₂); riebeckite (Na₂Fe²⁺₃Fe³⁺₂Si₈O₂₂(OH)₂; crocidolite or "blue asbestos").
 
• Geologic Occurrence
• Overall, amphiboles are very common rock-forming minerals in a wide variety of igneous and metamorphic rocks, particularly hornblende.
anthophyllite - metamorphosed Mg-rich rocks such as mafic and ultramafic igneous rocks (e.g. basalts, gabbros, peridotites) and impure
        dolomitic shales.
cummingtonite-grunerite - regionally metamorphosed mafic and ultramafic igneous and rocks and Mg- and Fe-rich metasediments.
hornblende - very common in intermediate igneous rocks such as andesites and diorites; diagnostic mineral of med. to high grade
        metamophism (amphibolite facies), and thus is very common in schists and gneisses.
tremolite - metamorphosed siliceous , Mg-rich limestones and dolomites (e.g. 'dirty limestomes').  Can write simple reaction expressing this
        as:   5CaMg(CO₃)₂ + 8SiO₂ + H₂O   >>>>  Ca₂Fe₅Si₈O₂₂(OH)₂ + 3CaCO₃ +  7CO₂
                   dolomite         quartz                            tremolite                    calcite
actinolite - one of a several green minerals that characteristic of medium grade metamophism (greenschist facies), and thus is very common
           in schists; mainly derived from metamorphosed dolomitic shales rich in Ca, Fe, and Mg.
 
 
• Structure
• Based on double Si tetrahedral chains that run parallel to c-axis and are bonded to a complex "strip" that contains the A, and M1 through M4 sites that also runs parallel to the c-axis (see Fig. 13.66 in KH).
• Looking down c-axis (in a-b plane; see Fig 13.67 in KH), the M1, M2, and M3 sites are edge-sharing octahedra that from bands parallel to the c-axis and are coordinated by opposing apices of the Si tetrahedral chains, and thus produce tetrahedral-octahedral-tetrahedral ("t-o-t") strips.
• The t-o-t strips are cross-linked by the larger M4 sites (6-8 fold coordination) which link the edges of opposing Si double chains (Fig. 13.67 in KH).
• The A site (10-12 fold coordination) is located on the "back side" (apical oxygens are pointing away) of the Si tetrahedral chains (Fig. 13.67 in KH).
• OH^- is housed in the center of the apical oxygens that form the six-member "ring" portion of the Si double chains (Fig. 13.66 in KH).
• The spaces that makes up the A sites in the a-b plane is the weakest bond and leads to the ~60-120 degree cleavage in amphiboles.  This b-dimension of the unit cell is ~2x larger than pyroxenes.

• Serpentine Group   (usually shades of dark green)
• Chemistry and Geologic Occurrence
• Three polymorphs with the composition (Mg₃Si₂O₅(OH)₄):
• antigorite and lizardite are fine-grained, massive varieties; chrysotile is fibrous, asbestoform variety, commonly mistaken for cancerous asbsestos.
• Formed from the alteration of anhydrous Mg-rich silicates (olivine, enstatite), thus are commonly found in altered or low-grade metamorphosed mafic and ultramafic igneous rocks.  Is commonly associated with chlorite group minerals and talc.

• Chlorite Group  (usually shades of dark green)
• Chemistry and Geologic Occurrence
• several minerals fall into this group, but need x-ray techniques to distinguish them; chemical variation due to extensive cation substitution.
• the most common variety chlorite has formula (Mg,Fe)₃(Si,Al)₄O₁₀(OH)₂l(Mg,Fe)₃(OH)₆.
• Formed from the alteration of anhydrous Mg-rich silicates (olivine, enstatite), thus are commonly found in altered or low-grade metamorphosed mafic and ultramafic igneous rocks.  Diagnostic mineral of lower greenschist facies metamorphism (low-med. P-T).  Is commonly associated with serpentine minerals and talc.

 
• Mica Group

- tabular crystals with perfect basal cleavage; they are monoclinic; however, b ~90 degrees so that they look hexagonal (pseudohexagonal).
• Chemistry and Geologic Occurrence
• muscovite -  KAl₂(AlSi₃O₁₀)(OH)₂;  common in granites and granite pegmatites and major constituent of mica schists.
• phlogopite -  KMg₃(AlSi₃O₁₀)(OH)₂;  metamorphosed Mg-rich limestones, dolomites, and ultramafic igneous rocks.
• biotite -  K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂;  very common rock-forming mineral in a wide variety of igneous and metamorphic rocks.  Mostly in intrusive
                        intermediate and felsic igneous rocks (diorite and granite) and is an index mineral for medium grade metamorphism of shale protoliths.
• lepidolite -  K(Li,Al)_2-3(AlSi₃O₁₀)(O,OH,F)₂;  found in granite pegmatites associated with beryl, tourmaline, and muscovite.
• margarite - CaAl₂(Al₂Si₂O₁₀)(OH)₂

• Clay Mineral Group

- Hydrous aluminosilicates; "clay" is a rock term that refers to a fine-grained material (<2mm) consisting of different minerals, which generally includes a few
    species of clay minerals, quartz, feldspar, and other "junk".
- Small size and sheet structure give clays unique (and desirable) physical and chemical properties that include; (1) cation exchange capability, (2) plastic behavior
    when wet, (3) swelling behavior, (4) low hydraulic permeabilities.
 
• Chemistry and Geologic Occurrence
• kaolinite -  Al₂Si₂O₅(OH)₄;  pyrophyllite - Al₂Si₄O₁₀(OH)₂; talc - Mg₃Si₄O₁₀(OH)₂.
• Related clays include vermicullite, illite, montmorillonite (main constituent in bentonite which is formed from the alteration of volcanic ash).
• Formed by the chemical weathering or hydrothermal alteration of aluminosilicate minerals, particularly feldspars.  Compositional difference between clays minerals primarily a function of original weathered minerals. For example, talc is produced by the alteration of Mg-rich minerals like olivine and enstatite and is commonluy associated with serpentine and chlorite minerals.

• Sheet Silicate Structures (a systematic way of "building" all the different groups)

• Step 1 - Building simple "t-o" sheets (lizardite and kaolinite structure)
• Si₂O₅ tetrahedral sheets are arranged in 6-fold rings of Si tetrehedra, the OH^- molecule is the center of the ring and is aligned with the apical oxygens ("points" or the apex of the Si tetrahedra).
• Si₂O₅ tetrahedral sheets are bonded a layer (or a sheet) of octahedron where the cation in the octahedron is coordinated to two apical oxygens and the OH^- of the Si₂O₅ sheet and three OH^- from the octahedral sheet (see Fig. 13.81 in KH).
• Cations in the octahedral sheets can be of two types:
(1) Trivalent (3+) -  mainly Al³⁺; octahedral sheet is called a gibbsite layer with the formula Al(OH)₃; in order to maintain charge balance
                               only 2 out of 3 octahedra are occupied by the 3+ cation.  This leads to the name DIOCTAHEDRAL sheet silicates.

(2) Divalent (2+) -  mainly Mg²⁺; octahedral sheet is called a brucite layer with the formula Mg(OH)₂; charge balance is maintained
                               when 3 out of 3 (all) octahedra are occupied.  This leads to the name TRIOCTAHEDRAL sheet silicates.

 
• The stacking of Si tetrahedral and octahedral sheets leads "t-o" layers (see Fig. 13.84 in KH).  As a structural unit, these t-o layers are electrical neutral and are bound to other t-o layers by weak van der Waals bonds. This leads to excellent basal cleavage, easy gliding, and greasy feel of minerals with t-o structure (mostly clay minerals).
 
Trioctehedral  >>>>   lizardite    Mg₃Si₂O₅(OH)₄       minerals with t-o structures have Si₂O₅ (one "t" layer) and 4 OH's in their formulas
Dioctahedral   >>>>    kaolinite    Al₂Si₂O₅(OH)₄
 
• Step 2 - Build minerals with "t-o-t" structures (talc and pyrophyllite structure; see Fig. 13.82 in KH)
• Derive t-o-t structures by simply adding a second Si₂O₅ tetrahedral layer to the octahedral layer (like an octahedral "sandwich").  Like the t-o structure, the t-o-t structural unit is electrically neutral and only weak van der Waals bonds t-o-t units together and thus have similar physical properties as minerals with t-o structures.
 
Trioctehedral  >>>>   talc                Mg₃Si₄O₁₀(OH)₂   minerals with t-o-t structures Si₄O₁₀ (two "t" layers) and 2 OH's in their formulas
Dioctahedral   >>>>    pyrophyllite    Al₂Si₄O₁₀(OH)₂
 
• Step 3 - Build mica minerals  (phlogopite and muscovite structure; see Fig. 13.83 in KH)
• Derive micas (with t-o-t structures) by simply substituting Al³⁺ for Si⁴⁺ in the tetrahedral sheets.  If 1/4 of the Si are replaced by Al, a 1^- charge develops on the surface of each t-o-t sheet (i.e. it is no longer electrical neutral!).  This excess charge is balanced by interlayer cations (mostly K+ and Na+) that are in 12-fold coordination.   Bonding between t-o-t sheets is now ionic and a bit stronger, which gives micas distinct physical properties from clay minerals.
 
Trioctehedral  >>>>   phlogopite    KMg₃(AlSi₃O₁₀)(OH)₂
Dioctahedral   >>>>   muscovite     KAl₂(AlSi₃O₁₀)(OH)₂
 
• If 1/2 of the Si are replaced by Al, a 2^- charge is developed on the t-o-t sheets. You can put in Ca²⁺ (or sometimes Ba²⁺) as the interlayer cation.

 
Trioctehedral  >>>>   clintonite    CaMg₃(Al₂Si₂O₁₀)(OH)₂
Dioctahedral   >>>>   margarite     CaAl₂(Al₂Si₂O₁₀)(OH)₂
 
• Step 4 - Build chlorite minerals  (chlorite structure; see Fig. 13.85 in KH)
• Derive chlorite group minerals by simply sandwiching a brucite octahedral layer (or gibbsite) between two sheets of talc (or pyrophyllite); thus you have a (t-o-t) - "brucite"- (t-o-t) structural unit.
• For example, this leads to a formula like:     Mg₃Si₄O₁₀(OH)₂lMg,₃(OH)₆ which is pure Mg-chlorite endmember.

                                                                        talc          +       brucite
 
• Extensive cation substitution leads to a general formula for chlorite minerals:   (Mg,Fe²⁺,Fe³⁺,Al)₃(Si,Al)₄O₁₀(OH)₂l(Mg,Fe²⁺,Fe³⁺,Al)₃(OH)₆

• Interlayer Cation Exchange in Montmorillonite   (an environmental application)

 
• Montmorillonite is the dominant clay mineral found in bentonites (altered volcanic ash).  Bentonite is commonly used for lining the base of waste disposal sites primarily because of it's (montmorillonite) low permeability, cation-exchange capabilities, and long-term chemical stability.
 
• Montmorillonite can exchange a variety of ions, mostly Na⁺, K⁺, Ca²⁺, and Mg²⁺; other important molecules include significant amounts of H₂O and some organic contaminants that have a positive (+) charge.
 
• In all clays, there are "internal" and "external" surfaces where ions can be exchanged or attracted to.
• Internal surfaces -  have negative (-) charges developed on the surfaces of the t-o and t-o-t layers.  In some clays, these charges are balanced by interlayer cations, but in some clays (i.e. montmorillonite) these charges are not completely balanced.
Montmorillonite formula:  (Mg_0.4,Al_1.6)Si₄O₁₀(OH)₂     this produces a slight (-) charge on the t-o-t layers.
 
• External surfaces -  have negative (+) charges developed on the broken edges of the t-o and t-o-t layers which exposes the Si⁴⁺ and Al³⁺ in the tetrahedral and octahedral sheets.  This tends to attract anions, like Cl^-.
 
• Let's take an situation where saline groundwater is flowing through a sandy aquifer with some clay in it (mostly montmorillonite).  The dissolved Na⁺ would exchange (along with H₂O) with the existing interlayer cation (possibly Ca).  If Na⁺ was the only ion in the groundwater, then the montmorillonite crystals would tend to disperse inside the pores of the sand grains.  However, if Cl^- is present in the groundwater as well, then the montmorillonite crystals would be attracted to the Cl^- and attach along external edges and cause "clumping" or flocculation inside the pores of the sand grains.
 
• Dispersion tends to reduce the permeability of an aquifer by closing up the connected pores.
• Flocculation tends to increase permeability.
 
• Conclusion:

The chemical composition of groundwater will affect permeability when clay minerals are involved.  An aquifer or waste liner will have low permeabilities when dilute GW is flowing through, but if saline GW is suddenly introduced, then the permeability will increase.  This will have obvious affects for modeling groundwater flow and contaminant transport.