8: Silicate-Based Minerals

$\text{Si(OH)}_4$

the mineral subclass in which isolated $\text{SiO}_4$ tetrahedra are linked by the bonding of oxygens to cations

$\text{H}_6 \text{Si}_2 \text{O}_7$, the product of the condensation of two $\text{Si(OH)}_4$ units, losing a water molecule [H atoms not shown in Fig]

the mineral subclass in which one O is shared between two $\text{SiO}_4$ tetrahedra

$\text{SiO}^{2-}_3$, the product of $\text{SiO}_4$ tetrahedra condensating two water molecules, so that the tetrahedra share two O atoms

the mineral subclass in which cycles are formed by a few metasilicate ($\text{SiO}^{2-}_3$) units, usually 4 to 6--btwn 3 and 8 have been observed

the mineral subclass in which linear chains are formed by metasilicate ($\text{SiO}^{2-}_3$) units

$\text{Si}_4 \text{O}^{6-}_{11}$, the mineral subclass consisting of double-chain inosilicates (condensation of single-chains)

at or forming the bottom of something

situated at the top or tip of something

$\text{Si}_2 \text{O}^{2-}_5$, the mineral structure in which three O atoms per $\text{SiO}_4$ tetrahedron are shared, forming six-member rings--the extension of the double-chain structure in two dimensions

shared oxygens in a tetrahedral sheet

nonshared oxygens in a tetrahedral sheet

$\text{SiO}_2$, the mineral subclass in which all four O atoms between $\text{SiO}_4$ tetrahedra are shared, resulting in three-dimensional structures (e.g., quartz and minerals such as feldspars and zeolites)

the structure in which a central cation slightly larger than Si(IV) can accommodate six O2- ions (actually usually hydroxyls [OH- ions]). The cation is usually Al(III) but also Mg(II), Fe(II), etc

an octahedral sheet in which the cation has valence of 3 [e.g., Al(III)]: all OH- groups are shared between *two* Al centers, giving the minimum formula $\text{Al(OH)}_3$ as in gibbsite; note the "holes" in the structure, required by the stoichiometry

an octahedral sheet in which the cation has valence of 2 [e.g., Mg(II)]: all OH- groups are shared between *three* Mg centers giving the minimum formula $\text{Mg(OH)}_2$

the mineral subclass in which tetrahedral and octahedral sheets are bound by sharing the tetrahedral apical O atoms with the octahedral sheet; these atoms replace OH groups, forming layers characteristic of these types of minerals

the phyllosilicate structure in which one tetrahedral sheet is bound to an octahedral (hydroxyl and siloxane groups exposed)

the phyllosilicate structure in which one octahedral sheet is "sandwiched" between two tetrahedral sheets (only siloxane groups exposed)

the surface of an octahedral sheet in which (except for the O atoms shared between the two sheets) OH groups are exposed

the surface of a tetrahedral sheet in which basal O atoms are exposed

1. "dangling" O atoms, which, in the presence of water, each accept an H+ ion to form an OH group
2. "exposed" Si4+/Al3+ centers (are Lewis acid sites [with positive charge]), which, in the presence of water, each form an OH group and release an H+ ion

silanol
aluminol

-border OH groups bound to a silicon (Si) center
-can release an H+ ion to give a negative charge, but are too acidic to become positively charged (in the normal pH range of soil)

-border OH groups bound to an aluminum (Al) center
-can either bind or release an H+ ion to acquire (+ve) or (-ve) charge, respectively, depending on medium pH

kaolin (having Al(III) dioctahedral sheet; stable)
serpentine (having Mg(II) trioctahedral sheet; less stable)

- almost no isomorphic substitution (only Fe substituting 0.25-1% of Al)
- 1:1 layers held together by H bonding
- low border-to-surface ratio => little reactivity except at edges (silanol/aluminol groups)
- low CEC

- low permanent charge 2:1 phyllosilicates
- pyrophyllite: Al dioctahedral sheet, talc: Mg trioctahedral sheet
- layers are held together only by weak van der Waals interactions

- 2:1 phyllosilicate minerals
- 1 of 4 Si atoms replaced by Al
- permanent negative charge is balanced by dehydrated monovalent ions in the interlayer (mainly K+, fitting well in the voids of the siloxane surface)
- not expandable (due to the strong interactions)
- low CEC

muscovite (primary, dioctahedral)
illite (secondary, dioctahedral)
biotite (primary, trioctahedral)

- similar to micas, but with lower permanent charge, which is compensated by hydrated cations in the interlayer
- 15-25% Si atoms replaced by Al
- hydrated Mg2+ or Ca2+, etc, in the interlayer
- formed by weathering of biotite
- high CEC

the presence of hydroxyl-cations in the interlayer, usually Al-OH, which lowers CEC (preference of K over Ca) and swelling is reduced

- expansible clay minerals with lower (weaker) permanent charge, leading to easy expansion/shrinkage depending on medium conditions
- hydrated Ca2+, Mg2+, etc, and water molecules in the interlayer
- high CEC
- formed by weathering of other 2:1 phyllosilicates (esp micas) or other primary minerals

montmorillonite (dioctahedral)
beidelite (dioctahedral)

- 2:1 phyllosilicates with isomorphic substitution (or 2:1:1 phyllosilicates; see below)
- negative charge is compensated by a hydroxide sheet (not stoichiometric, having OH- deficiency or water molecules) in the interlayer
- hydroxide sheet usually contains Al(III) or Mg(II) (gibbsite or brucite sheet respectively), also can find Fe(III)
- unstable (due to hydroxide sheet); weather to smectites or vermicultes & release interlayer cations (Mg2+) to environment
- low surface area, low CEC

- rigid tectosilicates with some Si to Al isomorphic substitution
- inner spaces normally filled by water and exchangeable cations
- important components of sedimentary rocks, mainly of volcanic origin, also found in cavities of igneous rocks such as basalt

N2, being a nonpolar molecule, interacts only with low polarity surfaces; in particular, it is not able to enter the interlayer regions of clay minerals. Thus, the nitrogen area is often considered as the external area of phyllosilicates

- mainly given by electrostatic interactions (i.e., ionic exchange)
- there is relatively little specific binding, mainly through edge silanol and aluminol groups