Question

Explain why the surface area of 2:1 phyllosilicates decreases in the order:

montmorillonite > di-vermiculite > Talc > chlorite

Answer 1

reason for the rate o decrease in surface area is due to the difference in expansion capacity of each material. it is explained below,

• Smectites commonly refer to a group of expandable dioctahedral 2:1 minerals with a charge of 0.2-0.6 per formula unit. Montmorillonite, the most familiar member of this group, derives its charge from the octahedral substitution of Mg2+ for Al3+ .The 2:1 layers in smectites are held jointly by van der Waals bonds and weak cation-to-oxygen linkages. The presence of exchangeable cations located between water molecules in the interlayer allocate for expansion of the crystal lattice as the mineral hydrates. When the mineral is saturated with water, the basal spacing between layers can approach 2nm, while under dry conditions, the basal spacing may be reduced to less than 1 ran This expansion and contraction attribute found in smectites, often referred to as shrink-swell potential.
• The weathering of mica, via replacement of interlayer K + with hydrated exchangeable cations, results in the creation of vermiculite. In soils, vermiculite exists as an Al3+ dominated dioctahedral and, to a lesser extent, Mg2+ dominated trioctahedral mineral. A charge of 0.6-0.9 per formula unit is derived in these minerals by tetrahedral substitution of Al3+ for Si4+ . As a result, vermiculites exhibit a high cation exchange capacity because of the tetrahedral charge origin, water molecules and exchangeable cations—primarily Mg2+ and Ca2+—are strongly adsorbed within the interlayer space of vermiculites. Unlike in smectites, the strong bonding of the interlayer cations holds the 2:1 layers together in vermiculites, thus preventing expansion of the basal spacing to 1.5 nm.
• Chlorites are a group of minerals that exhibits a basic 2:1 layer structure analogous to that described for talc or pyrophyllite, but by an interlayer brucite- or gibbsitelike sheet, which forms a 2:1:1 structural arrangement. Isomorphic substitutions within the interlayer hydroxide sheet create a net positive charge that balance the negative charge come up from the 2:1 layers. A typical formula for the interlayer sheet is (MgFeAl)(OH)g There is no water adsorption within the interlayer space; thus, chlorites are consider non expansive minerals