Question

What is the net charge found in minerals where there has been an isomorphic substitution of Al ion for Si ion occure in tetrahedral sheet?​

Answer 1

Answer:

Clay minerals refers to a group of hydrous aluminosili-

cates that predominate the clay-sized (<2 |xm) fraction of

soils. These minerals are similar in chemical and structural

composition to the primary minerals that originate from

the Earth's crust; however, transformations in the

geometric arrangement of atoms and ions within their

structures occur due to weathering. Primary minerals form

at elevated temperatures and pressures, and are usually

derived from igneous or metamorphic rocks. Inside the

Earth these minerals are relatively stable, but transform-

ations may occur once exposed to the ambient conditions

of the Earth's surface. Although some of the most resistant

primary minerals (quartz, micas, and some feldspars) may

persist in soils, other less resistant minerals (pyroxenes,

amphiboles, and a host of accessory minerals) are prone to

breakdown and weathering, thus forming secondary

minerals. The resultant secondary minerals are the

culmination of either alteration of the primary mineral

structure (incongruent reaction) or neoformation through

precipitation or recrystallization of dissolved constituents

into a more stable structure (congruent reaction). These

secondary minerals are often referred to as phyllosilicates

because, as the name implies (Greek: phyllon, leaf), they

exhibit a platy or flaky habit, while one of their

fundamental structural units is an extended sheet of SiO4

tetrahedra.

STRUCTURE OF CLAY MINERALS

The properties that determine the composition of a mineral

are derived from its chemical foundation, geometric

arrangement of atoms and ions, and the electrical forces

that bind them together (1). Given that there are eight

elements that constitute over 99% of the Earth's crust

(Table 1), the inclusion of these in the elemental makeup

of soil minerals is understandable. Notwithstanding, the

prevalence of silicon and oxygen in the phyllosilicate

structure is logical. The SiC>4 tetrahedron is the foundation

of all silicate structures. It consists of four O2

~~ ions at the

apices of a regular tetrahedron coordinated to one Si4+

at

the center (Fig. 1). An interlocking array of these

tetrahedral connected at three corners in the same plane

by shared oxygen anions forms a hexagonal network

called the tetrahedral sheet (2). When external ions bond to

the tetrahedral sheet they are coordinated to one hydroxyl

and two oxygen anion groups. An aluminum, magnesium,

or iron ion typically serves as the coordinating cation and

is surrounded by six oxygen atoms or hydroxyl groups

resulting in an eight-sided building block termed an

octohedron (Fig. 1). The horizontal linkage of multiple

octahedra comprises the octahedral sheet. The minerals

brucite Mg(0H)2 and gibbsite A1(OH)3 are similar to the

octahedral sheets found in many clay minerals; however,

phyllosilicates may contain coordinating anions other than

hydroxyls. Cations in the octahedral layer may exist in a

divalent or trivalent state. When the cations are divalent

(Mg, Fe2+), the layer exhibits a geometry similar to

brucite, such that electrical neutrality is maintained. In this

arrangement the ratio of divalent cations to oxygens is 1:2

and all three possible cation sites in the octahedron are

occupied. This configuration and the respective sheet

formed from an array of such as octahedral are referred to

as trioctahedral. When the cations are trivalent (Al, Fe3+),

the charge balance is maintained by leaving one of every

three octahedral cation sites empty. Under this configur-

ation, the ratio of trivalent cations to oxygens is 1:3 and the

layer exhibits a gibbsite-like dioctahedral arrangement. A

combination of tetrahedral and di- or trioctahedral sheets

bound by shared oxygen atoms forms aluminosilicate

layers that comprise the basic structural units of

phyllosilicates (Fig. 2). Sheet arrangement within the

aluminosilicate layers varies between clay mineral types

resulting in variable physical and chemical properties that

differentiate the clay mineral classes.