Question

Which of the following silicate is called amphibole?
A. Single chain silicates
B. Double chain silicates
C. 2D silicates
D. Cyclic silicates

Answer 1

Hint: The single and double chain silicates are collectively known as inosilicates. Amphibole is a group of inosilicate minerals. Amphibole is composed of double chain $Si{{O}_{4}}$ tetrahedral linked at the vertices and usually contains ions like magnesium or iron or both.

Complete step by step answer:
Single chain silicates: General formula for this type of silicates is \[{{(Si{{O}_{3}})}_{n}}^{2n-}\]. In single chain silicates silica tetrahedra are linked together in a single chain, where one oxygen ion from each tetrahedron is shared with the adjacent tetrahedron, hence there are fewer oxygens in the structure.
Double chain silicates: General formula for this type of silicates is \[{{(S{{i}_{4}}{{O}_{11}})}_{n}}^{6n-}~\]. In this type of silicate structures Silicon tetrahedra link into double chains of indefinite lengths in which two simple chains are joined together through the third oxygen atom of the silicon tetrahedral.
2D Silicates or sheet silicates: The general formula of Sheet or 2-D silicates is \[{{\left( S{{i}_{2}}{{O}_{5}} \right)}_{n}}^{2n-}.\] \[{{\left( S{{i}_{2}}{{O}_{5}} \right)}_{n}}^{2n-}\]. Each Silicon tetrahedron shares three oxygen atoms with others and thus by forming two-dimensional sheets. These silicates can be cleaved easily just like graphite. The layers are held together by weak van der Waals forces. They are called amphibole.
Cyclic Silicates: It contain \[{{(Si{{O}_{3}})}_{n}}^{2n-}~\] ions which are formed by linking three or more tetrahedral SiO44- units cyclically. Each unit shares two oxygen atoms with other units.

Correct answer is Option (B).

Additional information:
Amphiboles crystallize into two types of crystal systems: monoclinic and orthorhombic.

Note: In chemical composition and general characteristics they are similar to the pyroxenes that are single chain silicates but major difference between these two is; amphiboles contain essential hydroxyl ($OH$) or halogen ($F, Cl$) and the basic structure is a double chain of tetrahedra as compared to pyroxenes single chain structure