Question

The anion, \[{{\left( S{{i}_{6}}{{O}_{18}} \right)}^{12}}\]– is present in:
A.Pyroxene
B.Beryl
C.Mica
D.Albite

Answer 1

Hint: Cyclic silicates contain \[\left (Si{{O}_{3}} \right)_{n}^{2n-}\] ions which are formed by linking three or more tetrahedral \[SiO_{4}^{4-}\]units cyclically. Each unit shares two oxygen atoms with other units.

Complete step by step answer:
There are the minerals containing silicon and oxygen in tetrahedral \[SiO_{4}^{4-}\] units, which are linked together in several patterns. Depending on the way the tetrahedral units are linked, the silicates are classified as following-
Orthosilicates, Pyro silicates, Cyclic silicates, Chain silicates, Double silicates, Sheet silicates, 3D or tectosilicates, Orthosilicates:
These are the simplest silicates which contain discrete \[SiO_{4}^{4-}\] tetrahedral units, ex – Phenacite –\[B{{e}_{2}}Si{{O}_{4}}\].
Ortho silicates minerals are rare and only found with cations which form highly insoluble salts.
Pyro silicates:
These contain \[S{{i}_{2}}O_{7}^{6-}\] ions which are formed by joining two tetrahedral \[SiO_{4}^{4-}\]which share one oxygen atom at one corner.
Ex – Thortveitite – \[S{{c}_{2}}S{{i}_{2}}{{O}_{7}}\]
Thortveitite is the only material in nature containing pyro-silicate ions.
Chain silicates:
These contain \[\left( Si{{O}_{3}} \right)_{n}^{2n-}\] ions which are formed by linking ‘n’ number of tetrahedral \[SiO_{4}^{4-}\]units linearly.
Ex – Diopside \[\left( Ca\text{ }Mg\text{ }{{\left( Si{{O}_{3}} \right)}_{2}} \right)\]
These are also known as Pyroxenes.
Double Chain silicates:
General formula is\[\left( S{{i}_{4}}{{O}_{11}} \right)_{n}^{6n-}\]. Ex-Asbestos
Sheet silicates:
These are 2 dimensional silicates with general formula\[\left( S{{i}_{2}}{{O}_{5}} \right)_{n}^{2n-}\]. These silicates can be cleaved easily like graphite.
Ex – Talc – \[M{{g}_{3}}S{{i}_{4}}{{O}_{10}}{{\left( OH \right)}_{2}}\]
3D silicates
These are also known as Tecto or Framework silicates. General formula is\[{{\left( Si{{O}_{2}} \right)}_{n}}\]. Ex – Quartz.
Cyclic silicates
These contain \[\left( Si{{O}_{3}} \right)_{n}^{2n-}\]ions.
Beryl contains six – silicate rings \[\left( Si{{O}_{3}} \right)_{6}^{-2x6}\]
$={{\left( S{{i}_{6}}{{O}_{18}} \right)}^{12-}}$
Beryl is given as\[B{{e}_{3}}A{{l}_{2}}{{\left( Si{{O}_{3}} \right)}_{6}}\]. It is an aluminosilicate. Each aluminum is surrounded by 6 oxygen atoms octahedrally. Varieties of Beryl include Emerald and Aquamarine.

Hence, Option – B is correct.

Additional Information:
 Beryl is available in Green, Yellow, Blue, White, Red, Pink, Purple, Orange and Brown colors. It has a Hexagonal crystal system. It is the most allowing and popular mineral with several important gemstone varieties. The green variety, Emerald, is one of the most precious gems. Red beryl is one of the rarest and most expensive gems.

Note:
Concepts of silicates and their respective classification should be studied properly. One should know about all the Major Groups, their general formulas and how the structures are formed along with their diagrams. Cyclic or Ring silicates should be given a through reading. Molecular Geometry of Tetrahedral, Octahedral and all other structures involved should be studied properly.