Question

Which of the following is not an orthosilicate?
A.Phenacite
B.Willemite
C.Thortveitite
D.Forsterite

Answer 1

Orthosilicates are the simplest silicates which contain discrete $Si{O_4}^{4 - }$ tetrahedral units. Thortveitite is a scandium silicate which contains large amounts of scandium which is about $34\%$ , it basically is a silicate mineral of yttrium and scandium.

Complete step by step answer:
Orthosilicates, also known as Neso or Island silicates, are the simplest silicates which contain discrete $Si{O_{44}}^ -$ tetrahedral units. Ortho silicate ion is the strong conjugate base of weak Orth silicic acid. It does not persist in aqueous solutions. Hence in nature, ortho silicate minerals are rare and they are only found with cations which form highly insoluble salts.
Phenacite or Phenakite are potent high vibration crystals, a crystal of the Light. It is a fairly rare nesosilicate mineral consisting of beryllium orthosilicate, $B{e_2}Si{O_4}$ .
Williamite $(Z{n_2}Si{O_4})$ is a minor silicate ore of zinc and it is highly fluorescent (green) under short wave UV.
Thortveitite, a scandium silicate is the only which contains large amounts of scandium, about $34\%$ , but this mineral is quite rare and is not an important source of scandium unfortunately. The cosmic abundance of scandium is relatively high. Thortveitite is a silicate mineral of yttrium and scandium. It has the composition (Sc and Y) $2S{i_2}{O_7}$ .
Forsterite $(M{g_2}Si{O_4})$ is an orthosilicate and end member of the ubiquitous ${\left( {Mg,Fe} \right)_2}Si{O_4}$ (olivine) solid solution.

Therefore, the correct answer is option (C).

Note: Phenacite or phenakite is occasionally used as a gemstone, it occurs as isolated crystals, that are rhombohedral with parallel-faced hemihedrism, and are either lenticular or prismatic in habit. The lenticular habit is determined by the development of faces of several obtuse rhombohedra and the absence of prism faces.