Silicate Mineral Groups Questions and Answers

Answer: D

The ring silicate is Tourmaline.

Silicates are classified by how their SiO₄ tetrahedra link. In ring silicates (cyclosilicates) the tetrahedra join into closed rings; tourmaline and beryl both contain six-membered rings.

Pyrophyllite is a sheet silicate (phyllosilicate), enstatite a single-chain silicate (pyroxene, an inosilicate), and forsterite, the magnesium olivine, an island silicate (nesosilicate) whose tetrahedra are isolated from one another.

Answer: A

The fourth end member is ferrosilite.

Common pyroxenes are plotted in the pyroxene quadrilateral with four end members: diopside (CaMgSi₂O₆), hedenbergite (CaFeSi₂O₆), enstatite (Mg₂Si₂O₆) and ferrosilite (Fe₂Si₂O₆). Enstatite and ferrosilite are the magnesium and iron ends of the orthopyroxenes.

Ferropericlase is a magnesium-iron oxide of the lower mantle, not a pyroxene. Fayalite (spelt 'faylite') is the iron olivine, and jadeite (spelt 'jadite') is a sodium pyroxene outside the four-member quadrilateral.

Answer: C

Akermanite is a sorosilicate.

Akermanite, Ca₂MgSi₂O₇, is an end-member of the melilite group. Its structure is built of pairs of silica tetrahedra sharing one oxygen, Si₂O₇ groups, which is the defining feature of sorosilicates.

Nesosilicates such as olivine and garnet have isolated SiO₄ tetrahedra, cyclosilicates such as beryl have rings of tetrahedra, and tectosilicates such as quartz and feldspar have three-dimensional frameworks.

Answer: C

The brownish red alteration product of olivine is iddingsite.

Iddingsite is a mixture of iron oxides (goethite, hematite) and clay minerals that replaces olivine, starting from the rims and fractures, through oxidation of its iron during late-stage cooling or weathering of basalt. It gives the olivine crystals of many basalts their reddish brown borders.

Nephrite is a variety of jade (an amphibole), schorlomite is a titanium-rich garnet, and kaolinite is a clay formed mainly from the weathering of feldspar, not the characteristic product of olivine alteration.

Answer: B

Pigeonite typically occurs in volcanic rocks because at deeper conditions it exsolves into other pyroxenes.

Pigeonite is a low-calcium clinopyroxene stable only at high temperature. In rapidly cooled lavas it is quenched and preserved. In slowly cooled plutonic rocks it does not survive: it exsolves lamellae of augite and inverts to orthopyroxene, so plutonic rocks contain 'inverted pigeonite' instead of pigeonite itself.

Pigeonite is not light enough to float in magma, and its loss at depth is due to slow cooling and exsolution rather than to pressure destabilising it or to rapid reaction with other minerals.

Answer: A

The 1 : 1 clay group is kaolinite.

Each layer of kaolinite consists of one sheet of silica tetrahedra bonded to one sheet of alumina octahedra, a 1 : 1 or T-O structure, and the layers are held together by hydrogen bonds. That is why kaolinite does not swell and has a low cation exchange capacity.

Illite, smectite and vermiculite are 2 : 1 (T-O-T) clays, with an octahedral sheet sandwiched between two tetrahedral sheets; they differ in their interlayer cations and swelling behaviour.

Answer: D

Chrome garnet is uvarovite.

Uvarovite, Ca\(_{3}\)Cr\(_{2}\)(SiO\(_{4}\))\(_{3}\), is the calcium-chromium end member of the garnet group, an emerald-green mineral found with chromite in serpentinites and skarns.

Almandine is the iron-aluminium garnet, Fe\(_{3}\)Al\(_{2}\)(SiO\(_{4}\))\(_{3}\); grossular is the calcium-aluminium garnet; and spessartine (spessartite) is the manganese-aluminium garnet. None of them contains chromium as an essential component.

Answer: D

Glaucophane is an alkali amphibole.

Glaucophane, Na₂(Mg,Fe)₃Al₂Si₈O₂₂(OH)₂, is a sodic amphibole, with sodium filling the B site. It is the characteristic blue mineral of blueschists, formed at high pressure and low temperature in subduction zones.

Calcic amphiboles such as hornblende and tremolite have calcium in that site, and magnesium (Mg-Fe) amphiboles such as anthophyllite and cummingtonite have magnesium and iron. White mica is a sheet silicate, not an amphibole.

Answer: B

The keyed answer is MgSiO₄, the option that names the magnesium end-member.

Forsterite is the pure magnesium olivine. Its true formula is Mg₂SiO₄, two magnesium ions to each silica tetrahedron, and none of the printed options gives the subscript 2 correctly in this position. Option B is the only one that contains magnesium alone with an SiO₄ group, so it is the formula the key accepts.

Option C, with iron alone, stands for fayalite, the iron end-member (Fe₂SiO₄). Option D, with both Mg and Fe, stands for the olivine solid solution between them, not forsterite. Option A has a pyroxene-type SiO₃ group, so it is not an olivine at all.

Answer: B

Dravite is Mg-tourmaline.

The tourmaline group is named by the cations in its Y site. Dravite is the sodium-magnesium tourmaline, NaMg₃Al₆(BO₃)₃Si₆O₁₈(OH)₄, typically brown and common in metamorphosed sediments and carbonates.

The iron-rich tourmaline (option C) is schorl, the common black variety, and the lithium tourmaline is elbaite. Dravite is not a pyroxene, so the Ca-pyroxene and Mg-augite options are wrong.