Your Answer : (Not Answered)

Correct Answer : C

The d-spacing of (110) is 3.54 Å.

For a cubic crystal with lattice parameter a, the spacing of the (hkl) planes is

\[ \begin{aligned} d_{hkl} &= \frac{a}{\sqrt{h^{2} + k^{2} + l^{2}}} \\ d_{110} &= \frac{5}{\sqrt{1 + 1 + 0}} = \frac{5}{1.414} = 3.54\ \text{Å} \end{aligned} \]

7.07 Å is what you get by multiplying by \(\sqrt{2}\) instead of dividing. 2.5 Å is a/2, the spacing of (200), and 0.28 is the reciprocal of 3.54, from inverting the formula.

Your Answer : (Not Answered)

Correct Answer : D

The intensity of light does not control the interference colour.

Interference colour depends on the retardation between the two rays travelling through the crystal:

\[ \Delta = t\,(n_{1} - n_{2}) \]

where t is the thickness of the section and \(n_{1} - n_{2}\) the birefringence of the section, which depends on its orientation with respect to the crystallographic axes. The orientation of the vibration directions relative to the polariser and analyser also matters, since the mineral goes dark at extinction and shows its colour most brightly at 45°.

Changing the brightness of the lamp makes the colours brighter or dimmer but does not change which colour is seen.

Your Answer : (Not Answered)

Correct Answer : A

The correct matching is 1 - i, 2 - ii, 3 - iii, 4 - iv.

1. The Carlsbad twin of orthoclase is a penetration twin with the composition plane (010).
2. The Baveno twin (printed Beveno) has the twin plane (021).
3. The Manebach twin has the twin plane (001), the basal pinacoid.
4. The swallow-tail twin of gypsum is a contact twin on (100).

The other options assign the basal plane (001) to Carlsbad or Baveno, or move (021) to the Manebach twin, which mixes up the three feldspar laws.

Your Answer : (Not Answered)

Correct Answer : B

The defining symmetry of the cubic system is four axes of three-fold symmetry.

All five crystal classes of the cubic (isometric) system have four three-fold rotation axes, lying along the body diagonals of the cube. That is the element shared by every cubic class, so it defines the system.

Three four-fold axes are present only in the most symmetrical cubic classes; others have two-fold axes in those directions. Oblique and vertical mirror planes are not present in every class, and four two-fold axes alone do not define the cubic system.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct Answer : B

Cassiterite is the main ore mineral of tin.

Cassiterite is tin dioxide, SnO₂, and almost all the world's tin comes from it, either from veins and greisens around granites or from placer deposits, as in South-East Asia. In India it occurs in the Bastar region of Chhattisgarh.

Tungsten comes from wolframite and scheelite, titanium from ilmenite and rutile, and tantalum from tantalite (columbite-tantalite). None of them is obtained from cassiterite.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct Answer : C

The deposit is at Mangampetta.

Mangampet (Mangampeta) in the Kadapa (Cuddapah) district of Andhra Pradesh is a bedded, stratiform barite deposit in the rocks of the Cuddapah Supergroup and one of the largest single barite deposits in the world. It makes India a major producer of barite, used mainly as a weighting agent in oil-well drilling mud.

Guntur, Hyderabad and Nellore do not host this deposit; Nellore is known instead for mica.

Your Answer : (Not Answered)

Correct Answer : A

Sapphire is \(\text{Al}_{2}\text{O}_{3}\), aluminium oxide.

Sapphire is a gem variety of the mineral corundum, coloured blue mainly by traces of iron and titanium; the red variety of the same mineral is ruby. Corundum's hardness of 9 on the Mohs scale makes it the hardest natural mineral after diamond.

\(\text{CaTiO}_{3}\) is perovskite, a calcium titanium oxide. \(\text{Al}_{2}\text{SiO}_{5}\) is the formula shared by the aluminosilicates kyanite, andalusite and sillimanite. The hydrous aluminium silicate in option D is not corundum either.

Your Answer : (Not Answered)

Correct Answer : C

The correct matching is P-ii; Q-i; R-iv; S-iii.

• Zircon has refractive indices near 1.92 to 1.98, far above the mounting medium, so it shows very high positive relief (ii).
• Orthopyroxene (hypersthene) shows characteristic pink to green pleochroism (i).
• Muscovite has a birefringence of about 0.036 to 0.049, giving bright upper second to third order interference colours (iv).
• Cordierite forms cyclic (sector) twins that divide a grain into three or six segments (iii).

Options giving muscovite the cyclic twinning, or zircon the pleochroism, swap properties that belong to cordierite and orthopyroxene.

Your Answer : (Not Answered)

Correct Answer : B

The halide is fluorite.

Scratching calcite (hardness 3) but being scratched by quartz (hardness 7) puts the mineral between 3 and 7 on the Mohs scale. Fluorite, CaF₂, has hardness 4 and perfect octahedral cleavage in four directions, so it fits both clues.

Sylvite (KCl) and halite (NaCl) are halides, but both are soft (hardness about 2 to 2.5, so calcite scratches them) and both have cubic cleavage, not octahedral. Apatite has hardness 5, but it is a phosphate, not a halide, and has only poor cleavage.

Your Answer : (Not Answered)

Correct Answer : A

The mineral keyed as showing no change under UV light is apatite.

The other three are the textbook fluorescent minerals. Fluorite gave its name to fluorescence and commonly glows blue or violet. Scheelite (CaWO₄) fluoresces a bright bluish white under short-wave UV, which is why prospectors search for tungsten ore with a UV lamp at night. Calcite often fluoresces red, pink or orange, as in the well-known Franklin, New Jersey specimens.

Some apatite specimens do fluoresce, but the property is erratic and is not one of its characteristic features, so among these four it is the one the standard lists leave out.