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Karnataka II PUC Geology (KSEAB) practice questions

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Question 1Physical Geology and Geomorphology

In the context of the Karnataka II PUC Geology syllabus, which layer of the Earth lies directly below the crust and is separated from it by the Mohorovičić discontinuity?

  1. The outer core
  2. The asthenosphere only
  3. The mantle Correct answer.
  4. The inner core

Explanation

The Mohorovičić discontinuity, or Moho, marks the boundary between the crust and the mantle. Seismic P-wave velocities increase sharply across this boundary, confirming the change from silicate crustal rock to denser ultramafic mantle rock. The core boundaries lie far deeper.

Question 2Physical Geology and Geomorphology

A student observes a mineral specimen that scratches glass but is itself scratched by quartz. Which Mohs hardness value best fits this specimen?

  1. 2
  2. 6 Correct answer.
  3. 8
  4. 4

Explanation

Glass has a hardness of about 5.5 and quartz is 7 on the Mohs scale. A mineral that scratches glass but is scratched by quartz must therefore fall between 5.5 and 7, and 6 fits this range. Values of 2 and 4 are too soft to scratch glass.

Question 3Physical Geology and Geomorphology

Which type of plate boundary is characterised by the formation of new oceanic lithosphere and the creation of mid-oceanic ridges?

  1. Convergent boundary
  2. Transform boundary
  3. Passive margin
  4. Divergent boundary Correct answer.

Explanation

At divergent boundaries, plates move apart and magma rises to fill the gap, cooling to form new oceanic crust. This sea-floor spreading builds mid-oceanic ridges such as the Mid-Atlantic Ridge. Convergent and transform boundaries consume or slide crust rather than create it.

Question 4Physical Geology and Geomorphology

Which of the following best describes the process of chemical weathering known as hydrolysis in silicate rocks?

  1. Abrasion of rock particles carried by wind
  2. Reaction of minerals with water to form clay minerals and soluble ions Correct answer.
  3. Frost wedging of jointed rock by expanding ice
  4. Dissolution of limestone by carbonic acid alone

Explanation

Hydrolysis involves water reacting with silicate minerals, especially feldspars, to produce clay minerals and release soluble cations such as sodium and potassium. It is a chemical breakdown rather than a physical one. Frost wedging and abrasion are mechanical processes.

Question 5Physical Geology and Geomorphology

A river flows from a mountainous source to the sea. Which sequence of geomorphic features is most typical along its course?

  1. V-shaped valley, delta, meander
  2. Delta, meander, V-shaped valley
  3. V-shaped valley, meander, delta Correct answer.
  4. Meander, delta, V-shaped valley

Explanation

In the youthful stage, vertical erosion cuts steep V-shaped valleys. In the mature stage, lateral erosion produces meanders on gentler gradients. In the old stage, deposition at the mouth builds a delta. This order follows the downstream decline in energy.

Question 6Physical Geology and Geomorphology

Which seismic wave type is a compressional body wave that travels fastest and can pass through both solids and fluids?

  1. Rayleigh wave
  2. Love wave
  3. P-wave Correct answer.
  4. S-wave

Explanation

P-waves are longitudinal compressional waves and are the fastest seismic body waves, arriving first at a seismograph. They propagate through solids, liquids, and gases. S-waves are transverse and cannot pass through fluids.

Question 7Physical Geology and Geomorphology

Which of the following is a typical characteristic of a transform fault boundary?

  1. Two continental plates collide to form fold mountains
  2. Crust is created by rising magma
  3. Plates slide past each other with no crust created or destroyed Correct answer.
  4. Crust is destroyed by subduction

Explanation

Transform boundaries accommodate lateral motion between plates, such as along the San Andreas Fault. Lithosphere is neither created nor destroyed there, though earthquakes are common. Creation and destruction occur at divergent and convergent boundaries respectively.

Question 8Physical Geology and Geomorphology

Which rock type is formed by the cooling and crystallisation of magma or lava?

  1. Metamorphic rock
  2. Igneous rock Correct answer.
  3. Sedimentary rock
  4. Biochemical rock

Explanation

Igneous rocks crystallise from molten material, either below the surface as intrusive rocks or at the surface as extrusive rocks. Sedimentary rocks form from deposited material and metamorphic rocks form by alteration of pre-existing rocks under heat and pressure.

Question 9Physical Geology and Geomorphology

A geologist examines a sandstone bed containing well-rounded, well-sorted quartz grains with a small amount of feldspar. Which interpretation of the depositional environment is most reasonable?

  1. Deposition close to the source with minimal transport
  2. Deposition after prolonged transport, probably in a high-energy fluvial or beach setting Correct answer.
  3. Formation by rapid cooling of basaltic lava
  4. Direct precipitation from evaporating seawater

Explanation

Rounded, well-sorted quartz grains indicate extensive transport and winnowing, which removes unstable minerals and leaves quartz concentrated. High-energy fluvial or beach environments produce such mature sands. Proximity to source would give angular, poorly sorted, feldspar-rich sediment.

Question 10Physical Geology and Geomorphology

Which of the following best explains why the outer core is inferred to be liquid?

  1. P-waves are refracted at the core boundary but S-waves do not pass through the outer core Correct answer.
  2. Surface waves arrive before body waves from distant earthquakes
  3. Both P-waves and S-waves travel freely through the outer core
  4. The outer core generates compressional waves of its own

Explanation

Seismographs at distances beyond about 103 degrees record a P-wave shadow zone and no direct S-waves. Because S-waves cannot propagate through fluids, their absence indicates a liquid outer core, while P-wave refraction confirms the boundary. This is the classic seismic evidence for a fluid outer core.

Question 11Physical Geology and Geomorphology

In the rock cycle, which process converts a shale into a slate?

  1. Deposition and cementation of clastic grains
  2. Melting followed by slow crystallisation
  3. Chemical precipitation from solution
  4. Low-grade regional metamorphism under directed pressure Correct answer.

Explanation

Shale undergoes low-grade regional metamorphism to become slate, with aligned mica flakes producing slaty cleavage. Increasing metamorphism would produce phyllite, schist, and finally gneiss. Melting would produce an igneous rock instead.

Question 12Physical Geology and Geomorphology

Which of the following coastal landforms is produced mainly by wave erosion?

  1. Sand bar
  2. Sea arch Correct answer.
  3. Spit
  4. Tombolo

Explanation

A sea arch forms when wave action erodes a headland from both sides until a passage is cut through it. Spits, tombolos, and sand bars are depositional features built by longshore drift and sediment accumulation. Erosional coasts typically show cliffs, caves, arches, and stacks.

Question 13Physical Geology and Geomorphology

Which of the following describes the principle of superposition as used in stratigraphy?

  1. In an undisturbed sequence, younger layers lie above older layers Correct answer.
  2. Fossils are always found in the lowest layer
  3. Igneous intrusions are older than the rocks they cut
  4. Sedimentary layers are always horizontal

Explanation

The principle of superposition states that in an undisturbed sequence of sedimentary rocks, each layer is younger than the one below it. This allows relative ages to be assigned. Cross-cutting relationships deal separately with intrusions being younger than the rocks they cut.

Question 14Physical Geology and Geomorphology

A confined aquifer differs from an unconfined aquifer mainly because a confined aquifer is:

  1. Bounded above by an impermeable layer and stored under pressure Correct answer.
  2. Recharged only by rivers and never by rainfall
  3. Always shallower than an unconfined aquifer
  4. Composed entirely of impermeable rock

Explanation

A confined aquifer is sandwiched between impermeable layers, so its water is under pressure and rises in a well to form an artesian level. An unconfined aquifer has a water table open to the atmosphere through overlying permeable material. Both can be recharged by rainfall.

Question 15Physical Geology and Geomorphology

Which of the following best explains the formation of a hanging valley at the junction of a tributary glacier with a main glacial trough?

  1. The tributary glacier flows faster because it is smaller
  2. The tributary glacier deposits more moraine than the main glacier
  3. The main glacier erodes its floor faster and deeper than the smaller tributary glacier Correct answer.
  4. Meltwater cuts a canyon after the ice retreats

Explanation

The main glacier is thicker and more powerful, so it deepens its trough more rapidly than the smaller tributary. When the ice melts, the tributary floor is left perched above the main valley, forming a hanging valley often marked by a waterfall. Differential glacial erosion is the key control.

Question 16Physical Geology and Geomorphology

Which of the following minerals is a common ferromagnesian silicate found in basalt and gabbro?

  1. Quartz
  2. Augite pyroxene Correct answer.
  3. Muscovite mica
  4. Orthoclase feldspar

Explanation

Augite is a pyroxene rich in iron and magnesium and is a major constituent of mafic rocks such as basalt and gabbro. Quartz, orthoclase, and muscovite are felsic minerals typical of granitic rocks. Ferromagnesian minerals give mafic rocks their dark colour.

Question 17Physical Geology and Geomorphology

Which of the following landforms is characteristic of a karst terrain developed on limestone?

  1. Drumlin
  2. Butte
  3. Moraine
  4. Sinkhole Correct answer.

Explanation

Karst landscapes form by chemical dissolution of limestone, producing sinkholes, caves, swallow holes, and disappearing streams. Drumlins and moraines are glacial features, and buttes are erosional remnants of flat-lying sedimentary strata in arid regions.

Question 18Physical Geology and Geomorphology

An earthquake's epicentre is best defined as:

  1. The point within the Earth where rupture begins
  2. The zone where seismic waves are strongest in the mantle
  3. The point on the Earth's surface vertically above the focus Correct answer.
  4. The line along which the fault slips

Explanation

The focus, or hypocentre, is the point within the Earth where rupture initiates, and the epicentre is the point on the surface directly above it. Seismic waves radiate from the focus, and the epicentre is where surface effects are usually greatest. The two terms are frequently confused.

Question 19Physical Geology and Geomorphology

Which of the following best describes an unconformity in a rock sequence?

  1. A fault plane along which displacement has occurred
  2. A graded bed showing upward fining of grains
  3. A layer of igneous rock intruded between sedimentary beds
  4. A surface representing a gap in the geological record due to erosion or non-deposition Correct answer.

Explanation

An unconformity is a surface of erosion or non-deposition that represents missing time in the stratigraphic record. Angular unconformities, disconformities, and nonconformities are its main types. Faults and intrusions are structural features, not time gaps.

Question 20Physical Geology and Geomorphology

Which of the following is the most important agent of erosion in desert regions?

  1. Glacial ice
  2. Wave action
  3. Running water during flash floods Correct answer.
  4. Groundwater dissolution

Explanation

Although rainfall is scarce in deserts, sporadic intense storms produce flash floods that erode and transport large volumes of sediment. Wind is a visible agent but moves far less material than episodic running water. Glaciers and waves are absent from most hot deserts.

Question 21Physical Geology and Geomorphology

A rock specimen shows a glassy texture with no visible crystals and abundant vesicles. Which conclusion is most consistent with these observations?

  1. It is a well-sorted clastic sedimentary rock
  2. It is a regionally metamorphosed shale
  3. It is an extrusive volcanic rock that cooled rapidly at the surface Correct answer.
  4. It is an intrusive plutonic rock that cooled slowly at depth

Explanation

Glassy texture with no visible crystals indicates very rapid cooling, which prevents crystal growth and commonly produces volcanic glass. Vesicles are gas bubbles trapped as lava solidifies at the surface, confirming an extrusive origin. Slow cooling at depth would produce coarse crystals instead.

Question 22Physical Geology and Geomorphology

Using the principle of cross-cutting relationships, a dyke that cuts across a sedimentary sequence is:

  1. Younger than the sedimentary beds it cuts Correct answer.
  2. Older than the sedimentary beds it cuts
  3. Unrelated in age to the sedimentary beds
  4. The same age as the sedimentary beds

Explanation

The principle of cross-cutting relationships states that a feature cutting another must be younger than the feature it cuts. Therefore the dyke intruded after the sedimentary beds were deposited. This principle is fundamental to relative dating of geological events.

Question 23Physical Geology and Geomorphology

A river meander is eventually cut off to form an oxbow lake. Which process best explains this transformation?

  1. Erosion on the outer bank and deposition on the inner bank until the neck is breached Correct answer.
  2. Headward erosion at the river's source
  3. Downcutting that raises the river's gradient
  4. Deposition of a natural levee along the channel

Explanation

On a meander, the outer bank erodes by undercutting while point-bar deposition occurs on the inner bank. Continued migration narrows the meander neck until the river cuts across it, abandoning the loop as an oxbow lake. This reflects lateral erosion and deposition in the mature stage.

Question 24Physical Geology and Geomorphology

Which of the following best explains why the Himalaya is a zone of intense seismicity and crustal thickening?

  1. Continental collision between the Indian and Eurasian plates Correct answer.
  2. Transform sliding between two oceanic plates
  3. Rifting of a continental plate above a mantle plume
  4. Sea-floor spreading along a mid-oceanic ridge

Explanation

The Indian plate is converging with and underthrusting the Eurasian plate, producing crustal shortening, thrust faulting, and thickening that built the Himalaya. The collision generates frequent earthquakes along thrust systems such as the Main Central Thrust. Rifting and ridge spreading are extensional settings.

Question 25Physical Geology and Geomorphology

A groundwater well is drilled into a confined aquifer whose potentiometric surface lies above ground level. Which outcome is expected?

  1. The water level in the well will rise above ground level and flow without pumping Correct answer.
  2. The aquifer will be recharged only through the well itself
  3. The well will yield no water because the aquifer is confined
  4. The water level will remain at the depth of the aquifer

Explanation

When the potentiometric surface of a confined aquifer stands above ground level, hydrostatic pressure drives water up the well to that level, producing a flowing artesian well. This requires a confined aquifer with a recharge area at higher elevation. The aquifer still stores and transmits water normally.

Question 26Physical Geology and Geomorphology

Which of the following best describes the asthenosphere in relation to plate movement?

  1. A rigid outer shell that fractures to produce earthquakes
  2. A weak, partially molten layer over which lithospheric plates glide Correct answer.
  3. A solid iron-rich layer that generates the magnetic field
  4. A thin surface veneer of sediment and soil

Explanation

The asthenosphere lies beneath the lithosphere and is mechanically weak because it is close to its melting point, so it deforms plastically. Plates of rigid lithosphere ride over this weak layer, which is why it is described as the zone of isostatic adjustment and plate motion.

Question 27Physical Geology and Geomorphology

A stream flows from a mountainous region onto a broad plain, and its gradient suddenly decreases. Which feature is most likely to develop at that point?

  1. A waterfall formed by a resistant bed
  2. A plunge pool scoured at the base of a cliff
  3. A V-shaped gorge cut into bedrock
  4. An alluvial fan built of deposited sediment Correct answer.

Explanation

When gradient and velocity fall abruptly, the stream loses competence and deposits its coarse load in a fan-shaped body called an alluvial fan. The same process at the mouth of a canyon produces the fan surface, and continued deposition may merge fans into a bajada.

Question 28Physical Geology and Geomorphology

A bed of limestone is overlain by shale, and the contact between them is sharp and planar with no evidence of erosion or weathering. Which interpretation is most consistent with this field relationship?

  1. A conformable contact recording continuous or near-continuous deposition Correct answer.
  2. An angular unconformity produced by tilting and erosion
  3. A disconformity indicating a long period of non-deposition
  4. An intrusive contact where magma baked the limestone

Explanation

A sharp, planar contact without a weathered or eroded surface, and without a basal conglomerate, indicates that deposition continued without a significant break. Such a contact is conformable, even though the lithology changes abruptly because the depositional environment shifted.

Question 29Physical Geology and Geomorphology

Which of the following best explains why a shield volcano has gentle slopes while a composite volcano has steep slopes?

  1. Composite volcanoes form only beneath glaciers, which steepen their flanks
  2. Composite volcanoes are built from highly fluid lava that piles up close to the vent
  3. Shield volcanoes erupt only ash, which settles at a low angle
  4. Shield volcanoes are built from low-viscosity basaltic lava that spreads widely before solidifying Correct answer.

Explanation

Slope angle is controlled mainly by lava viscosity and by the mix of lava and pyroclastic material. Low-viscosity basaltic lava flows far from the vent, building broad, gently sloping shields, whereas viscous andesitic magma plus interbedded ash and lava builds steep-sided composite cones.

Question 30Physical Geology and Geomorphology

In a region of active normal faulting, a series of parallel faults with steep dips has dropped a central block between two uplifted blocks. Which feature has formed, and what stress regime produced it?

  1. A horst, produced by compressional stress
  2. A graben, produced by tensional stress Correct answer.
  3. A fold-thrust belt, produced by shear stress
  4. A thrust sheet, produced by compressional stress

Explanation

Normal faults dip steeply and form where the crust is extended, so the dropped block between two such faults is a graben. The uplifted blocks on either side are horsts, and the paired structure is typical of rift valleys such as the East African Rift.

Question 31Mineralogy

Which silicate mineral group is built from isolated SiO4 tetrahedra that are not linked to one another by shared oxygen atoms?

  1. Phyllosilicates
  2. Tectosilicates
  3. Nesosilicates Correct answer.
  4. Inosilicates

Explanation

The classification of silicates rests on how many oxygens each SiO4 tetrahedron shares with its neighbours. Nesosilicates share none, so the tetrahedra stay isolated and are held together only through metal cations such as Mg, Fe or Ca. Sharing of corners in chains, sheets or frameworks defines the other groups.

Question 32Mineralogy

The Mohs hardness scale ranks minerals by their resistance to scratching. Which mineral occupies position 7 on this scale?

  1. Orthoclase
  2. Corundum
  3. Topaz
  4. Quartz Correct answer.

Explanation

The Mohs scale runs talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9 and diamond 10. Quartz therefore scratches glass and orthoclase but is itself scratched by topaz.

Question 33Mineralogy

A mineral specimen shows perfect cleavage in three directions mutually at right angles, producing cubic fragments. Which mineral is most consistent with this observation?

  1. Calcite
  2. Quartz
  3. Muscovite
  4. Halite Correct answer.

Explanation

Halite has cubic cleavage because its sodium and chloride ions are arranged on a cubic lattice, giving three cleavage planes at 90 degrees. Muscovite cleaves into one perfect sheet direction, calcite into three rhombohedral directions, and quartz shows no true cleavage at all.

Question 34Mineralogy

Which of the following is a native element mineral, that is, one composed of a single chemical element?

  1. Magnetite
  2. Pyrite
  3. Galena
  4. Graphite Correct answer.

Explanation

Native elements occur in nature in the uncombined state, and graphite is simply carbon. Galena is lead sulphide, pyrite is iron disulphide and magnetite is iron oxide, so all three are compounds rather than native elements.

Question 35Mineralogy

Which physical property of a mineral is directly controlled by the strength and type of its chemical bonds, so that minerals with strong covalent bonding tend to be hard?

  1. Colour
  2. Hardness Correct answer.
  3. Specific gravity
  4. Magnetic susceptibility

Explanation

Hardness reflects how strongly atoms are held together in the crystal structure. Strong covalent bonds such as those in diamond resist scratching, while weak bonds such as those in talc are easily broken. Colour depends on trace impurities or electronic transitions, not on bond strength alone.

Question 36Mineralogy

A dark, heavy mineral is strongly attracted to a magnet and gives a black streak. Which mineral is it?

  1. Chromite
  2. Ilmenite
  3. Hematite
  4. Magnetite Correct answer.

Explanation

Magnetite (Fe3O4) is the common oxide that is strongly ferromagnetic and is readily picked up by a hand magnet. Hematite is magnetic only weakly or not at all and gives a red-brown streak, which distinguishes it from magnetite's black streak.

Question 37Mineralogy

Which mineral is the hardest naturally occurring substance and therefore defines position 10 on the Mohs scale?

  1. Corundum
  2. Quartz
  3. Topaz
  4. Diamond Correct answer.

Explanation

Diamond is pure carbon in which every carbon atom is covalently bonded to four others in a rigid tetrahedral framework, giving it the greatest scratch resistance. Corundum occupies position 9 and is scratched by diamond.

Question 38Mineralogy

Which mineral group is characterised by a three-dimensional framework of SiO4 tetrahedra in which all four oxygens of each tetrahedron are shared with neighbouring tetrahedra?

  1. Sorosilicates
  2. Tectosilicates Correct answer.
  3. Nesosilicates
  4. Cyclosilicates

Explanation

Tectosilicates, or framework silicates, share all four corners of every SiO4 tetrahedron, producing a continuous three-dimensional network. Quartz and the feldspars are typical members, and their framework structure explains their lack of cleavage and their moderate hardness.

Question 39Mineralogy

A specimen of galena is described as having a metallic lustre, a grey-black streak and perfect cubic cleavage. These properties are consistent with which mineral composition?

  1. Calcium carbonate
  2. Iron oxide
  3. Lead sulphide Correct answer.
  4. Potassium feldspar

Explanation

Galena is lead sulphide (PbS), which crystallises in the cubic system and cleaves along three planes at right angles, giving cubic fragments. Its high lead content also makes it unusually dense for a common sulphide.

Question 40Mineralogy

Which pair of minerals can be distinguished most reliably by their streak colour, with one giving a cherry-red to red-brown streak and the other a black streak?

  1. Calcite and dolomite
  2. Muscovite and biotite
  3. Quartz and orthoclase
  4. Hematite and magnetite Correct answer.

Explanation

Hematite consistently leaves a red-brown streak on unglazed porcelain even when the hand specimen is grey and metallic, whereas magnetite leaves a black streak. Streak is more reliable than body colour because it reflects the powdered mineral.

Question 41Mineralogy

A mineral is scratched by a copper coin but not by a fingernail. Which Mohs hardness range does it most probably fall in?

  1. 6 to 7
  2. 8 to 9
  3. 2.5 to 3.5 Correct answer.
  4. 1 to 2

Explanation

A fingernail has a hardness of about 2.5 and a copper coin about 3 to 3.5. A mineral that resists the fingernail but yields to the coin therefore lies between roughly 2.5 and 3.5, a range that includes calcite and gypsum varieties.

Question 42Mineralogy

Which statement correctly explains why micas show a single, highly perfect cleavage?

  1. Their crystal lattice is amorphous and lacks order.
  2. Their sheet structure is held together by weak bonds between layers. Correct answer.
  3. Their atoms are arranged in continuous chains.
  4. Their SiO4 tetrahedra are isolated from one another.

Explanation

Phyllosilicates form continuous sheets of linked SiO4 tetrahedra, and successive sheets are bound by comparatively weak bonds involving potassium or other interlayer cations. Cleavage occurs preferentially between the sheets, producing thin flexible flakes.

Question 43Mineralogy

Which mineral is the principal ore of aluminium and is extracted industrially by the Bayer process?

  1. Cassiterite
  2. Bauxite Correct answer.
  3. Chalcopyrite
  4. Sphalerite

Explanation

Bauxite is a residual rock rich in aluminium hydroxides and oxides, and the Bayer process dissolves it in hot sodium hydroxide to obtain pure alumina. Cassiterite is the ore of tin, sphalerite of zinc and chalcopyrite of copper.

Question 44Mineralogy

Two specimens of quartz from different localities show different colours, one milky and one rose. Which conclusion is most justified?

  1. They have the same composition but differ in trace impurities. Correct answer.
  2. They differ in hardness and therefore in structure.
  3. They differ in cleavage and therefore in crystal system.
  4. They belong to different mineral species.

Explanation

Varietal colour in quartz arises from minute impurities or structural defects, such as iron or titanium traces, while the essential composition SiO2 and the framework structure remain the same. Colour alone is therefore a poor criterion for identifying a mineral species.

Question 45Mineralogy

Which silicate group contains minerals whose structure consists of double chains of SiO4 tetrahedra, giving cleavage at about 120 and 60 degrees?

  1. Amphiboles Correct answer.
  2. Feldspars
  3. Pyroxenes
  4. Garnets

Explanation

Amphiboles are double-chain inosilicates, and their cleavage surfaces meet at approximately 120 and 60 degrees. Pyroxenes are single-chain silicates with cleavage near 90 degrees, which is the classic field distinction between the two groups.

Question 46Mineralogy

A carbonate mineral effervesces vigorously with dilute hydrochloric acid and shows rhombohedral cleavage. Which mineral is it?

  1. Fluorite
  2. Calcite Correct answer.
  3. Dolomite
  4. Gypsum

Explanation

Calcite (CaCO3) reacts readily with dilute hydrochloric acid, releasing carbon dioxide, and its rhombohedral cleavage reflects the trigonal symmetry of its structure. Dolomite reacts only weakly or after powdering, which distinguishes the two carbonates.

Question 47Mineralogy

Which property is described as the colour of a mineral in its powdered form, usually tested on unglazed porcelain?

  1. Lustre
  2. Streak Correct answer.
  3. Tenacity
  4. Diaphaneity

Explanation

Streak is the colour of the fine powder left when a mineral is rubbed on a porcelain plate, and it may differ markedly from the colour of the bulk specimen. Lustre describes the way light is reflected from a surface, tenacity describes resistance to breaking, and diaphaneity describes transparency.

Question 48Mineralogy

Which mineral is a common rock-forming ferromagnesian silicate with isolated tetrahedra, olive-green colour and a vitreous lustre, often found in gabbro and peridotite?

  1. Biotite
  2. Olivine Correct answer.
  3. Hornblende
  4. Augite

Explanation

Olivine is a nesosilicate of magnesium and iron with isolated SiO4 tetrahedra, and it is a major constituent of ultramafic and mafic rocks such as peridotite and gabbro. Its olive-green colour and glassy lustre are characteristic in hand specimen.

Question 49Mineralogy

Which statement best explains why diamond and graphite, both composed only of carbon, differ so greatly in hardness?

  1. One of them is amorphous and the other crystalline.
  2. They contain different chemical elements.
  3. They differ in the isotopes of carbon present.
  4. Their carbon atoms are bonded in different structural arrangements. Correct answer.

Explanation

Diamond has a three-dimensional tetrahedral framework in which each carbon is covalently bonded to four others, while graphite forms layers of hexagonal rings held together by weak forces. The difference in hardness is therefore a consequence of atomic arrangement, not of composition.

Question 50Mineralogy

A mineral shows two cleavages intersecting at nearly 90 degrees, is pink to white, and is the most abundant mineral in granite. Which mineral is it?

  1. Muscovite
  2. Plagioclase feldspar
  3. Quartz
  4. Potassium feldspar Correct answer.

Explanation

Potassium feldspar (orthoclase) is a framework silicate with two cleavages at right angles and is typically pink to white, forming a major component of granite. Its cleavage distinguishes it from quartz, which shows conchoidal fracture instead.