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Q181. The texture characterized by exsolved lamellae of albite in K-feldspar is known as
The correct answer is perthite. When a hot crystal of potassium-rich feldspar (K-feldspar) cools down, it can't hold all its sodium anymore. The sodium separates out into thin, zebra-like stripes of a different mineral called albite. This beautiful striped pattern of albite inside K-feldspar is officially called 'perthitic' texture.
Q182. Which one among the following planets in the Solar system is most similar in size to the Earth?
The correct answer is Venus. Venus is often called Earth's 'twin sister' because it is almost exactly the same physical size and weight as the Earth. Its radius is only about 300 kilometers smaller than Earth's. Mercury is much smaller, while Neptune and Uranus are massive gas giants.
Q183. Which one of the following processes is responsible for the formation of syngenetic Ni-Cu sulphide ore in gabbro-noritic rocks?
The correct answer is Liquid immiscibility. Deep underground, when magma gets too full of sulfur, heavy, dense liquid metal-sulfides spontaneously separate from the lighter, rocky silicate magma (like oil separating from water). This 'liquid immiscibility' causes heavy metal droplets to sink to the bottom of the magma chamber, forming massive Nickel-Copper deposits.
Q184. Which one of the following sedimentary structures is NOT used for determining top and bottom of beds?
The correct answer is Plane lamination. Geologists look for 'way-up' structures to tell if a rock layer has been flipped upside down. Mud cracks (which open upwards), wave ripples (which have sharp peaks pointing up), and load structures (which sag downwards) all clearly show which way was originally 'up'. Simple flat 'plane lamination' looks exactly the same right-side-up as it does upside-down, so it cannot be used.
Q185. A geological formation neither containing nor transmitting water is termed as
The correct answer is aquifuge. In groundwater science, rocks behave differently with water. An 'aquifuge' is a solid, completely impermeable rock (like dense granite) that cannot hold any water and refuses to let any flow through it. An 'aquifer' holds and transmits a lot of water. An 'aquiclude' holds water but won't let it flow (like clay). An 'aquitard' only lets a tiny trickle flow, making them incorrect.
Q186. A helically coiled ammonite Turrilites is differentiated from externally resembling Gastropoda Turritella by
The correct answer is chambered shell. Both the ammonite Turrilites (an extinct squid relative) and the gastropod Turritella (a sea snail) have tall, spiraling, cone-shaped shells that look incredibly similar on the outside. However, the inside of an ammonite shell is divided into many small rooms or 'chambers' used for floating, whereas a snail's shell is just one long, continuous hollow tube. The other options describe outer shapes that can be too similar to tell apart reliably.
Q187. A mineral with a point group symmetry 2/m
The correct answer is has two optic axes. A point group symmetry of '2/m' strictly belongs to the Monoclinic crystal system. In optical mineralogy, all Monoclinic minerals are mathematically 'biaxial', meaning light splitting through them creates exactly two optic axes. Uniaxial minerals belong to Hexagonal or Tetragonal systems. Because of how Monoclinic crystals slant, their extinction angles vary, so options B and C are not absolute rules.
Q188. A sandstone has < 5% matrix. The recalculated modal compositions of feldspar, quartz and rock fragments are 45%, 35%, 20%, respectively. The sandstone is classified as
The correct answer is lithic arkose. In classifying sandstones, less than 5% mud matrix means it is a clean 'arenite', instantly ruling out the 'wacke' options. Next, we look at the grains: it has a huge amount of feldspar (45%), which makes it an 'arkose'. Because it also has a notable chunk of rock fragments (20%), we add the prefix 'lithic'. It is too rich in feldspar to be a quartz arenite or subfeldsarenite.
Q189. Conservative plate boundary is represented by
The correct answer is transform fault. Tectonic plates can crash together (destructive) or pull apart (constructive). A 'conservative' boundary is where two plates simply slide past each other horizontally, neither creating nor destroying rock. A 'transform fault' is the geological term for this sliding crack. Normal and growth faults involve pulling apart, and reverse faults involve crashing together, making them incorrect.
Q190. En-echelon sigmoidal ‘gash’ veins indicate
The correct answer is brittle-ductile shear zone. When rock is pulled apart, it can crack and fill with minerals to form veins. 'En-echelon sigmoidal gash veins' are a specific S-shaped pattern of cracks. The cracking means the rock is partly snapping (brittle), but the S-shape bending means it is also stretching like plastic (ductile). A purely brittle zone just shatters, a purely ductile zone flows without cracking, and a saddle reef is a fold, not a shear zone.