This website is made to help students prepare for their competitive exams with ease and confidence.
Q1. The core-rim compositions of a normally zoned plagioclase crystal are as follows: Core: Ca0.6NaXAl1.6Si2.4O8 Rim: Ca0.4NaYAl1.4Si2.6O8 The amount of increase of Na atom from core to rim per formula unit of plagioclase is ___.
The correct answer is 0.2. Plagioclase is a mix of Calcium (Ca) and Sodium (Na) that always adds up to 1.0 total atom in its formula. In the core, Ca is 0.6, meaning the Na (X) must be the remaining 0.4. In the rim, Ca has dropped to 0.4, meaning the Na (Y) must have risen to the remaining 0.6. The increase of Sodium from the core (0.4) to the rim (0.6) is exactly 0.2 atoms.
Q2. Two limbs of a vertical chevron fold strike S70°E and N55°E. The value of the interlimb angle of the fold is ___ (degree).
The correct answer is 55. We need to find the angle between these two compass directions. N55°E is 55 degrees clockwise from North. S70°E means starting at South (180°) and turning 70 degrees backwards toward East, which is 110 degrees clockwise from North. The physical angle folded between these two rock sides is just the difference between their compass headings: 110 minus 55 equals exactly 55 degrees.
Q3. The birefringence of a mineral of thickness 30 μm and retardation 0.27 μm is ___ (give answer in three decimal places).
The correct answer is 0.009. Birefringence is a measure of how light splits inside a crystal. The formula is very straightforward: Birefringence = Retardation divided by Thickness. We are given the retardation as 0.27 μm and the thickness as 30 μm. By simply dividing 0.27 by 30, we get the final mathematical value of 0.009.
Q4. Aluminium (Al) can occur in both tetrahedral and octahedral co-ordinations in silicates. The amount of octahedral Al in a pyroxene crystal of composition Mg1.4Fe0.4Al0.4Si1.8O6 is ___ (give answer in one decimal place).
The correct answer is 0.2. In pyroxene minerals, the ideal structure needs exactly 2.0 silicon atoms in its 'tetrahedral' slots. Our formula only has 1.8 silicon atoms. To fill this gap, 0.2 of the Aluminium atoms must act like silicon and sit in those tetrahedral slots (1.8 + 0.2 = 2.0). The formula has a total of 0.4 Aluminium atoms. Subtracting the 0.2 used in the tetrahedral slots leaves exactly 0.2 Aluminium atoms to sit in the other 'octahedral' slots.
Q5. In a mineral with chemical formula AT4O8, the ionic radii of A and O are 1.12 Å and 1.40 Å, respectively. The co-ordination number of cation A is ___.
The correct answer is 8. In chemistry, how many oxygen atoms can physically pack around a metal atom (co-ordination number) depends strictly on their size ratio. Divide the radius of the metal A (1.12) by the radius of Oxygen (1.40) to get a ratio of 0.80. In crystallography rules, any ratio between 0.732 and 1.0 dictates that exactly 8 oxygen atoms will perfectly fit around the central metal atom in a cubic shape.
Q6. The Weiss symbol of a crystal face is 4a: 2b: c. The value of h in the corresponding Miller Index (hkl) is ___.
The correct answer is 1. Geologists translate 'Weiss symbols' into 'Miller Indices' to describe crystal shapes. The Weiss numbers here are 4, 2, and 1. First, flip them upside down (reciprocals): 1/4, 1/2, and 1/1. Next, to remove the fractions, find a common multiplier (which is 4). Multiply all by 4: (1/4)*4=1, (1/2)*4=2, (1/1)*4=4. The final Miller Index is (124). The first number 'h' is simply 1.
Q7. The mole% of forsterite component in olivine with chemical formula Mg1.8Fe0.2SiO4 is ___.
The correct answer is 90. Olivine is a mix of Magnesium (Forsterite) and Iron (Fayalite). The formula tells us there are 1.8 atoms of Magnesium and 0.2 atoms of Iron. The total number of these metal atoms is 2.0 (1.8 + 0.2). To find the percentage of Magnesium (Forsterite), divide the Magnesium part (1.8) by the total (2.0) and multiply by 100. This mathematically equals exactly 90%.
Q8. An object is spotted at S60°E front bearing from the observer. If the position is interchanged, the front bearing value in degree from North (measured clockwise) is ___.
The correct answer is 300. A compass is a 360-degree circle. S60°E means you face South (180°) and turn 60 degrees towards East, aiming at 120 degrees. If you swap places with the object, you must look exactly in the opposite direction. You simply add 180 degrees to turn completely around. 120 + 180 equals exactly 300 degrees clockwise from North.
Q9. The half life of a radionuclide A is double that of a radionuclide B. The fraction of A remaining when B is reduced to 1/64 is _________. Give answer in three decimal places.
The correct answer is 0.125. A half-life is the time it takes for half of a substance to decay. If B is reduced to 1/64, it means it has halved 6 times (1/2, 1/4, 1/8, 1/16, 1/32, 1/64). Since A's half-life is twice as long, it decays half as fast as B. So in the time B went through 6 half-lives, A only went through 3 half-lives. Halving A three times gives 1/2, then 1/4, then 1/8. The fraction 1/8 converted to a decimal is 0.125.
Q10. Fine muds are deposited at a rate of 1 cm per 1000 y. Assuming constant sedimentation rate and absence of compaction, a 1 km thick sequence would be deposited in _______million years.
The correct answer is 100. First, we figure out how long it takes to build 1 meter of mud. If 1 centimeter takes 1,000 years, then 100 centimeters (1 meter) takes 100,000 years. A sequence that is 1 kilometer thick is 1,000 meters. Multiplying 100,000 years by 1,000 gives 100,000,000 years. Converting this to millions gives exactly 100 million years.