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Crystallography Fundamentals Questions and Answers

1.

The d-spacing of a plane (110) of a cubic crystal with lattice parameter a = 5 Å is :

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

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.

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2.

Solid angle is an angle between at least :

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

A solid angle is formed between at least Three facies, that is, three crystal faces.

In crystallography a solid angle is the corner where crystal faces meet at a point. Two faces meet only along an edge, forming an interfacial (dihedral) angle; it takes at least three faces meeting at a point to enclose a solid angle, as at the corner of a cube.

Lines bound plane angles, not solid angles, so the options in terms of lines are wrong, and two faces give an edge, not a solid angle.

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3.

Which of the following have curved crystal facies ?

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: C

The mineral with curved crystal faces is Siderite.

Siderite (iron carbonate), like dolomite, commonly forms rhombohedral crystals with curved, saddle-shaped faces, because the crystal lattice is slightly distorted as it grows. Curved faces are a recognised feature of the rhombohedral carbonates.

Hypersthene (an orthopyroxene) and actinolite (an amphibole) form prismatic crystals with flat faces, and lepidolite, a lithium mica, forms flat tabular plates, so none of them is known for curved faces.

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