Question

Monoclinic crystal has dimension

• A. a=b=c; \(\alpha\)≠\(\beta\)≠\(\gamma\)≠90⁰
• B. a=b≠c; \(\alpha\)=\(\beta\)=\(\gamma\)=90⁰
• C. a=b=c; \(\alpha\)=\(\beta\)=\(\gamma\)=90⁰
• D. a≠b≠c; \(\alpha\)=\(\gamma\)= 90⁰; \(\beta\)≠ 90⁰

Hint:

In the monoclinic system, the crystal is described by vectors of unequal lengths. The two combinations of vectors are perpendicular (right angles), while the third pair forms an angle other than 90°. 

Answer 1

The Correct Option is D

Monoclinic crystal has dimension: a≠b≠c; \(\alpha\)=\(\gamma\)= 90⁰; \(\beta\)≠ 90⁰

For monoclinic crystals, axial lengths are not equal. Hence  a≠b≠c and the axial angles \(\alpha\) =\(\gamma\)= 90⁰; \(\beta\)≠ 90⁰ 

The monoclinic crystal system is one of the practical crystal systems in crystallography. Three vectors are used to describe the crystal system. For example, quartz is described by vectors of inequitable lengths forming a rectangular type prism with a parallelogram base. Some of the common characteristics of the monoclinic crystals are:

• Orientation of a crystal has few constraints – where b is the only fixed axis by symmetry.
• Axis C is generally chosen based on cleavage and habit.
• α and γ = 90⁰
• In some cases, the b axis will be 90⁰(pseudo-orthorhombic form)
• Symmetry operation in a monoclinic system, the unprecedented operation is 2/m – a twofold axis of rotation with a mirror plane.
• The axis b is the rotation, while c and a lie in the mirror plane.
• Monoclinic crystals have two forms: pinacoids and Monoclinic shape crystals have two shapes: pinacoidal and prisms.
• Common monoclinic rock-forming crystals include clinopyroxene, orthoclase, mica, and titanite.

Also Read: Amorphous and Crystalline Solids