Materials structure terminology: a substance whose atomic arrangement is ordered in some directions (exhibiting layered or directional order) but not in others is termed 'mesomorphous'. Which of the following is a classic example of such a material?

Introduction:
Atomic order in solids ranges from fully crystalline (long-range order in all three dimensions) to amorphous (no long-range order). Between these extremes are materials that show order in particular directions or planes while being disordered in others. Understanding these categories clarifies why certain materials cleave easily, exhibit anisotropic properties, or behave uniquely under stress or heat.

Given Data / Assumptions:

• 'Mesomorphous' here denotes materials with directional or layered order.
• We compare common examples: metals, glasses, layered silicates, and polymers.
• We select the example that best illustrates anisotropic structural order.

Concept / Approach:
Mica, a family of phyllosilicates, exhibits strong two-dimensional order within sheets of silicate tetrahedra, with weak bonding between layers. This gives pronounced anisotropy—easy cleavage into thin flakes and distinctive electrical and thermal behavior along planes versus through thickness. Such layered order contrasts with glasses (amorphous), which lack long-range periodicity, and typical metals (lead, silver), which are crystalline and isotropic on the macroscopic scale.

Step-by-Step Solution:
Identify structural hallmark: layered, anisotropic order.Map candidates: mica fits due to phyllosilicate sheets and weak interlayer bonds.Exclude fully crystalline isotropic metals and fully amorphous glass.

Verification / Alternative check:
Crystallography references classify micas as layered silicates with perfect basal cleavage and strong anisotropy, consistent with the mesomorphic description in many materials texts.

Why Other Options Are Wrong:

• Lead and silver: Typical metallic crystals with 3D periodicity.
• Glass: Amorphous; lacks long-range order in any direction.
• Polyethylene: Semi-crystalline polymer, but the defining layered, sheet-like mineral structure is better represented by mica.

Common Pitfalls:
Equating any anisotropy with mesomorphous behavior; the layered mineral structure in mica is the archetypal example.

Final Answer:
mica