Particle Size Classification Criteria

The question asks to classify particles based on their size, specifically focusing on particles with a diameter smaller than $0.002 \text{ mm}$. Particle size is a fundamental characteristic used in various scientific fields, including soil science and geology, to categorize materials.

Analyzing Particle Size Categories

Let's examine the typical size ranges for the given options:

• Cobble: These are large stones, generally defined as having a diameter greater than 64 mm. They are significantly larger than $0.002 \text{ mm}$.
• Pebble: Pebbles are smaller than cobbles but still relatively large, typically ranging from 4 mm to 64 mm in diameter. They are much larger than $0.002 \text{ mm}$.
• Silt: Silt particles fall into a finer category, generally ranging from $0.002 \text{ mm}$ to $0.0625 \text{ mm}$ (or 2 to 62.5 micrometers). The lower bound of silt is often considered the upper bound for the next smaller category.
• Colloids: Colloids represent extremely fine particles. While definitions vary slightly, they are typically considered to have diameters less than $0.001 \text{ mm}$ (1 micrometer). This size range is definitively smaller than $0.002 \text{ mm}$. In many soil classification systems, particles smaller than $0.002 \text{ mm}$ are termed 'clay', and colloids are often associated with or smaller than this clay fraction.

Identifying Particles Smaller Than 0.002 mm

Based on the standard classifications and the options provided:

• Particles larger than $0.0625 \text{ mm}$ include sand, pebbles, and cobbles.
• Particles in the range of $0.002 \text{ mm}$ to $0.0625 \text{ mm}$ are classified as silt.
• Particles smaller than $0.002 \text{ mm}$ are classified as clay or, in this context representing the finest fraction, as colloids.

Therefore, particles with a diameter smaller than $0.002 \text{ mm}$ are classed as Colloids.