Minerals and ores: identify the principal ore source of thorium that is widely associated with beach sand placer deposits.

Introduction / Context:
Thorium is a fertile material of strategic interest for advanced nuclear fuel cycles. Knowing its geological sources is essential for resource planning and fuel-cycle education. In many coastal regions, thorium is recovered from monazite-bearing beach sands.

Given Data / Assumptions:

• Monazite is a rare-earth phosphate mineral that often contains thorium and sometimes uranium.
• Other listed minerals are principal ores of different metals (e.g., galena for lead, pitchblende for uranium).

Concept / Approach:
Monazite concentrates recovered from heavy-mineral beach sands are a traditional source of thorium compounds. After beneficiation, chemical processing extracts thorium (commonly as ThO2 or related compounds) for industrial or research uses.

Step-by-Step Solution:

Verification / Alternative check:
Resource literature and historical production records consistently cite monazite as a primary thorium source, especially in India and Brazil.

Why Other Options Are Wrong:

• Pitchblende is primarily uranium ore, not thorium.
• Limonite is an iron ore; galena is lead sulfide; cassiterite is tin oxide.

Common Pitfalls:
Confusing uranium and thorium ores due to both being actinides; overlooking that “monazite sand” refers to heavy-mineral concentrates within beach sands.

Final Answer:
Monazite sand