TL;DR: Helium-3 is a main entry in the 外星矿物知识引擎 database.
Helium-3 is deposited in the upper few centimeters of lunar regolith by the solar wind. While concentrations are extremely low (1-50 ppb), the total lunar inventory may exceed 1 million tonnes. Economic viability depends entirely on practical He-3 fusion reactor development.
Crystal System
N/A (gas/noble gas)
Occurrence
Lunar surface regolith (1-50 ppb); concentrated in mature mare soils; implanted by solar wind
Significance
Potential fusion fuel worth $3-5B/tonne; depends on fusion reactor development; long-term energy resource
Source
Apollo sample analysis, lunar regolith studies