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How do bastnasite and monazite differ as rare-earth ores?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

Bastnasite and monazite are both significant sources of rare-earth elements, but they differ fundamentally in their chemical composition and geological occurrence.

Chemical Composition

Bastnasite is a fluorocarbonate mineral with the general formula ${CeCO}_{3} (OH,F)$ . It serves as a primary source of light rare earths.12
Monazite is a phosphate mineral, typically expressed as $(Ce, La) {PO}_{4}$ . Unlike bastnasite, monazite frequently contains 10–12 percent thorium dioxide, making it a major commercial source of thorium as well as cerium.3

Geological Occurrence and Mining

Bastnasite is often found in contact metamorphic zones and pegmatites. Major deposits are mined in locations such as Mountain Pass, California, and various sites in China.12
Monazite typically occurs as small, resinous crystals in granitic and gneissic rocks. It is also commonly found in detritus known as monazite sands. It is mined in countries including India, Brazil, Vietnam, and Malaysia.3

While both minerals are rich in cerium and lanthanum, their specific rare-earth distributions vary by deposit.23