According to the US Geological Survey, nearly all of the world's lithium reserves are in Chile/Argentina, China, and Australia. Why is this? Lithium seems to be mostly harvested from brine pools, so are these just the areas where lithium can be economically harvested, or does it represent an uneven distribution in Earth's crust?
The term "reserves" doesn't mean what most people think it does. A mineral deposit is simply a concentration of a mineral; a reserve is a deposit that is both known and economical to extract. As an example, oil shale and tar sands have both been known for decades, but only recently became oil reserves as improving technology and rising prices made it practical to extract the crude oil from them.
Chile, Argentina, Australia, and China have quantified their deposits, and local economic/geologic conditions make it profitable to extract those deposits, so they've got most of the world's reserves.
Start by reading my answer on a different website:
This is a quick explainer on what "reserves" actually are.
Lithium is mined from two main sources: a type of granitic rock called "pegmatite", and brine pools.
Pegmatites are rare rocks, and lithium-rich pegmatites are even rarer and you don't find them in a lot of places. Even when you do, they are most likely not included in the "reserves" because there is no extraction infrastructure, know-how, or economic drive to produce lithium.
Lithium-rich brines only form when you have evaporite lakes in dry regions, that also happen to have pegmatites, granites, and other lithium-rich rocks nearby. This commonly occurs in South America because there are deserts and plenty of granites. In contrast, the Dead Sea (Israel/Jordan) is one of the most saline lakes in the world, but it has very little lithium in it because the nearby rocks are limestones, not granites.