Lithium can be hosted within a range of minerals in pegmatitic...

Lithium can be hosted within a range of minerals in pegmatitic rocks. A list of the main lithium-bearing minerals is provided in Table 1. Olympus pXRD can rapidly identify and quantify all these lithium-bearing minerals, irrespective of their reflectance. Quantitative mineralogy can then be back calculated to chemistry to provide geologists with excellent estimates of lithium concentrations while in the field (Figure 1). Rather than waiting days or weeks for lithium assay data from a laboratory, exploration geologists can use Olympus pXRD

I say nope on the pXRD. The problem is that the portable pXRD analyzes a very small pinch of a sample (maybe 15 milligrams). A grain of rice is 25 milligrams. Pegmatites are very coarse-grained by nature, so you need a large sample to get proper representation of spodumene, lepidolite, or other lithium bearing minerals. A lab-based XRD would work somewhat better than a portable one, and they work with say 2 to 3 grams of sample instead of 15 milligrams (thousands of times more sample), but that sample has to have been pulverised to a fine powder. You would likely need to pulverise around 250gr of a well crushed and homogenized sample that would best start out as several kilos. Even after all that, the lab based XRD is still mostly qualitative. Using a visual estimation of the percent of spodumene, lepidolite, or other lithium bearing minerals would be more accurate. Those minerals should be easy enough for an experienced pegmatite explorationist to identify.