Trying to find high content gypsum samples

How can I find the high content gypsum samples in the LUCAS spanish database?. We have the spectrum of pure gypsum, so we can try to correlate (or meassure a distance) every spectrum of the database with the pure gypsum spectra and see if we find some threshold or gap that splits these high content gypsum samples. 

I try in this case the correlation, because it is the easiest algorithm, and  I don´t have a population of gypsum soil spectra (just one reference pure spectrum).

Let´s run the correlation:

corSG1 <- as.numeric(cor(t(lucas_spain$spcnir_SG[,21:546]),
                     mineralRef_nir_2nm_SG1[3, 21:546]))

Now we have a correlation value for every spectrum, so we can plot the histogram:

hist(corSG1, breaks = 1000)

We can see the distribution and the tail on the right with some grouping over 0.85, so this can be a way to look at the samples over 0.8 correlation and make a fine tune selection.