Are there soil samples with gypsum in our sample set?

One way to see it is overplotting the reference spectrum of gypsum on our soil sample set (in this case treated with Detrend to remove some of the scatter effects). We don´t see it clearly due to the high quantity of samples, but it seems clearly that are samples with gypsum on our sample set. 

Now we could find some ways to check which of our samples have higher correlation with the gypsum reference spectrum or looking other metrics like distances.

Having a reference sample set with gypsum, calcite, kaolinite, iron oxide, etc, is a good way to explore and play with the data, overplotting them over our sample set.

In this case due that the reference gypsum spectrum and the soil sample set are scanned on different NIR instruments, a common range was used (400 to 2500 nm)

matplot(x =colnames(lucas_spain_spcdt), t(lucas_spain_spcdt), 

        type = "l", col = rgb(red = 0.5, green = 0.5, 

        blue = 0.5, alpha = 0.3), ylim = c(-1.5, 4.0), 

        xlab = "wavelength", ylab = "Absorbance")

par(new = TRUE)

matplot(wavelength2, mineralRef2, type = "l", col = "red",                    xlab = "", ylab = "", main = " ", ylim = c(-1.5, 4.0))
abline(v = c(994, 1204, 1445, 1489, 1537, 1748, 

             1944, 2215, 2266), col = "green", 

             lty = "dotted")