I am sure that after having dated alunite by some of your friends the friendship will seriously be at risk due to the spoiling and cleaning of the measuring device. Nevertheless it is a good idea (see paper below with lab. handling this method):
DILL, H. G., BOSSE, H.-R., HENNING, K-H. , FRICKE, A. and AHREND , H. (1997) Mineralogical and chemical variations in hypogene and supergene kaolin deposits in a mobile fold belt--The Central Andes of northwestern Peru.- Mineralium Deposita, 32: 149-163.