Figure 4
Mineralogy of dome lava samples. (a) Pie charts of mineral contents determined by XRD from fresh dome lava (left), moderately altered dome lava (centre) and strongly altered dome lava (right) based on data in Supplementary Table S2. The amount of andesine feldspar (mid- Ca plagioclase) is steadily decreasing while the proportion of alteration minerals (e.g. natro-alunite, gypsum) is seen to increase, documenting progressive replacement of the original rock mass with an acidic sulphurous alteration mineral assemblage. (b) BSE image of a lined vug in the strongly altered dome rock sample. (c–e) Chemical element maps (Na, Fe, and S respectively) showing Fe and S enrichment in the secondary minerals that are lining the vesicle, but low Na content relative to primary minerals. The combined mineralogical evidence from XRD and elemental mapping implies that hydrothermal alteration at Merapi progressively replaces strong silicate minerals (e.g. plagioclase) with sulfate mineralization and additional precipitations in fractures and vesicle spaces. While this will progressively reduce porosity of the altered rock, we show that the combined mineralogical changes will cause an overall decrease in the mechanical strength of the dome rock (see Fig. 5).
