Table 1.

Physicochemical data of the five substrates

Substrate		Volcanic Glass	Volcanic Asha	Carbonate reef sandb	Calciteb	Quartzc
Chemical composition (wt. %)	SiO2	58.2	58.5	0.2	0.9	>99.5
	Al2O3	16.8	16	0.1	0.3	n.s.
	Fe2O3	7.9	7.9	0	0	n.s.
	MnO	0.2	0.2	0	0	n.s.
	MgO	3.4	3.6	1.3	0.4	n.s.
	CaO	7.4	7.2	52.2	54.4	n.s.
	Na2O	3.4	3	0.3	0.1	n.s.
	K2O	1.4	1.4	0	0	n.s.
	TiO2	0.8	0.8	0	0	n.s.
	P2O5	0.2	0.2	0.1	0	n.s.
	Total	99.6	100.1	99.0	100.0	>99.5
	L.O.I	−0.17	0.02	44.49	44.43	<0.5
Physical properties	SSA (mb/g)	0.02	0.1	0.49	0.22	0.11
	Median diameter (μm)	234	221	224	230	263
	Interquartile diameter (μm)	180–260	190–260	190–270	190–260	220–310
	Colour	Dark	Dark	White	White	White
	Morphology	Angular	Subangular	Aggregate	Aggregate	Subangular
	Surface appearance	Smooth	Rough	Rough	Rough	Rough

^aComprised of glass, plagioclase, pyroxenes, Fe–Ti oxides (Miwa et al., 2013).

^bComprised of CaCO₃.

^cCompositional data of pure quartz from Sigma–Aldrich product specifications (n.s. not stated).