Table 2.

Cadmium contents in rocks.

Rock type	Average Cd content (mg/kg)	Reference
Igneous
Granitic rocks	0.12	a
Mafic rocks	0.11	b
Ultramafic rocks	0.02	b
Obsidian	0.25	a
Basalt	0.22	a
Metamorphic
Gneisses	0.04	c
Schists	0.02	c
Sedimentary
Bituminous shale (black shales)	0.8	a
Red shales	0.03	b
Bentonite	1.4	a
Marlstone	2.6	a
Red clay	0.56	c
Shale and claystone	1	a
Limestone	0.1	d
Sandstone	0.028	b
Carbonate stone	0.012	b
Organic sediment	0.5	d
Oceanic manganese oxides	8	d
Phosphorites	25	d

^a:Thornton (1986)

^b:Gong et al. (1977)

^c:Page et al. (1987)

^d:Hammons et al. (1978).