Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 129

Density (g/cm 3 )

0.00 1.00 2.00 3.00 4.00 5.00

2.67

Range of rock-forming
minerals

Serpentinised

Dunite

(Ultramafic)

Serpentinised

Peridotite

Gabbro

(Mafic) Dolerite

Basalt

Vesicular

Andesite

Trachyte

(Intermediate)

Diorite

Igneous
rocks

Syenite

Rhyolite

(Felsic)

Granite

Pyroclastics

Kimberlite

Eclogite

Serpentinised
Felsic

Mafic

Granulite

Amphibolite

Felsic Mafic Gneiss

Schist

Metamorphic
rocks

Slate

Phyllite

(Meta-sedimentary)

Marble

Quartzite

Iron formation

Dolomite

Limestone

Shales

Sedimentary
rocks

Sandstone

Evaporite

Lignite Anthracite Coal

Laterite

Saprolite
Sand - wet

Superficial
deposits

Sand - dry
Clay

Bauxite Metal oxides & sulphides

Mineral
deposits

Metallic mineralisation

Disseminated Massive

0.00 1.00 2.00 3.00 4.00 5.00

Density (g/cm 3 )

Figure 3.30 Density ranges of various rock types. Based mainly on diagrams and data in Emerson ( 1990 ) , Schon ( 1996 ) and Wohlenberg
( 1982 ).

structure of diamond results in a density of 3.52 g/cm 3
whereas the more loosely packed atoms in graphite result
in a density of 2

3.8.2 Densities of porous rocks

The pore contents of porous rocks are an important con-
trol on the bulk density of the rock because the density of
the pore contents is much lower than that of any of the
matrix minerals ( Fig. 3.29 ) . Consequently, a change in
porosity exerts much more in

2.3 g/cm 3 . The density difference of other
relevant dimorphic substances is usually much less, but still
significant. For example, the two forms of CaCO 3 (arago-
nite and calcite) and FeS 2 (pyrite and marcasite) both have
a density difference of 0.22 g/cm 3 .
Figure 3.29 demonstrates that the densities of the
main rock-forming minerals lie mostly in the range 2.5 to
3.5 g/cm 3 . Felsic minerals, containing lighter cations such
as Na and K, are noticeably less dense than ma

-

uence than a change in the
mineralogy of the matrix. The relationship between bulk
density ( ρ bulk ), and the (average) density of the matrix

( ρ matrix ) and the density of the pore contents ( ρ pore contents )

is given by:

c minerals
whose cations are mainly heavier elements such as Fe, Mg
and Ca. Several common metamorphic minerals, e.g.
garnet and kyanite, have unusually high densities.

ρ bulk ¼ ρ matrix ð 1 ϕÞ + ρ pore contents ϕ ð 3 : 26 Þ

where ϕ is the fractional porosity (0 to 1).