Rock Density

Rock density definition

Rock density (ρ) is a measure of mass of the rock contained in a given unit volume (density = mass/volume). The unit of expression of rock density is in g/cm3. Rock density is also identically with bulk density. Since most of rocks are porous media, bulk density of porous rocks depends on not only the density of the solid matrix material. Porous rocks also depends on the density of pore fluids as well as saturation, as shown in the following equation:

 

ρb = (1-Φ)ρma + Φ ρf

 

Where ρb is the rock or bulk density; ρma is the matrix or solid density, ρf  is the fluid density, Φ is the porosity.

For most rocks and soils, the particle density is about 2.65 g/cm3. The number roughly the density of quartz and clay minerals. The density of water is 1 g/cm3. Thus, the bulk density of the rock can be calculated when the formation porosity is known. The rock density changes with depth below the sea floor. The calculation of rock density in the table 1 is coming from the core porosity data (Ostermeier et al. 2001). The data gained by using equation above. Given this points it shows that the density of formations increases as the burial depth increases.

List of rock density for common minerals

Tables 1.1 and 1.2 list matrix density for common minerals (Schön 1996) and bulk density for various rock types.

 

Minerals Density
Quartz   2.65
Calcite   2.71
Dolomite   2.87
Montmorillonite   2.06
Illite   2.64
Kaolinite   2.59
Chlorite   2.88
K-Feldspar   2.56
Biotite   2.90
Halite   2.17
Anhydrite   2.96
Sylvite   1.99
Plagioclase (Na)   2.62
Plagioclase (Ca)   2.76
Barite   4.48

Table 1.1. Matrix density for common minerals

 

Rock type Density Porosity
Granite 2.5 – 2.8 0.5 – 1.5
Dolerite 3.0 – 3.1 0.1 – 0.5
Limestone 2.5 – 2.8 5 – 20
Dolomite 2.5 – 2.6 1 – 5
Quartzite 2.65 0.1 – 0.5
Sandstone 2.0 – 2.6 5 – 25
Shale 2.0 – 2.7 10 – 30
Coal – Anthracite 1.3 – 1.6
Coal – Bituminous 1.1 – 1.4
Sediments 1.7 – 2.3
Metamorphic rocks 2.6 – 3.0

Table 1.2. Bulk density and porosity for various rock types

Rock density measurement

We can do the measurement of rock density in 2 different ways. First in the lab using core samples or second by in-situ test. The sample either for lab measurement or in-situ, similarly can be collected since the very first time of exploration. For underground rocks, it is more convenient to determine rock bulk density by geophysical methods. Gardner et al. (1974) presented the following relationship between bulk density and seismic wave velocity in sedimentary rocks:

ρb = 1.741 Vfp

where ρb is the bulk density, in g/cm3; f = 0.25vp is the velocity of the compressional wave, in km/s.

In summary, this equation represents a fair average for a large number of laboratory. This is also represents field observations of different brine saturated rock types.

rock density is 2.71 for calcite
Figure 1. Calcite, one kind of rock, with density 2.71

Factor control the rock density

Rock density is correspondingly controlled by densities and volumetric fractions of components which the rock is composed. Therefore, density differences between minerals, fluids, and gases cause a strong correlation between rock density and porosity. For underground rocks, as depth increases the rock compaction increases, causing porosity reduction. This induces an increase in rock density as depth increases, as shown in Fig. 2.

density variations in the Gulf of MExico
Figure 2. Density variations with depth below the sea floor in the Gulf of Mexico

It needs to mention that specific gravity ( g) is very useful for calculating rock overburden stress. Specific density is dimensionless, equal to the density of the material divided by the density of water.

Rock density is very sensitive to the minerals that compose a particular rock type. In the same way, sedimentary rocks (and granite), which are rich in quartz and feldspar, tend to be less dense than volcanic rocks. And if you know your igneous petrology, you will see that the more mafic a rock the greater its density.

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