# What is a stone in measurement

The density of rock is one of the most important geological and material properties of rock in addition to hardness, mineral composition, structure (visible structure), gloss and elasticity.

### Basics

The density is by the ratio of the mass m of a body to its volume V. Are defined:

(Mass by volume).
Formula symbol is usually ρ (Greek. rho)

In fact, rocks are not ideal homogeneous bodies, but more or less porous, and therefore contain water or absorb it in addition during the measurement. In the first case, a bulk density is determined that deviates from the (theoretical) true density of the rock, and a statement about the water content about it. In the second case, the measurement falsifies the result and that should be avoided. Highly porous rocks and soils are difficult to recover in their original state (such samples are called Handpiece), so the density will be whenever possible in situ measured, and the earth sciences use a number of special methods.

In particular, only mean values ​​can be measured, as the groundwater content of the aquifer can be subject to strong fluctuations in terms of location and time.

### Density measurement and influence of humidity

At Minerals and Rocks In principle, the density can be determined as with other solids by measuring the mass with a fine balance and determining the volume of the handpiece by means of the water displacement (Archimedean principle). However, it is important to proceed quickly so that too much water does not penetrate or flow out into the pores and fine crevices of the rock and falsify the volume measurement.

In borehole geophysics, in addition to other parameters, the density is often determined using the DL density log (also known as the density log or gamma log), which is based on the measurement of scattered, high-energy gamma radiation. Its intensity depends on the density of the surrounding rock (absorption by the electron shells of the atoms involved in the structure of the rock). The non-absorbed part (Compton effect) is reflected back to the measuring device. The method should not be confused with the similarly named Gammaray, which is used to distinguish between sand and clay.

Such rapid procedures would be useful for the geologist's field work, but are not yet in sight or would be too dangerous. So far, geological surveys have mostly had to do without in situ density determinations.

In the geosciences, rock density is given either in g / cm³ (i.e. relative to water) or in kg / m³ (which means a value 1000 times higher). Granite - the most common rock in the earth's crust - has a density of around 2.7 g / cm³ or 2700 kg / m³.

With porous handpieces (especially with sediments and clay rocks) a distinction has to be made - as mentioned above - between the gross density (including the cavities) and the true density (volume without cavities). With many rocks and drill cores there is also between mountain damp and dried State to differ. The dry density (which is comparable to the kiln density in wood) can e.g. B. be determined after pulverization.

### Some density values ​​and possible deviations

material Density in g / cm³ comment
Pumice sand, pumice gravel 0,7–0,8
Sand (pit-damp) 1,2–1,5
Crushed stone / gravel 1,3–1,5 mean grain size
Gravel sand 1,4–1,6
Clays 1,3–2,0 depending on the water content
Slate 1,8–2,8
concrete 1,8–2,4
Tegel (Vienna Basin) 1,9–2,2 depending on depth and age
graphite 2,25
plaster 2,3
Molasse (Switzerland) 2,42–2,67
Sandstone 2,35–2,7 Mean 2.5; depending on the compaction
Quartz porphyry 2,59–2,67
Greywacke 2,65–2,75
Slate 2,77–2,84
Granite ("sour") 2,55–2,81 Default value: 2.67
Granodiorite 2,67–2,78 Average: 2.72
Quartz diorite 2,68–2,96 Average: 2.81
Gneiss 2,66–2,72
Syenite (intermediate) 2,65–2,85 Average: 2.75
Diorite 2,72–2,96 Average: 2.84
Gabbro ("basic") 2,75–3,10 Average: 2.98
limestone 2,45–2,80 Average: ~ 2.7
marble 2,62–2,84
dolomite 2,3–2,95 Average: ~ 2.8
basalt 2,9–3,15
Peridotite (ultrabas.) 3,15–3,28 Mean: 3.23; upper coat
Iron oxide (rust) 5,1
Density leap Moho 2.8 to 3.2 / 3.3 Crust-mantle border

The tabulated values ​​are only to be understood as guidelines. They can vary in nature due to various influences, including among others:

• Porosity and fissures, water content,
• mineral additions and impurities, micro-growth
• Differences in structure, foliation etc.,
• Weathering, density increase due to compaction (rock pressure), etc.

In contrast, the influence of temperature, which is mostly observed in laboratory physics, can be neglected because the natural variance of the density for one and the same rock makes an accuracy better than about ± 0.01 g / cm³ meaningless. The (theoretical) difference between weighing in air and in a vacuum is even less important.