# Properties - Density

(grams per cubic centimeter)

Density data for a suite of samples from a study area near Silverton Colorado.

Data from McCafferty, A.E., Horton, R.J., Stanton, M.R., McDougal, R.R., and Fey, D.L., 2011, Geophysical, geochemical, mineralogical, and environmental data for rock samples collected in a mineralized volcanic environment, upper Animas River watershed, Colorado: U.S. Geological Survey Data Series 595, 13 p.

Sample density is a function of composition, porosity and saturation. Density is calculated from mass and volume measurements. Mass is determined by carefully weighing the sample on an analytical balance. Volume is determined a number of different ways. For uniform shaped samples, such as spheres, cubes, rectangular prisms, and cylinders, dimensions can be measured with a caliper and the volume determined using geometric volume formulas. For irregular shaped samples, volume is usually determined using the buoyancy method, which is based on Archimedes' principle. The lab is also equipped with a stereo pycnometer used to determine the volume of fine-grained sediment, powders and samples having extremely small pore spaces. The stereo pycnometer determines volume based on the displacement of gas.

## Porosity

Porosity is the volume of air or liquid space within a solid sample matrix. Porosity can be due to pore space between mineral grains, gas bubbles, vugs, and fractures. Porosity is an important factor controlling a sample's water content; and thus the sample's density and electrical properties. Water accessible porosity is determined by weighing the sample dry and then saturated. To completely saturate a sample, it is submerged in deionized water and placed under vacuum pressure to evacuate air trapped in pore spaces. The difference in weight, saturated minus dry, equals the amount of water contained in the sample's pore space. The sample's porosity is the pore water weight divided by the sample's bulk volume.