What is Pumice?

Brown pumice stone sourced from volcanic rock.
Brown pumice stone sourced from volcanic rock.

5. Description

The word pumice was one derived from the Latin word pumex, meaning "foam". Pumice is a pyroclastic (meaning it is formed from volcanic matter), igneous rock. The chemical properties of Pumice vary between deposits, but it is primarily composed of amorphous aluminium silicate (silicon dioxide), aluminium oxide, and trace amounts of other oxides, such as ferric and ferrous oxide, sodium oxide, calcium oxide, and magnesium oxide. Its pale, light color, ranging from white to grey to green-brown, indicates that this volcanic rock is high in silica content, and low in iron and magnesium content. Pumice has a vesicular matrix structure with primarily two types of vesicles. Namely, these are tubular micro-vesicles, and spherical or sub-spherical vesicles. With a porosity of 90%, pumice is also the only known rock that floats on water. However, after absorbing sufficient amounts of water, it will in fact eventually sink.

4. Location

Pumice has an extremely wide distribution across the Earth’s surface, and in fact covers all of the continents. It is frequently found in deposits discovered in the deepest portions of the ocean floor, and is a common occurrence in abyssal red clay. The abundant oceanic deposits of pumice can be explained by submarine volcanic eruptions, as well as the distribution of floating pumice by sea winds and currents. Afghanistan, Indonesia, Japan, and (the Eastern half of) Russia are Asian countries with significantly large reserves of pumice. In Europe, Italy, Turkey, Greece, Hungary, and Iceland have large deposits of pumice. The U.S.A., Canada, and Mexico in North America have pumice reserves, as do a number of the Caribbean Islands. A few African (Kenya, Ethiopia, Tanzania and South American countries (Peru, Argentina, Ecuador, Chile also have deposits of pumice to their respective names.

3. Formation

Pumice formation takes place when highly pressurized, super-heated molten rock, with its incorporated water and gases, violently erupts to the surface during volcanic explosions. This rock then undergoes simultaneous processes of rapid cooling and rapid depressurization to form the frothy, vesicle-riven structure of pumice. Depressurization lowers the solubility of gases trapped in the molten rock, triggering the formation of bubbles, and causing the gases trapped inside to "exsolve", or separate from the solid solution. The simultaneous cooling, however, freezes these bubbles into the matrix of the rock, preventing their escape, thus creating the solidified, foamy matrix of pumice.

2. Uses

The friable, separable nature of pumice makes it an ideal choice as an abrasive material. It is used in cosmetics for exfoliate purposes, as well as in polishes, pencil erasers, and for softening and aging textiles like denims. In beauty salons, pumice stones are often used during manicure and pedicure procedures to exfoliate the skin. It also finds use as a lightweight concrete material. The porous, vesicular nature of pumice makes it an ideal material to be used for blast mitigation. Thus, it is often used to manufacture bomb encasements where the porous chambers of this material hold the potential to absorb blast energy, thus limiting the damage arising from accidental blasts. Pumice is also used in large-scale water filtration systems around the world. The horticulture industry makes use of the porous structure of pumice to aerate soils and increase the water retention property of dense soils, as well as utilizing the rock to serve as a growing medium in "soilless" hydroponics crops. This material is also used in the paint, rubber, and plastics industries as a functional filler.

1. Production

Italy is the largest producer of pumice in the world. The other top five leading pumice producers are Spain, Greece, Turkey, and Chile. Pumice is also mined extensively in the United States, with Oregon, Arizona, and California being especially major producers of the rock in the country. Pumice mining has a lower impact on the environment than other rocks and minerals. Extracting pumice usually involves surface mining procedures, where the top soil layer is simply removed to get to the deposits, and the soil is stored for later reclamation of the mining site following the cessation of operations. The pumice layer is then scraped or ripped out, and subjected to crushing for producing "mine grade" pumice. This mine grade produce is then transported to the plant for additional processing. Drying procedures at the plant removes moisture from pumice to favorable levels so that it may undergo further refining. The mined rock is then passed through further processes involving crushers, shakers, and screens to produce "industrial grade" pumice. Further processing still is often needed to meet the standards for other commercial applications, such as in the cosmetics industry.


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