Is pumice stone an igneous stone


Author: Torsten Purle ( | Last update: 10.05.2021

Pumice - properties, formation and use

English: pumice | French: ponce

Pumice Stone - A volcanic glass

The word pumice originally comes from Latin. Out Pumex developed from the old high German term bumiz today's word pumice. The name Pumex is due to the look and origin with Foam stone translated.

Properties of pumice stone

Definition: Pumice stones are counted among the igneous rocks of extrusive origin, which are characterized by a high number of pores and the associated low density.

The color of pumice varies from white, light gray and yellowish to reddish or dark Colours. Over time, the rock surface weathers, so that older pumice stones are light brown in color.

The chemical composition of pumice can be both acidic and basic. It is not uncommon for pumice to contain microcrystalline silicate minerals such as feldspars and iron-magnesium minerals.

The texture of pumice stone is very porous, the individual pores do not normally adjoin each other. The Pore ​​volume of pumice is up to 80%. The grain size is fine-grained, the Mohs hardness is 5.

Due to their formation, pumice stones are amorphous, i.e. glass-like and without any noticeable crystallization of the mineral components.

The low density of 0.24 to about 0.3 g / cm3 causes pumice to float on water and can therefore be transported over long distances (comparison of the density of water at 20 ° C: approx. 1 g / cm3).

Origin and distribution of pumice stone

The foamy rock glass pumice is created by the rapid cooling of molten rock on the surface of the earth. As a result of the sudden eruption of volcanoes, the emerging lava is very rich in gas. Water vapor, carbon dioxide and other volatile substances in the lava cause the molten rock to expand and foam.

The gases contained escape and remain after cooling as cavities in the solidified rock. The newly formed rock is still deformable at this point, so the possible shape of the bubble cavities provides information about the direction of flow of the lava flow. Due to the rapid cooling, a shiny, glass-like coating forms on the surface of pumice stones, because the rock-forming minerals were unable to form crystals in this short time and instead solidified into amorphous glass.

The occurrence of pumice extends worldwide to regions whose formation is connected with volcanism - both in the distant past as well as currently taking place.

Notable localities are for example in Iceland; Auvergne / France; Neuwied basin and at the Laacher See / Germany, Lipari Islands / Italy, Australia, Kos / Greece and Turkey.

Pumice and tuff

Pumice and tuff are two rocks that are characterized by their pore-rich, fine-grained character.

In contrast to pumice, inclusions of other minerals or rock fragments are clearly visible in tuffs, while pumice is optically more homogeneous. The appearance of tuff can be compared to that of concrete. The pore cavities are also more pronounced in pumice stones than in comparison with tuff.

Use and meaning of pumice

Pumice is a rock that is used for many purposes. Finely ground pumice is added to abrasives, but is also part of lightweight concrete / pumice concrete. The reason why pumice is used as an aggregate in concrete is due to the thermal insulation of the rock.

Pumice has also made a name for itself as a substrate in the garden area. Due to the high water storage capacity and the supporting effect on rooting and aeration of the soil, pumice is used to improve the quality of soils. In combination with other rocks of volcanic origin, pumice is used to produce purely mineral cactus soil, which can be bought ready-to-use in stores.

Also interesting:
⇒ volcanic glass
⇒ Pumice stone in foot care
⇒ Plutonism and Neptunism

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⇒ Schumann, W. (1991): Minerals rocks - characteristics, occurrence and use. FSVO nature guide. BLV Verlagsgesellschaft mbH Munich
⇒ Maresch, W., Medenbach, O .; Trochim, H.-D. (1987): The colored natural guide rocks. Mosaik Verlag GmbH Munich *
⇒ Murawski, H. (1992): Geological Dictionary. Ferdinand Enke Verlag Stuttgart
⇒ Schumann, W. (1994): Collecting stones and minerals; find, prepare, determine. BLV Verlag Munich