Name/Title
PumiceEntry/Object ID
LN2012.1.7Description
Assemblage Zone: pegmatite dikes
Chemical Composition: Si
Crystal System: Orthorhombic System
Description: Common Name: Pumice
Group Name: Native Elements
Chemistry: Silicon
Location: unknown
Description: Roughly rounded aggregate of porous-nature volcanic debris with very light weight.
PHYSICAL CHARACTERISTICS:
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering the solubility of gases (including water and CO2) dissolved in the lava, causing the gases to rapidly exsolve (like the bubbles of CO2 that appear when a carbonated drink is opened). The simultaneous cooling and depressurization freezes the bubbles in the matrix.
Pumice is composed of highly microvesicular glass pyroclastic with very thin, translucent bubble walls of extrusive igneous rock. It forms when volcanic gases exsolving from viscous magma nucleate bubbles which cannot readily decouple from the viscous magma prior to chilling to glass. Pumice has an average porosity of 90%, and initially floats on water.
Scoria differs from pumice in being denser. With larger vesicles and thicker vesicle walls, it sinks rapidly. The difference is the result of the lower viscosity of the magma that forms scoria. When larger amounts of gas are present, the result is a finer-grained variety of pumice known as pumicite. Pumice is considered a glass because it has no crystal structure. Pumice varies in density according to the thickness of the solid material between the bubbles; many samples float in water.
There are two main forms of vesicles. Most pumice contains tubular microvesicles that can impart a silky or fibrous fabric. The elongation of the microvesicles occurs due to ductile elongation in the volcanic conduit or, in the case of pumiceous lavas, during flow. The other form of vesicles are subspherical to spherical and result from high vapor pressure during eruption.
Fracture: Conchoidal
Hardness: 2 Gypsum
Luster: Resinous
Occurrence: Pumice is a common product of explosive eruptions (plinian and ignimbrite-forming) and commonly forms zones in upper parts of silicic lavas.
Rock Type: Igneous
Specific Gravity: 2-2.1
Streak: Whie
Variety: Pumice is commonly, but not exclusively of silicic or felsic to intermediate in composition (e.g., rhyolitic, dacitic, andesite, pantellerite, phonolite, trachyte), but basaltic and other compositions are known. Pumice is commonly pale in color, ranging from white, cream, blue or grey, to green-brown or black.Collection
Carol Swisher Mineral-Uses CollectionDimensions
Width
1-5/16 inDepth
1-3/16 inLength
2 inDimension Notes
Dimension taken at widest pointsLocation
Location
Container
front rowDrawer
centerShelf
topCabinet
Mineral UsesWall
west wallBuilding
Museum Entrance HallwayCategory
Permanent
