A vug, vugh, or vugg () is a small- to medium-sized cavity inside

rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wale ...

. It may be formed through a variety of processes. Most commonly, cracks and

fissure A fissure is a long, narrow crack opening along the surface of Earth. The term is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in Earth's crust, on ice sheets and glaciers, and on volcanoes. Ground fissure A ...

s opened by

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

activity (

folding Fold, folding or foldable may refer to: Arts, entertainment, and media * ''Fold'' (album), the debut release by Australian rock band Epicure * Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot *Abov ...

and

faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

) are partially filled by

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

, and other secondary

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

s. Open spaces within

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

s formed by an ancient collapse are another important source of vugs. Vugs may also form when mineral

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s or

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s inside a rock matrix are later removed through

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

or dissolution processes, leaving behind irregular voids. The inner surfaces of such vugs are often coated with a crystal druse. Fine crystals are often found in vugs where the open space allows the free development of external crystal form. The term ''vug'' is not applied to veins and fissures that have become completely filled, but may be applied to any small cavities within such veins.

Geode A geode (; ) is a geology, geological secondary formation within sedimentary rock, sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter (which may include crystals) are secluded. The crys ...

s are a vug-formed rock, although that term is usually reserved for more rounded crystal-lined cavities in

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s and ancient

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s.Geology Buzz: Vug
/ref> The word ''vug'' was introduced to the English language by Cornish

miners A miner is a person who extracts ore, coal, chalk, clay, or other minerals from the earth through mining. There are two senses in which the term is used. In its narrowest sense, a miner is someone who works at the rock face (mining), face; cutt ...

, from the days when Cornwall was a major supplier of

tin Tin is a chemical element; it has symbol Sn () and atomic number 50. A silvery-colored metal, tin is soft enough to be cut with little force, and a bar of tin can be bent by hand with little effort. When bent, a bar of tin makes a sound, the ...

.''The history of Cornwall, civil, military, religious, ...'' (Page 56),

Richard Polwhele Richard Polwhele (6 January 1760 – 12 March 1838) was a Cornish clergyman, poet and historian of Cornwall and Devon. Biography Richard Polwhele's ancestors long held the manor of Treworgan, 4 3/4 miles south-east of Truro in Cornwall, w ...

- 1816 The Cornish word was , which meant "cave".

Images

File:Conichalcite-Calcite-65643.jpg, A vug in a

limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...

matrix is the host for a layer of green botryoids of

conichalcite Conichalcite, Ca Cu( As O4)(O H), is a relatively common arsenate mineral related to duftite ( PbCu(AsO4)(OH)). It is green, often botryoidal, and occurs in the oxidation zone of some metal deposits. It occurs with limonite, malachite, beudant ...

and colorless, translucent rhombs of calcite File:Azurite-Malachite-163967.jpg, Vug with rosettes of deep blue

azurite Azurite or '' Azure spar'Krivovichev V. G.'' Mineralogical glossary. Scientific editor A. G. Bulakh. — St.Petersburg: St.Petersburg Univ. Publ. House. 2009. — 556 p. — ISBN 978-5-288-04863-0. ''(in Russian)'' is a soft, deep-blue copp ...

on a field of malachite File:Goosecreekite-Heulandite-Ca-Quartz-indi-41a.jpg, Goosecreekite with

heulandite Heulandite is the name of a series of tectosilicate minerals of the zeolite group. Prior to 1997, heulandite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a seri ...

-Ca on quartz File:Rosasite-rosa08a.jpg, Intergrown botryoids of blue-green

rosasite Rosasite is a carbonate mineral with minor potential for use as a zinc and copper ore. Chemically, it is a copper zinc carbonate hydroxide with a copper to zinc ratio of 3:2, occurring in the secondary oxidation zone of copper-zinc deposits. It wa ...

inside the curve of a limonite vug File:Epistilbite-indi-15a.jpg, Example of open space in a vug allowing the formation of crystallized minerals File:Pyromorphite-t07-109a.jpg, Apple green

pyromorphite Pyromorphite is a mineral species composed of lead chlorophosphate: Pb5( P O4)3 Cl, sometimes occurring in sufficient abundance to be mined as an ore of lead. Crystals are common, and have the form of a hexagonal prism terminated by the basal p ...

cluster on limonite matrix File:Wulfenite-181388.jpg, Vug in a limonitic matrix is the host for bi-pyramidal, brownish-red crystals of

wulfenite Wulfenite is a lead molybdate mineral with the formula Pb Mo O4. It often occurs as thin tabular crystals with a bright orange-red to yellow-orange color, sometimes brown, although the color can be highly variable. In its yellow form it is some ...

File:Groutite-tuc1048a.jpg, Crystallized

groutite Groutite is a manganese oxide mineral with formula Mn3+O(OH). It is a member of the diaspore group and is trimorphous with manganite and feitknechtite. It forms lustrous black crystals in the orthorhombic system. It occurs in weathered banded i ...

scattered about a hollow vug File:Aurichalcite-Calcite-aur06a.jpg, In a vug of limonite are intergrown clusters of rhombohedral calcite with

inclusion Inclusion or Include may refer to: Sociology * Social inclusion, action taken to support people of different backgrounds sharing life together. ** Inclusion (disability rights), promotion of people with disabilities sharing various aspects of lif ...

s of fibrous

aurichalcite Aurichalcite is a carbonate mineral, usually found as a secondary mineral in copper and zinc deposits. Its chemical formula is . The zinc to copper ratio is about 5:4. Copper (Cu2+) gives aurichalcite its green-blue colors. Occurrence Aurichalci ...

File:Epistilbite-Fluorapophyllite-Stellerite-indi-20a.jpg,

Stellerite Stellerite is a rare mineral discovered by and named after Georg Wilhelm Steller, a German explorer and zoologist. The mineral has a general formula of Ca Aluminum">Al2Silicon">Si7Oxygen">O18�7Hydrogen">H2O. Like most rare minerals, there are f ...

, epistilbite, and eluorapophyllite in a basaltic vug File:Sulfur-tmix07-162b.jpg, Nestled in its matrix vug are five golden-yellow crystals of

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

File:Quartz-Chrysocolla-193915.jpg, In an open pocket in a matrix of limonite there is a cluster of deep robin's-egg blue

chrysocolla Chrysocolla ( ) is a hydrous copper phyllosilicate mineral and mineraloid with the formula (''x'' < 1) or . The structure of the mineral has been questioned, as a 2006 spectrographic study suggest ma ...

with a covering of

drusy In geology and mineralogy, druse is a crystal habit represented by the coating of fine crystals on a rock fracture surface, or vein or within a vug or geode. See also * Miarolitic cavity Miarolitic cavities (or miarolitic texture) are typicall ...

quartz

References

External links

*{{wiktionary-inline Petrology Geology terminology Cornish words and phrases