Variolite
Encyclopedia
Variolites are a group of dark green basic igneous rock
s that, especially on weathered
surfaces, exhibit pale colored spots, or spherules, that give them a pockmarked appearance. In some conditions these spots weather out prominently; they are grey, pale green, violet or yellowish, while the matrix of the rock is usually dark green.
The variolites are related most closely to the basalt
s or diabase
s. They are nearly always much decomposed, and, since they are also fine-grained rocks, their original composition may be much obscured by secondary changes. The variolitic spots are rounded in outline and are often about a quarter-inch in diameter, but may exceed this size. They have a radiate structure and are sometimes, though not generally, zoned with concentric circles of different appearance and composition. Many authors have compared them with the spherules of the acid
rocks (obsidian
s and rhyolite
s), and undoubtedly some kinds of variolite are merely glassy spherulitic varieties of basalt. The tachylyte
selvages of the dolerite dike
s of the west of Scotland
, for example, often contain large brown spherules that are easily visible in hand specimens. These spherulites consist of very thin divergent fibers, and their nature is often difficult to determine on account of the indefiniteness of the optical characteristics of minerals in this state. It seems probable, however, that they are mostly feldspar
embedded in dark brown glass. Small phenocryst
s or skeleton crystals of olivine
, augite
and plagioclase
feldspar may occur in these tachylytes.
growths, branching and curved, which spread through the glassy base and do not interfere with the feldspar spherules. Occasionally olivine-dolerites have a coarsely spherulic structure with long rods of plagioclase feldspar converging to a point; one example of these rocks from Skye contains variolites over three inches in diameter.
Another group of variolites includes the most famous rock of this type, which comes from the Durance, in France
. Pebbles of this were well known to collectors for a long time before they were traced to their source at Mont Genvre. They were proved to belong to a diabasic rock that shows well-marked pillow-structure or spheroidal jointing. Each pillow has a marginal portion which is variolitic, but towards the center of the block-shaped masses the structure becomes coarse and groups of radiate feldspars make their appearance. It is doubtful whether the variolite is an intrusive rock or a lava flow. Many of these pillow lava
s (or spilities) occur in the Devonian
rocks of Germany
, and they often have variolitic facies which seem to belong to the same group as the rock of the Durance. Their spherulites are very often oligoclase feldspar or decomposition products after a felspathic mineral. In other cases they consist of chlorite
or pale green amphibole
, both of which may be secondary after pyroxene
. The ground mass is very fine grained and is filled with chlorite, epidote
, leucoxene
, and other secondary minerals. There is much reason to believe that it was originally in large measure vitreous, but has suffered devitrifaction. Sometimes little steam cavities occur and may serve as a nucleus from which the variolite has grown. The radiate structure of the varioles is often nearly obliterated in these much-decomposed rocks, in fact it may never have been very perfect. Variolites are found also in several parts of the Swiss Alps
at Jatluga on Lake Onega, in Anglesey, the Lleyn district and Fishguard in Wales
, in Cornwall
, and in more than one place in Ireland
. They are very common on beaches of the Strait of Juan de Fuca
, along the northern edge of the Olympic Peninsula
of Washington State, USA. This would appear to indicate an origin within the extensive Eocene
basalts of the Crescent Formation of the northern Olympic Mountains
.
Finally, there is a group of spotted rocks formerly known to French
petrographers as the variolites du Drac from the locality in which they are found, but they have been proved to be merely vesicular, rotten diabases, with steam cavities filled with white calcite
and other secondary minerals.
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s that, especially on weathered
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
surfaces, exhibit pale colored spots, or spherules, that give them a pockmarked appearance. In some conditions these spots weather out prominently; they are grey, pale green, violet or yellowish, while the matrix of the rock is usually dark green.
The variolites are related most closely to the basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
s or diabase
Diabase
Diabase or dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material...
s. They are nearly always much decomposed, and, since they are also fine-grained rocks, their original composition may be much obscured by secondary changes. The variolitic spots are rounded in outline and are often about a quarter-inch in diameter, but may exceed this size. They have a radiate structure and are sometimes, though not generally, zoned with concentric circles of different appearance and composition. Many authors have compared them with the spherules of the acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
rocks (obsidian
Obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
s and rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
s), and undoubtedly some kinds of variolite are merely glassy spherulitic varieties of basalt. The tachylyte
Tachylyte
Tachylite is a vitreous form of basaltic volcanic glass. This "glass" is formed naturally by the rapid cooling of molten basalt. It is a basic type of igneous rock that is decomposable by acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous luster...
selvages of the dolerite dike
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
s of the west of Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, for example, often contain large brown spherules that are easily visible in hand specimens. These spherulites consist of very thin divergent fibers, and their nature is often difficult to determine on account of the indefiniteness of the optical characteristics of minerals in this state. It seems probable, however, that they are mostly feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
embedded in dark brown glass. Small phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...
s or skeleton crystals of olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
, augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
and plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldspar may occur in these tachylytes.
Varieties and occurrence
Other variolites are glassy or partly crystalline facies of olivine-free dolerites, occurring as thin dikes or intrusions or at the margins of dolerite masses. In these the feldspars are well crystallized as thin rods with square or forked ends radiating outwards from a center. They are commonly oligoclase, and sometimes assume branching or feathery forms. Some authors would call these sphero-crystals rather than spherules; they are an intermediate stage between the latter and the stellate groupings of feldspar that occurs frequently in igneous rocks. In the same rocks augite spherules occur also, but this mineral forms plumoseFeather
Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...
growths, branching and curved, which spread through the glassy base and do not interfere with the feldspar spherules. Occasionally olivine-dolerites have a coarsely spherulic structure with long rods of plagioclase feldspar converging to a point; one example of these rocks from Skye contains variolites over three inches in diameter.
Another group of variolites includes the most famous rock of this type, which comes from the Durance, in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
. Pebbles of this were well known to collectors for a long time before they were traced to their source at Mont Genvre. They were proved to belong to a diabasic rock that shows well-marked pillow-structure or spheroidal jointing. Each pillow has a marginal portion which is variolitic, but towards the center of the block-shaped masses the structure becomes coarse and groups of radiate feldspars make their appearance. It is doubtful whether the variolite is an intrusive rock or a lava flow. Many of these pillow lava
Pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...
s (or spilities) occur in the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
rocks of Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, and they often have variolitic facies which seem to belong to the same group as the rock of the Durance. Their spherulites are very often oligoclase feldspar or decomposition products after a felspathic mineral. In other cases they consist of chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
or pale green amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
, both of which may be secondary after pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
. The ground mass is very fine grained and is filled with chlorite, epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...
, leucoxene
Leucoxene
Leucoxene is a fine granular alteration product of titanium minerals. It varies in color from yellow to brown.It consists mainly of rutile or anatase...
, and other secondary minerals. There is much reason to believe that it was originally in large measure vitreous, but has suffered devitrifaction. Sometimes little steam cavities occur and may serve as a nucleus from which the variolite has grown. The radiate structure of the varioles is often nearly obliterated in these much-decomposed rocks, in fact it may never have been very perfect. Variolites are found also in several parts of the Swiss Alps
Swiss Alps
The Swiss Alps are the portion of the Alps mountain range that lies within Switzerland. Because of their central position within the entire Alpine range, they are also known as the Central Alps....
at Jatluga on Lake Onega, in Anglesey, the Lleyn district and Fishguard in Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
, in Cornwall
Cornwall
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
, and in more than one place in Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
. They are very common on beaches of the Strait of Juan de Fuca
Strait of Juan de Fuca
The Strait of Juan de Fuca is a large body of water about long that is the Salish Sea outlet to the Pacific Ocean...
, along the northern edge of the Olympic Peninsula
Olympic Peninsula
The Olympic Peninsula is the large arm of land in western Washington state of the USA, that lies across Puget Sound from Seattle. It is bounded on the west by the Pacific Ocean, the north by the Strait of Juan de Fuca, and the east by Puget Sound. Cape Alava, the westernmost point in the contiguous...
of Washington State, USA. This would appear to indicate an origin within the extensive Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
basalts of the Crescent Formation of the northern Olympic Mountains
Olympic Mountains
The Olympic Mountains is a mountain range on the Olympic Peninsula of western Washington in the United States. The mountains, part of the Pacific Coast Ranges, are not especially high - Mount Olympus is the highest at - but the western slopes of the Olympics rise directly out of the Pacific...
.
Finally, there is a group of spotted rocks formerly known to French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
petrographers as the variolites du Drac from the locality in which they are found, but they have been proved to be merely vesicular, rotten diabases, with steam cavities filled with white calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
and other secondary minerals.