Leucite
Encyclopedia
Leucite is a rock
-forming mineral
composed of potassium
and aluminium
tectosilicate
K[AlSi2O6]. Crystal
s have the form of cubic icositetrahedra but, as first observed by Sir David Brewster
in 1821, they are not optically isotropic, and are therefore pseudo-cubic. Goniometric
measurements made by Gerhard vom Rath
in 1873 led him to refer the crystals to the tetragonal system. Optical investigations have since proved the crystals to be still more complex in character, and to consist of several orthorhombic or monoclinic individuals, which are optically biaxial and repeatedly twinned
, giving rise to twin-lamellae and to striations on the faces. When the crystals are raised to a temperature of about 500 °C they become optically isotropic and the twin-lamellae and striations disappear, although they reappear when the crystals are cooled again. This pseudo-cubic character of leucite is very similar to that of the mineral boracite
.
The crystals are white or ash-grey in colour, hence the name suggested by A. G. Werner in 1701, from 'λευκος', '(matt) white'. They are transparent and glassy when fresh, albeit with a noticeably subdued 'subvitreous' lustre due to the low refractive index, but readily alter to become waxy/greasy and then dull and opaque; they are brittle and break with a conchoidal fracture. The Mohs hardness is 5.5, and the specific gravity
2.47. Inclusions of other minerals, arranged in concentric zones, are frequently present in the crystals. On account of the color and form of the crystals the mineral was early known as white garnet. French authors in older literature may employ René Just Haüy
's name amphigène, but 'leucite' is the only name for this mineral species that is recognised as official by the International Mineralogical Association.
being entirely without them. However, they are of wide distribution, occurring in every quarter of the globe. Taken collectively, they exhibit a considerable variety of types and are of great interest petrographically. For the presence of this mineral it is necessary that the silica percentage of the rock should be low, since leucite is incompatible with free quartz
and reacts with it to form potassium feldspar
. Because it weathers rapidly, leucite is most common in lava
s of recent and Tertiary
age, which have a fair amount of potassium, or at any rate have potassium equal to or greater than sodium; if sodium is abundant nepheline
occurs rather than leucite.
In pre-Tertiary rocks leucite readily decomposes and changes to zeolite
s, analcite
and other secondary minerals. Leucite also is rare in plutonic rocks and dike
rocks, but leucite syenite
and leucite tinguaite bear witness to the possibility that it may occur in this manner. The rounded shape of its crystals, their white or grey color, and absence of planar cleavage make the presence of leucite easily determinable in many of these rocks by inspection, especially when the crystals are large.
"Pseudoleucites" are rounded areas consisting of feldspar
, nepheline
, analcite
, &c., which have the shape, composition and sometimes even the outward crystalline shape of leucite; they are probably pseudomorph
s or paramorphs, which have developed from leucite because this mineral is not stable at ordinary temperatures and can be expected under favorable conditions to undergo spontaneous change into an aggregate of other minerals. Leucite is very often accompanied by nepheline, sodalite
or nosean
; other minerals which make their appearance with some frequency are melanite, garnet
and melilite
.
The plutonic leucite-bearing rocks are leucite syenite and missourite. Of these the former consists of orthoclase
, nepheline, sodalite, diopside
and aegirine
, biotite
and sphene. Two occurrences are known, one in Arkansas
, the other in Sutherland, Scotland
. The Scottish rock has been called borolanite
. Both examples show large rounded spots in the hand specimens; they are pseudoleucites, and under the microscope prove to consist of orthoclase, nepheline, sodalite and decomposition products. These have a radiate arrangement externally, but are of irregular structure at their centres; it is interesting to note that in both rocks melanite is an important accessory. The missourites are more mafic
and consist of leucite, olivine
, augite
and biotite
; the leucite is partly fresh partly altered to analcite
, and the rock has a spotted character recalling that of the leucite-syenites. It has been found only in the Highwood Mountains
of Montana
.
The leucite-hearing dike-rocks are members of the tinguaite and monchiquite groups. The leucite tinguaites are usually pale grey or greenish in color and consist principally of nepheline, alkali feldspar and aegirine. The latter forms bright green moss-like patches and growths of indefinite shape, or in other cases scattered acicular prisms, among the feldspars and nephelines of the ground mass. Where leucite occurs, it is always euhedral in small, equant, many-sided crystals in the ground mass, or in larger masses which have the same characters as the pseudoleucites. Biotite occurs in some of these rocks, and melanite also is present. Nepheline decreases in amount as leucite increases since the abundances of the two reflect the Na:K ratio of the rock. Rocks of this group are known from Rio de Janeiro
, Arkansas, Kola
(in Finland
), Montana
and a few other places., In Greenland
there are leucite tinguaites with much arfvedsonite
, (hornblende
) and eudialyte. Wherever they occur they accompany leucite- and nepheline syenites. Leucite monchiquites are fine-grained dark rocks consisting of olivine, titaniferous augite and iron oxides, with a glassy ground mass in which small rounded crystals of leucite are scattered. They have been described from Czechoslovakia
.
By far the greater number of the rocks which contain leucite are lava
s of Tertiary
or recent geological age. Although these never contain quartz, but feldspar
is usually present, though there are certain groups of leucite lavas which are non-feldspathic. Many of them also contain nepheline, sodalite, hauyne and nosean; the much rarer mineral melilite
appears also in some examples. The commonest ferromagnesian mineral is augite
(sometimes rich in sodium
), with olivine in the more basic varieties. Hornblende
and biotite
occur also, but are less common. Melanite is found in some of the lavas, as in the leucite syenites.
The rocks in which orthoclase
(or sanidine) is present in considerable amount are leucite-trachytes, leucite-phonolites and leucitophvres. Of these groups the two former, which are not sharply distinguished from one another by most authors, are common in the neighborhood of Rome
. They are of trachytic appearance, containing phenocysts of sanidine, leucite, augite and biotite. Sodalite or hauyne may also be present, but nepheline is typically absent. Rocks of this class occur also in the tuff
s of the Phlegraean Fields, near Naples
. The leucitophyres are rare rocks which have been described from various parts of the volcanic district of the Rhine (Olbrck. Laacher See, etc.) and from Monte Vulture in Italy. They are rich in leucite, but contain also some sanidine and often much nepheline with hauyne or nosean. Their pyroxene is principally aegirine or aegirine-augite; some of them are rich in melanite. Microscopic sections of some of these rocks are of great interest on account of their beauty and the variety of feldspathoid
minerals which they contain. In Brazil
leucitophyres have been found which belong to the Carboniferous
period.
Those leucite rocks which contain abundant essential plagioclase feldspar are known as leucite tephrites and leucite basanites. The former consist mainly of plagioclase, leucite and augite, while the latter contain olivine in addition. The leucite is often present in two sets of crystals, both porphyritic and as an ingredient of the ground mass. It is always idiomorphic with rounded outlines. The feldspar ranges from bytownite to oligoclase, being usually a variety of labradorite
; orthoclase is scarce. The augite varies a good deal in chemnistry and optical character, being green, brown or violet (suggesting high Na and Ti content), but it is rarely high enough in Na and Fe to qualify as aegirine-augite or aegirine
. Among the accessory minerals biotite, brown hornblende, hauyne, iron oxides and apatite
are the commonest; melanite and nepheline may also occur. The ground mass of these rocks is only occasionally rich in glass. The leucite-tephrites and leucite-basanites of Vesuvius and Somma are familiar examples of this class of rocks. They are black or ashy-grey in color, often vesicular, and may contain many large grey phenocysts of leucite. Their black augite and yellow green olivine are also easily observed in hand specimens. From Volcan Ello, Sardinia and Roccamonfina similar rocks are obtained; they occur also in Bohemia
, in Java, Celebes
, Kilimanjaro
(Africa) and near Trebizond in Asia Minor
.
Leucite lavas from which feldspar is absent are divided into the leucitites and leucite basalt
s. The latter contain olivine, the former do not. Pyroxene is the usual ferromagnesian mineral, and resembles that of the tephrites and basanites. Sanidine, melanite, hauyne and perovskite
are frequent accessory minerals in these rocks, and many of them contain melilite in some quantity, The well-known leucitite of the Capo di Bove, near, Rome, is rich in this mineral, which forms irregular plates, yellow in the hand specimen, enclosing many small rounded crystals of leucite. Bracciano and Roccamonfina are other Italian localities for leucitite, and in Java, Montana, Celebes and New South Wales
similar rocks occur, The leucite basalts belong to more basic types and are rich in olivine and augite. They occur in great numbers in the Rhenish volcanic district (Eifel, Laacher See) and in Bohemia, and accompany tephrites or leucitites in Java, Montana, Celebes and Sardinia. The peperino of the neighborhood of Rome is a leucitite tuff
.
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
-forming mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
composed of potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
and aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
tectosilicate
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
K[AlSi2O6]. Crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s have the form of cubic icositetrahedra but, as first observed by Sir David Brewster
David Brewster
Sir David Brewster KH PRSE FRS FSA FSSA MICE was a Scottish physicist, mathematician, astronomer, inventor, writer and university principal.-Early life:...
in 1821, they are not optically isotropic, and are therefore pseudo-cubic. Goniometric
Goniometer
A goniometer is an instrument that either measures an angle or allows an object to be rotated to a precise angular position. The term goniometry is derived from two Greek words, gōnia, meaning angle, and metron, meaning measure....
measurements made by Gerhard vom Rath
Gerhard vom Rath
Gerhard vom Rath , was a German mineralogist, born at Duisburg in Prussia.He was educated at Cologne, at Bonn University, and finally at Berlin, where he graduated Ph.D. in 1853. In 1856 he became assistant to Johann Jakob Nöggerath in the mineralogical museum at Bonn, and succeeded to the...
in 1873 led him to refer the crystals to the tetragonal system. Optical investigations have since proved the crystals to be still more complex in character, and to consist of several orthorhombic or monoclinic individuals, which are optically biaxial and repeatedly twinned
Crystal twinning
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals....
, giving rise to twin-lamellae and to striations on the faces. When the crystals are raised to a temperature of about 500 °C they become optically isotropic and the twin-lamellae and striations disappear, although they reappear when the crystals are cooled again. This pseudo-cubic character of leucite is very similar to that of the mineral boracite
Boracite
Boracite is a magnesium borate mineral with formula: Mg3B7O13Cl. It occurs as blue green, colorless, gray, yellow to white crystals in the orthorhombic - pyramidal crystal system. Boracite also shows pseudo-isometric cubical and octahedral forms. These are thought to be the result of transition...
.
The crystals are white or ash-grey in colour, hence the name suggested by A. G. Werner in 1701, from 'λευκος', '(matt) white'. They are transparent and glassy when fresh, albeit with a noticeably subdued 'subvitreous' lustre due to the low refractive index, but readily alter to become waxy/greasy and then dull and opaque; they are brittle and break with a conchoidal fracture. The Mohs hardness is 5.5, and the specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
2.47. Inclusions of other minerals, arranged in concentric zones, are frequently present in the crystals. On account of the color and form of the crystals the mineral was early known as white garnet. French authors in older literature may employ René Just Haüy
René Just Haüy
René Just Haüy – 3 June 1822 in Paris) was a French mineralogist, commonly styled the Abbé Haüy after he was made an honorary canon of Notre Dame. He is often referred to as the "Father of Modern Crystallography." -Biography:...
's name amphigène, but 'leucite' is the only name for this mineral species that is recognised as official by the International Mineralogical Association.
Leucite rocks
Rocks containing leucite are scarce, many countries such as EnglandEngland
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
being entirely without them. However, they are of wide distribution, occurring in every quarter of the globe. Taken collectively, they exhibit a considerable variety of types and are of great interest petrographically. For the presence of this mineral it is necessary that the silica percentage of the rock should be low, since leucite is incompatible with free quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and reacts with it to form potassium feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. Because it weathers rapidly, leucite is most common in lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
s of recent and Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
age, which have a fair amount of potassium, or at any rate have potassium equal to or greater than sodium; if sodium is abundant nepheline
Nepheline
Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
occurs rather than leucite.
In pre-Tertiary rocks leucite readily decomposes and changes to zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
s, analcite
Analcite
Analcime or analcite is a white, grey, or colourless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is NaAlSi2O6·H2O. Minor amounts of potassium and calcium substitute for sodium...
and other secondary minerals. Leucite also is rare in plutonic rocks and 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...
rocks, but leucite syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
and leucite tinguaite bear witness to the possibility that it may occur in this manner. The rounded shape of its crystals, their white or grey color, and absence of planar cleavage make the presence of leucite easily determinable in many of these rocks by inspection, especially when the crystals are large.
"Pseudoleucites" are rounded areas consisting of feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, nepheline
Nepheline
Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
, analcite
Analcite
Analcime or analcite is a white, grey, or colourless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is NaAlSi2O6·H2O. Minor amounts of potassium and calcium substitute for sodium...
, &c., which have the shape, composition and sometimes even the outward crystalline shape of leucite; they are probably pseudomorph
Pseudomorph
In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form , resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another...
s or paramorphs, which have developed from leucite because this mineral is not stable at ordinary temperatures and can be expected under favorable conditions to undergo spontaneous change into an aggregate of other minerals. Leucite is very often accompanied by nepheline, sodalite
Sodalite
Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent...
or nosean
Nosean
Nosean, also known as Noselite, is a mineral of the feldspathoid group with formula: Na8Al6Si6O24. It forms isometric crystals of variable color: white, grey, blue, green, to brown. It has a Mohs hardness of 5.5 to 6 and a specific gravity of 2.3 to 2.4. It is fluorescent. It is found in low silica...
; other minerals which make their appearance with some frequency are melanite, garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
and melilite
Melilite
Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is 2[SiO7]. Discovered in 1793 near Rome, it has a yellowish, greenish brown color...
.
The plutonic leucite-bearing rocks are leucite syenite and missourite. Of these the former consists of orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
, nepheline, sodalite, diopside
Diopside
Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class...
and aegirine
Aegirine
Aegirine is a member of the clinopyroxene group of inosilicates. Aegirine is the sodium endmember of the aegirine-augite series. Aegirine has the chemical formula NaFeSi2O6 in which the iron is present as Fe3+. In the aegirine-augite series the sodium is variably replaced by calcium with iron and...
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
and sphene. Two occurrences are known, one in Arkansas
Arkansas
Arkansas is a state located in the southern region of the United States. Its name is an Algonquian name of the Quapaw Indians. Arkansas shares borders with six states , and its eastern border is largely defined by the Mississippi River...
, the other in Sutherland, 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...
. The Scottish rock has been called borolanite
Borolanite
Borolanite is one of the most remarkable rocks of the British Isles, found on the shores of Loch Borolan in Sutherland, after which it has been named. In this locality there is a considerable area of granite rich in red alkali feldspar, and passing, by diminution in the amount of its quartz, into...
. Both examples show large rounded spots in the hand specimens; they are pseudoleucites, and under the microscope prove to consist of orthoclase, nepheline, sodalite and decomposition products. These have a radiate arrangement externally, but are of irregular structure at their centres; it is interesting to note that in both rocks melanite is an important accessory. The missourites are more mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
and consist of leucite, 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 biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
; the leucite is partly fresh partly altered to analcite
Analcite
Analcime or analcite is a white, grey, or colourless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is NaAlSi2O6·H2O. Minor amounts of potassium and calcium substitute for sodium...
, and the rock has a spotted character recalling that of the leucite-syenites. It has been found only in the Highwood Mountains
Highwood Mountains
The Highwood Mountains cover approximately 4,659 km² in north central Montana in the U.S., east of Great Falls and Benton Lake National Wildlife Refuge, at the northern end of the Lewis and Clark National Forest...
of Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
.
The leucite-hearing dike-rocks are members of the tinguaite and monchiquite groups. The leucite tinguaites are usually pale grey or greenish in color and consist principally of nepheline, alkali feldspar and aegirine. The latter forms bright green moss-like patches and growths of indefinite shape, or in other cases scattered acicular prisms, among the feldspars and nephelines of the ground mass. Where leucite occurs, it is always euhedral in small, equant, many-sided crystals in the ground mass, or in larger masses which have the same characters as the pseudoleucites. Biotite occurs in some of these rocks, and melanite also is present. Nepheline decreases in amount as leucite increases since the abundances of the two reflect the Na:K ratio of the rock. Rocks of this group are known from Rio de Janeiro
Rio de Janeiro
Rio de Janeiro , commonly referred to simply as Rio, is the capital city of the State of Rio de Janeiro, the second largest city of Brazil, and the third largest metropolitan area and agglomeration in South America, boasting approximately 6.3 million people within the city proper, making it the 6th...
, Arkansas, Kola
Kola
Kola can refer to:*Kola nut, a genus of about 125 species of trees**Inca Kola, a cola soft drink made in Peru**Kola Real, a Peruvian soft drink**Kola Inglesa , a Peruvian soft drink...
(in Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
), Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
and a few other places., In Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
there are leucite tinguaites with much arfvedsonite
Arfvedsonite
Arfvedsonite is a sodium amphibole mineral with composition: [Na][Na2][4Fe3+][2|Si8O22]. It crystallizes in the monoclinic prismatic crystal system and typically occurs as greenish black to bluish grey fibrous to radiating or stellate prisms....
, (hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
) and eudialyte. Wherever they occur they accompany leucite- and nepheline syenites. Leucite monchiquites are fine-grained dark rocks consisting of olivine, titaniferous augite and iron oxides, with a glassy ground mass in which small rounded crystals of leucite are scattered. They have been described from Czechoslovakia
Czechoslovakia
Czechoslovakia or Czecho-Slovakia was a sovereign state in Central Europe which existed from October 1918, when it declared its independence from the Austro-Hungarian Empire, until 1992...
.
By far the greater number of the rocks which contain leucite are lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
s of Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
or recent geological age. Although these never contain quartz, but feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
is usually present, though there are certain groups of leucite lavas which are non-feldspathic. Many of them also contain nepheline, sodalite, hauyne and nosean; the much rarer mineral melilite
Melilite
Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is 2[SiO7]. Discovered in 1793 near Rome, it has a yellowish, greenish brown color...
appears also in some examples. The commonest ferromagnesian mineral is 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...
(sometimes rich in sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
), with olivine in the more basic varieties. Hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
and biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
occur also, but are less common. Melanite is found in some of the lavas, as in the leucite syenites.
The rocks in which orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
(or sanidine) is present in considerable amount are leucite-trachytes, leucite-phonolites and leucitophvres. Of these groups the two former, which are not sharply distinguished from one another by most authors, are common in the neighborhood of Rome
Rome
Rome is the capital of Italy and the country's largest and most populated city and comune, with over 2.7 million residents in . The city is located in the central-western portion of the Italian Peninsula, on the Tiber River within the Lazio region of Italy.Rome's history spans two and a half...
. They are of trachytic appearance, containing phenocysts of sanidine, leucite, augite and biotite. Sodalite or hauyne may also be present, but nepheline is typically absent. Rocks of this class occur also in the tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
s of the Phlegraean Fields, near Naples
Naples
Naples is a city in Southern Italy, situated on the country's west coast by the Gulf of Naples. Lying between two notable volcanic regions, Mount Vesuvius and the Phlegraean Fields, it is the capital of the region of Campania and of the province of Naples...
. The leucitophyres are rare rocks which have been described from various parts of the volcanic district of the Rhine (Olbrck. Laacher See, etc.) and from Monte Vulture in Italy. They are rich in leucite, but contain also some sanidine and often much nepheline with hauyne or nosean. Their pyroxene is principally aegirine or aegirine-augite; some of them are rich in melanite. Microscopic sections of some of these rocks are of great interest on account of their beauty and the variety of feldspathoid
Feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....
minerals which they contain. In Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
leucitophyres have been found which belong to the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period.
Those leucite rocks which contain abundant essential plagioclase feldspar are known as leucite tephrites and leucite basanites. The former consist mainly of plagioclase, leucite and augite, while the latter contain olivine in addition. The leucite is often present in two sets of crystals, both porphyritic and as an ingredient of the ground mass. It is always idiomorphic with rounded outlines. The feldspar ranges from bytownite to oligoclase, being usually a variety of labradorite
Labradorite
Labradorite , a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It is usually defined as having "%An" between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573....
; orthoclase is scarce. The augite varies a good deal in chemnistry and optical character, being green, brown or violet (suggesting high Na and Ti content), but it is rarely high enough in Na and Fe to qualify as aegirine-augite or aegirine
Aegirine
Aegirine is a member of the clinopyroxene group of inosilicates. Aegirine is the sodium endmember of the aegirine-augite series. Aegirine has the chemical formula NaFeSi2O6 in which the iron is present as Fe3+. In the aegirine-augite series the sodium is variably replaced by calcium with iron and...
. Among the accessory minerals biotite, brown hornblende, hauyne, iron oxides and apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
are the commonest; melanite and nepheline may also occur. The ground mass of these rocks is only occasionally rich in glass. The leucite-tephrites and leucite-basanites of Vesuvius and Somma are familiar examples of this class of rocks. They are black or ashy-grey in color, often vesicular, and may contain many large grey phenocysts of leucite. Their black augite and yellow green olivine are also easily observed in hand specimens. From Volcan Ello, Sardinia and Roccamonfina similar rocks are obtained; they occur also in Bohemia
Bohemia
Bohemia is a historical region in central Europe, occupying the western two-thirds of the traditional Czech Lands. It is located in the contemporary Czech Republic with its capital in Prague...
, in Java, Celebes
Sulawesi
Sulawesi is one of the four larger Sunda Islands of Indonesia and is situated between Borneo and the Maluku Islands. In Indonesia, only Sumatra, Borneo, and Papua are larger in territory, and only Java and Sumatra have larger Indonesian populations.- Etymology :The Portuguese were the first to...
, Kilimanjaro
Mount Kilimanjaro
Kilimanjaro, with its three volcanic cones, Kibo, Mawenzi, and Shira, is a dormant volcano in Kilimanjaro National Park, Tanzania and the highest mountain in Africa at above sea level .-Geology:...
(Africa) and near Trebizond in Asia Minor
Asia Minor
Asia Minor is a geographical location at the westernmost protrusion of Asia, also called Anatolia, and corresponds to the western two thirds of the Asian part of Turkey...
.
Leucite lavas from which feldspar is absent are divided into the leucitites and leucite 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. The latter contain olivine, the former do not. Pyroxene is the usual ferromagnesian mineral, and resembles that of the tephrites and basanites. Sanidine, melanite, hauyne and perovskite
Perovskite
A perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers. Perovskites take their name from this compound, which was first discovered in the Ural mountains of...
are frequent accessory minerals in these rocks, and many of them contain melilite in some quantity, The well-known leucitite of the Capo di Bove, near, Rome, is rich in this mineral, which forms irregular plates, yellow in the hand specimen, enclosing many small rounded crystals of leucite. Bracciano and Roccamonfina are other Italian localities for leucitite, and in Java, Montana, Celebes and New South Wales
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...
similar rocks occur, The leucite basalts belong to more basic types and are rich in olivine and augite. They occur in great numbers in the Rhenish volcanic district (Eifel, Laacher See) and in Bohemia, and accompany tephrites or leucitites in Java, Montana, Celebes and Sardinia. The peperino of the neighborhood of Rome is a leucitite tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
.
See also
- ultrapotassic
- PhonolitePhonolitePhonolite is a rare igneous, volcanic rock of intermediate composition, with aphanitic to porphyritic texture....
- Igneous rockIgneous rockIgneous 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...
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