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Stilbite is the name of a series of
tectosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...
s of the
zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These p ...
group. Prior to 1997, stilbite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a series name, with the mineral species being named: * Stilbite-Ca * Stilbite-Na Stilbite-Ca, by far the more common of the two, is a hydrous calcium sodium and aluminium silicate, Na Ca4( Si27 Al9) O72·28( H2 O). In the case of stilbite-Na, sodium dominates over calcium. The species are visually indistinguishable, and the series name stilbite is still used whenever testing has not been performed.


History

At one time
heulandite Heulandite is the name of a series of tecto-silicate 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 se ...
and stilbite were considered to be identical minerals. After they were found to be two separate species, in 1818, the name desmine ("a bundle") was proposed for stilbite, and this name is still employed in Germany. The English name "stilbite" is from the Greek stilbein = to shine, because of the pearly luster of the faces.


Chemistry and related species

Stilbite shows a wide variation in exchangeable cations:
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
and
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ...
ions occupy equivalent sites and can substitute for each other. Since silicon and aluminium have a different charge (Si4+ and Al3+) the ions occupying the sodium/calcium site have to adjust to maintain charge balance. There is a continuous series between stellerite, whose formula can be written as Ca4(Si28Al8)O72·28(H2O), and stilbite, and another continuous series between stilbite and barrerite, Na8(Si28Al8)O72·26(H2O). Epistilbite is a distinct zeolite species unrelated to stilbite.


Crystal class

Stilbite is usually
monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic sys ...
2/m, meaning that it has one twofold axis of rotational symmetry perpendicular to a mirror plane. The twofold axis is the crystal axis b, and the a and c crystal axes lie in the mirror plane. For a monoclinic crystal a and c are inclined to each other at an angle β which is not a right angle. For stilbite β is nearly 130°. Stilbite crystals, however, appear to be almost
orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...
, and a larger
unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessari ...
can be chosen, containing two formula units (Z = 2) such that resembles an orthorhombic cell, with all three crystal axes very nearly mutually perpendicular. The mineral is said to be pseudo-orthorhombic. Non-
endmember An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity of its chemical composition. Minerals often can be described as solid solutions with varying compositions of ...
forms of stilbite may be
triclinic 180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ ) In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal is ...
or even truly orthorhombic, indeed the framework can have symmetry ranging from orthorhombic to triclinic in a single crystal.


Habit

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, macr ...
s are typically thin tabular, flattened parallel to the dominant cleavage and elongated along the an axis. Aggregates may be sheaf-like or in bow-ties, also fibrous and globular. Twinning, cruciform and penetration, is extremely common on .


Physical and optical properties

The color is usually colorless or white, also yellow, brown, pink, salmon, orange, red, green, blue or black. The luster is generally vitreous, and on the perfect cleavage parallel to the plane of symmetry it is markedly pearly. The streak is white and crystals are transparent to translucent. The
hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example har ...
is to 4 and the
specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
2.12 to 2.22. Cleavage is perfect on , poor on . The mineral is brittle, with a conchoidal or uneven
fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displ ...
. It is not radioactive. Stilbite is biaxial (-) with refractive indices: *Nx = 1.479 to 1.492, Ny = 1.485 to 1.500, Nz = 1.489 to 1.505 *Nx = 1.484 to 1.500, Ny = 1.492 to 1.507, Nz = 1.494 to 1.513


Unit cell and structure

Where sources give cell parameters for stilbite-Na, they are the same as those for stilbite-Ca. The
unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessari ...
can be considered as a monoclinic cell with β close to 130° and one formula unit per unit cell (Z = 1), or as a larger pseudo-orthorhombic cell with β close to 90° and Z = 2.
Cell Parameters for the monoclinic cell: *a = 13.595 to 13.69 Å, b = 18.197 to 18.31 Å, c = 11.265 to 11.30 Å, β = 127.94 to 128.1° *a = 13.63 Å, b = 18.17 Å, c = 11.31 Å, β = 129.166° *a = 13.60 to 13.69 Å, b = 18.20 to 18.31 Å, c = 11.27 Å, β = 128° Cell parameters for the pseudo-orthorhombic cell: *a = 13.595 to 13.69 Å, b = 18.197 to 18.31 Å, c = 17.775 to 17.86 Å, β = 90.00 to 90.91° *a = 13.595 to 13.657 Å, b = 18.197 to 18.309 Å, c = 17.775 to 17.842 Å, β = 90:05 to 90.91° (Z is doubled to Z = 4 because the formula unit halved to NaCa2Al5Si13O36.14H2O) *a=13.69 Å, b=18.25 Å, c=11.31 Å, β =128.2°American Mineralogist (1970) 55: 387–397 *a = 13.60 to 13.69 Å, b = 18.20 to 18.31 Å, c = 17.78 to 17.86 Å, β = 90.0 to 90.91° The framework of stilbite is pseudo-orthorhombic with the open channels typical of
zeolites Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These p ...
. It has 10-member rings and 8-member rings forming channels parallel to a and pseudo-orthorhombic c respectively.


Uses

The open channels in the stilbite structure act like a molecular sieve, enabling it to separate hydrocarbons in the process of petroleum refining.


Environment

Stilbite is a low-temperature secondary hydrothermal mineral. It occurs in the amygdaloidal cavities of
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low- viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More tha ...
ic
volcanic rocks Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...
, in andesites,
gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...
and hydrothermal veins. It also forms in hot springs deposits, and as a cementing agent in some
sandstones Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. ...
and conglomerates. Stilbite has not been found in sedimentary
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...
deposits or deep-sea deposits. Associated minerals are other zeolites,
prehnite Prehnite is an inosilicate of calcium and aluminium with the formula: Ca2Al(AlSi3O10)(OH)2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most often forms as stalactitic or bo ...
,
calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scrat ...
and
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical fo ...
.


Localities

Stilbite is abundant in the volcanic rocks of
Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its ...
, Faroe Islands,
Isle of Skye The Isle of Skye, or simply Skye (; gd, An t-Eilean Sgitheanach or ; sco, Isle o Skye), is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated b ...
, Bay of Fundy,
Nova Scotia Nova Scotia ( ; ; ) is one of the thirteen provinces and territories of Canada. It is one of the three Maritime provinces and one of the four Atlantic provinces. Nova Scotia is Latin for "New Scotland". Most of the population are native E ...
(where it is the provincial mineral), northern
New Jersey New Jersey is a U.S. state, state in the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. It is bordered on the north and east by the state of New York (state), New York; on the ea ...
and
North Carolina North Carolina () is a state in the Southeastern region of the United States. The state is the 28th largest and 9th-most populous of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, Georgia and ...
. Salmon-pink crystals occur with pale green
apophyllite The name apophyllite refers to a specific group of phyllosilicates, a class of minerals. Originally, the group name referred to a specific mineral, but was redefined in 1978 to stand for a class of minerals of similar chemical makeup that compris ...
in the
Deccan Traps The Deccan Traps is a large igneous province of west-central India (17–24°N, 73–74°E). It is one of the largest volcanic features on Earth, taking the form of a large shield volcano. It consists of numerous layers of solidified flood ...
near
Mumbai Mumbai (, ; also known as Bombay — the official name until 1995) is the capital city of the Indian state of Maharashtra and the '' de facto'' financial centre of India. According to the United Nations, as of 2018, Mumbai is the secon ...
(Bombay) and
Pune Pune (; ; also known as Poona, ( the official name from 1818 until 1978) is one of the most important industrial and educational hubs of India, with an estimated population of 7.4 million As of 2021, Pune Metropolitan Region is the largest i ...
,
India India, officially the Republic of India ( Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the ...
; white sheaf-like groups encrust the calcite ( Iceland-spar) of Berufjord near Djupivogr in Iceland; brown sheafs are found near
Paterson, New Jersey Paterson ( ) is the largest city in and the county seat of Passaic County, in the U.S. state of New Jersey.United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territo ...
; and crystals of a brick-red color are found at Old Kilpatrick,
Scotland Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...
. Iceland is generally considered to be the type locality for stilbite-Ca. It is presumed to be the Helgusta Iceland Spar Mine, along Reydarfjordur. Excellent white bow ties of stilbite are found here on calcite and quartz, associated with heulandite and
laumontite Laumontite is a mineral, one of the zeolite group. Its molecular formula is , a hydrated calcium-aluminium silicate. Potassium or sodium may substitute for the calcium but only in very small amounts. It is monoclinic, space group C2/m. It forms ...
in cavities. The type locality for stilbite-Na is Cape Pula,
Pula Pula (; also known as Pola, it, Pola , hu, Pòla, Venetian; ''Pola''; Istriot: ''Puola'', Slovene: ''Pulj'') is the largest city in Istria County, Croatia, and the seventh-largest city in the country, situated at the southern tip of the ...
,
Cagliari Cagliari (, also , , ; sc, Casteddu ; lat, Caralis) is an Italian municipality and the capital of the island of Sardinia, an autonomous region of Italy. Cagliari's Sardinian name ''Casteddu'' means ''castle''. It has about 155,000 inhabitant ...
Province,
Sardinia Sardinia ( ; it, Sardegna, label=Italian language, Italian, Corsican language, Corsican and Tabarchino ; sc, Sardigna , sdc, Sardhigna; french: Sardaigne; sdn, Saldigna; ca, Sardenya, label=Algherese dialect, Algherese and Catalan languag ...
, Italy. Small, lustrous, white or pink, pointed blades of stilbite-Na, and formless masses, up to 5 cm in diameter, have been found there, covering a thin crust of reddish
heulandite Heulandite is the name of a series of tecto-silicate 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 se ...
in large fractures and cavities in the highly weathered volcanic andesite or rhyolite. The
Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non- avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
Deccan basalts of western India are the most prolific sources of stilbite in the world. Stilbite is the most abundant zeolite in the tholeiitic basalt plateaux near
Nasik Nashik (, Marathi: aːʃik, also called as Nasik ) is a city in the northern region of the Indian state of Maharashtra. Situated on the banks of river Godavari, Nashik is the third largest city in Maharashtra, after Mumbai and Pune. Nashik ...
and Pune and decreases in abundance toward the coast at Mumbai.


Photo gallery

File:Stilbite-Ca-Celadonite-178645.jpg, Green Stilbite-Ca, which gets its color from inclusions of celadonite File:Fluorapophyllite-Stilbite-Ca-indi-71a.jpg, Clustered on a matrix of white chalcedony are a group of light salmon stilbite crystals; adjoining is a cluster of mint green fluorapophyllite crystals File:Stilbite-20450.jpg, Intergrown pinkish stilbite crystals showing the "bow-tie" tendency from Jalgaon District, Maharashtra, India. File:Stilbite Orange Fans.jpg, Stilbite crystal aggregates in fan shapes from Wassons Bluff, Nova Scotia. File:Stilbite-Ca-251147.jpg, Reddish-orange Stilbite-Ca from Jalgaon District, Maharashtra, India


References

;Attribution *


External links


Structure type STI
{{Authority control Tectosilicates Sodium minerals Calcium minerals Aluminium minerals Zeolites Monoclinic minerals Minerals in space group 12 Orthorhombic minerals Minerals in space group 63