Stilbite

{{Short description|Tectosilicate mineral of the zeolite group}} {{Other uses|Stiblite}} {{Infobox mineral | name = Stilbite | category = Tectosilicate minerals | group = Zeolite group, stilbite subgroup

| boxwidth = 24 | boxbgcolor = | image = Fluorapophyllite-Stilbite-Ca-indi-52b.jpg | imagesize = 260px | caption = | formula = Stilbite-Ca: NaCa₄(Si₂₇Al₉)O₇₂·28(H₂O)<br />Stilbite-Na: Na₉(Si₂₇Al₉)O₇₂·28(H₂O) | IMAsymbol = Stb<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = Stilbite-Ca: 2,840 g/mol <br />Stilbite-Na: 2,864 g/mol | strunz = 9.GE.10 (10 ed) <br /><small>8/J.23-30 (8 ed)</small> | dana = 77.1.4.3 | system = Monoclinic, <br />also triclinic and orthorhombic | class = Prismatic (2/m) <br /><small>(same H-M symbol)</small> | symmetry = ''C2/m'' (monoclinic) <br />''Amma'' (orthorhombic) | color = Usually colorless, white or pink | habit = Thin tabular, aggregates sheaf-like or in bow-ties, also fibrous and globular. | lattice = | twinning = Very common on {001} | cleavage = Perfect on {010} | fracture = Conchoidal or uneven | tenacity = Brittle | mohs = {{frac|3|1|2}} to 4 | luster = Vitreous, pearly on {010} | refractive = Nx = 1.479 to 1.492, Ny = 1.485 to 1.500, Nz = 1.489 to 1.505<ref name=ZW>Rudy W. Tschernich (1992) Zeolites of the World. Geoscience Press</ref><ref name=WebminCa/> Nx = 1.484 to 1.500, Ny = 1.492 to 1.507, Nz = 1.494 to 1.513<ref name=MindatCa/><ref name=HOM/> | opticalprop = Biaxial (−) | birefringence = | pleochroism = | streak = White | gravity = 2.12 to 2.22 | density = | melt = | fusibility = Easily fused by blowpipe (~1050 °C) to produce a white "enamel"<ref name=Klein>Klein, Cornelis and Cornelius S. Hurlbut, 1985, ''Manual of Mineralogy,'' Wiley, pp. 465–466 {{ISBN|0-471-80580-7}}</ref> | diagnostic = | solubility = Decomposes in HCl<ref name=Klein/> | diaphaneity = Transparent to translucent | other = | references = <ref name=WebminCa>{{Cite web|url=http://webmineral.com/data/Stilbite-Ca.shtml|title = Stilbite-Ca Mineral Data}}</ref><ref name=MindatCa>http://www.mindat.org/min-7313.html stilbite-Ca</ref> <ref name=HOM>{{Cite web |url=http://www.handbookofmineralogy.org/pdfs/stilbiteca.pdf |title=Archived copy |access-date=2010-07-29 |archive-date=2020-03-29 |archive-url=https://web.archive.org/web/20200329041312/http://www.handbookofmineralogy.org/pdfs/stilbiteca.pdf |url-status=dead }}</ref><ref name=Dana>Gaines et al (1997) Dana’s New Mineralogy Eighth Edition. Wiley</ref> <ref name=MindatNa>http://www.mindat.org/min-7314.html stilbite-Na</ref><ref name=WebminNa>{{Cite web|url=http://webmineral.com/data/Stilbite-Na.shtml|title = Stilbite-Na Mineral Data}}</ref><ref>{{cite journal |last1=Slaughter |first1=M. |journal=American Mineralogist |volume=55 |year=1970 |pages=387–397 |title=Crystal structure of stilbite |url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/55/3-4_Part_1/387/540613 |access-date=30 December 2020}}</ref><ref>{{cite journal |last1=Cruciani |first1=Giuseppe |last2=Artioli |first2=Gilberto |last3=Gualtieri |first3=Alessandro |last4=Stahl |first4=Kenny |last5=Hanson |first5=Jonathan C. |title=Dehydration dynamics of stilbite using synchrotron X-ray powder diffraction |journal=American Mineralogist |date=1 August 1997 |volume=82 |issue=7–8 |pages=729–739 |doi=10.2138/am-1997-7-810|bibcode=1997AmMin..82..729C |s2cid=99122598 }}</ref>}} '''Stilbite''' is the name of a series of tectosilicate minerals of the zeolite 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, sometimes also called ''stiblite''<ref name="kriv">''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</ref>{{rp|325}}

Stilbite-Ca, by far the more common of the two, is a hydrous calcium sodium and aluminium silicate, NaCa₄(Si₂₇Al₉)O₇₂·28(H₂O). 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 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 {010} faces.

== Chemistry and related species == Stilbite shows a wide variation in exchangeable cations: silicon and aluminium 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 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, and a larger unit cell 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 forms of stilbite may be triclinic or even truly orthorhombic, indeed the framework can have symmetry ranging from orthorhombic to triclinic in a single crystal.<ref name=ZW/>

== Habit == Crystals are typically thin tabular, flattened parallel to the dominant cleavage and elongated along the a axis. Aggregates may be sheaf-like or in bow-ties, also fibrous and globular.<ref name=ZW/> Twinning, cruciform and penetration, is extremely common on {001}.<ref name=Dana/>

== 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 is {{frac|3|1|2}} to 4 and the specific gravity 2.12 to 2.22. Cleavage is perfect on {010}, poor on {001}.<ref name=ZW/> The mineral is brittle, with a conchoidal or uneven fracture. 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<ref name=ZW/><ref name=WebminCa/> *Nx = 1.484 to 1.500, Ny = 1.492 to 1.507, Nz = 1.494 to 1.513<ref name=MindatCa/><ref name=HOM/>

== Unit cell and structure== Where sources give cell parameters for stilbite-Na, they are the same as those for stilbite-Ca.

The unit cell 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.<br /> 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°<ref name=Dana/> *a = 13.63 Å, b = 18.17 Å, c = 11.31 Å, β = 129.166°<ref name=WebminCa/><ref name=MindatCa/><ref name=MindatNa/><ref name=WebminNa/> *a = 13.60 to 13.69 Å, b = 18.20 to 18.31 Å, c = 11.27 Å, β = 128°<ref name=ZW/> 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°<ref name=Dana/> *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°<ref name=HOM/> (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°<ref name=AM55>American Mineralogist (1970) 55: 387–397</ref> *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°<ref name=ZW/>

The framework of stilbite is pseudo-orthorhombic with the open channels typical of zeolites. It has 10-member rings and 8-member rings forming channels parallel to a and pseudo-orthorhombic c respectively.<ref name=AM55/>

== Uses == The open channels in the stilbite structure act like a molecular sieve, enabling it to separate hydrocarbons in the process of petroleum refining.{{Citation needed|date=May 2023}}

== Environment == Stilbite is a low-temperature secondary hydrothermal mineral. It occurs in the amygdaloidal cavities of basaltic volcanic rocks, in andesites, gneiss and hydrothermal veins. It also forms in hot springs deposits, and as a cementing agent in some sandstones and conglomerates.<ref name=HOM/> Stilbite has not been found in sedimentary tuff deposits or deep-sea deposits.<ref name=ZW/> Associated minerals are other zeolites, prehnite, calcite and quartz.<ref name=HOM/>

== Localities == Stilbite is abundant in the volcanic rocks of Iceland, Faroe Islands, Isle of Skye, Bay of Fundy, Nova Scotia (where it is the provincial mineral), northern New Jersey and North Carolina. Salmon-pink crystals occur with pale green apophyllite in the Deccan Traps near Mumbai (Bombay) and Pune, India; white sheaf-like groups encrust the calcite (Iceland-spar) of Berufjord near Djupivogr in Iceland; brown sheafs are found near Paterson, New Jersey in the United States; and crystals of a brick-red color are found at Old Kilpatrick, Scotland.{{sfn|Spencer|1911}}

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 in cavities.<ref name=ZW/>

The type locality for stilbite-Na is Cape Pula, Pula, Cagliari Province, Sardinia, Italy. Small, lustrous, white or pink, pointed blades of stilbite-Na, and formless masses, up to 5&nbsp;cm in diameter, have been found there, covering a thin crust of reddish heulandite in large fractures and cavities in the highly weathered volcanic andesite or rhyolite.

The Tertiary 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 and Pune and decreases in abundance toward the coast at Mumbai.<ref name=ZW/>

==Photo gallery== <gallery widths="165px" heights="135px"> 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 </gallery>

==References== {{Reflist}}

;Attribution *{{EB1911|first=Leonard James |last=Spencer|wstitle=Stilbite}}

==External links== *[http://helios.princeton.edu/zeomics/cgi-bin/view_structure.pl?src=iza&id=STI Structure type STI]

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Category:Sodium minerals Category:Calcium minerals Category:Aluminium minerals Category:Stilbite subgroup Category:Monoclinic minerals Category:Minerals in space group 12 Category:Orthorhombic minerals Category:Minerals in space group 63 Category:Provincial symbols of Nova Scotia