Stilbite

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Stilbite
Specific gravity
2.12 to 2.22
Optical propertiesBiaxial (−)
Refractive indexNx = 1.479 to 1.492, Ny = 1.485 to 1.500, Nz = 1.489 to 1.505[2][3] Nx = 1.484 to 1.500, Ny = 1.492 to 1.507, Nz = 1.494 to 1.513[4][5]
FusibilityEasily fused by blowpipe (~1050 °C) to produce a white "enamel"[6]
SolubilityDecomposes in HCl[6]
References[3][4]

[5][7]

[8][9] [10][11]

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

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.[2]

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.[2] Twinning, cruciform and penetration, is extremely common on {001}.[7]

Physical and optical properties

The color is usually colorless or white, also yellow, brown, pink, salmon, orange, red, green, blue or black. The

Cleavage is perfect on {010}, poor on {001}.[2] The mineral is brittle, with a conchoidal or uneven fracture
. It is not radioactive.

Stilbite is biaxial (-) with

refractive indices
:

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.
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°[7]
  • a = 13.63 Å, b = 18.17 Å, c = 11.31 Å, β = 129.166°[3][4][8][9]
  • a = 13.60 to 13.69 Å, b = 18.20 to 18.31 Å, c = 11.27 Å, β = 128°[2]

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°[7]
  • 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°[5] (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°[12]
  • 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°[2]

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.[12]

Uses

The open channels in the stilbite structure act like a molecular sieve, enabling it to separate

petroleum refining.[citation needed
]

Environment

Stilbite is a low-temperature

sandstones and conglomerates.[5] Stilbite has not been found in sedimentary tuff deposits or deep-sea deposits.[2] Associated minerals are other zeolites, prehnite, calcite and quartz.[5]

Localities

Stilbite is abundant in the volcanic rocks of

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.[13]

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.[2]

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 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

Nasik and Pune and decreases in abundance toward the coast at Mumbai.[2]

Photo gallery

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

References

  1. S2CID 235729616
    .
  2. ^ a b c d e f g h i j Rudy W. Tschernich (1992) Zeolites of the World. Geoscience Press
  3. ^ a b c d "Stilbite-Ca Mineral Data".
  4. ^ a b c d http://www.mindat.org/min-7313.html stilbite-Ca
  5. ^ a b c d e f "Archived copy" (PDF). Archived from the original (PDF) on 2020-03-29. Retrieved 2010-07-29.{{cite web}}: CS1 maint: archived copy as title (link)
  6. ^
    ISBN 0-471-80580-7
  7. ^ a b c d Gaines et al (1997) Dana’s New Mineralogy Eighth Edition. Wiley
  8. ^ a b http://www.mindat.org/min-7314.html stilbite-Na
  9. ^ a b "Stilbite-Na Mineral Data".
  10. ^ Slaughter, M. (1970). "Crystal structure of stilbite". American Mineralogist. 55: 387–397. Retrieved 30 December 2020.
  11. S2CID 99122598
    .
  12. ^ a b American Mineralogist (1970) 55: 387–397
  13. ^ Spencer 1911.
Attribution

External links

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