Jennite
Crystal structure of jennite: elementary unit cell viewed in 3D
General
CategorySilicate mineral
Formula
(repeating unit)		Ca9Si6O18(OH)6·8H2O
IMA symbolJnn[1]
Strunz classification9.DG.20
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit cella = 10.56, b = 7.25
c = 10.81 [Å]; α = 99.7°
β = 97.67°, γ = 110.07°; Z = 1
Identification
Formula mass1,063 g/mol
ColorWhite
Crystal habitBlade shaped crystals, fibrous aggregates, platy – sheet forms
CleavageDistinct on [001]
Mohs scale hardness3.5
LusterVitreous (glassy)
StreakWhite
DiaphaneityTransparent to translucent
Density2.32–2.33
Optical propertiesBiaxial (−)
Refractive indexnα = 1.548 – 1.552 nβ = 1.562 – 1.564 nγ = 1.570 – 1.571
Birefringenceδ = 0.022
2V angleMeasured: 74°
Ultraviolet fluorescenceWeak white
References[2][3][4][5]

Jennite is a calcium silicate hydrate mineral of general chemical formula: Ca9Si6O18(OH)6·8H2O.

Jennite occurs as an alteration mineral in metamorphosed limestone and skarn.[3] It typically occurs as vein and open space fillings as a late mineral phase.[5] It also occurs in hydrated cement paste.

A first specimen of jennite found in 1966 at the Crestmore quarries (Crestmore, Riverside County, California, US) was analysed and identified as a new mineral by Carpenter in 1966 (Carpenter, 1966). They named it in honor of its discoverer: Clarence Marvin Jenni (1896–1973) director of the Geological Museum at the University of Missouri.[3]

In contrast to the first analysis made by Carpenter, jennite was found to not contain appreciable amount of sodium when the Crestmore specimen was reexamined.[6]

The structure of jennite is made of three distinct modules: ribbons of edge-sharing calcium octahedra, silicate chains of wollastonite-type running along the b axis, and additional calcium octahedra on inversion centers. The hydroxyl groups are bonded to three calcium cations while no SiOH groups are observed.[7]

Jennite transforms to metajennite at 70–90 °C (158–194 °F) by losing four water molecules.[6]

Cement chemistry

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Jennite is often used in thermodynamical calculations to represent the pole of the less evolved calcium silicate hydrate (C-S-H). The value of its atomic Ca/Si or molecular CaO/SiO2 (C/S) ratio is 1.50 (9/6), as directly calculated from its elementary composition formula. Tobermorite represents the more evolved pole with a C/S ratio of 0.83 (5/6).

See also

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Jennite on Webmineral
  3. ^ a b c Jennite on Mindat
  4. ^ Jennite in the American Mineralogist Crystal Structure Database
  5. ^ a b Handbook of Mineralogy
  6. ^ a b Gard, J.A.; Taylor, H.F.W.; Cliff, G.; Lorimer, G.W. (1977), "A reexamination of jennite" (PDF), American Mineralogist, vol. 62, pp. 365–368, retrieved 2009-02-04
  7. ^ Carpenter, A.B.; Chalmers, R.A.; Gard, J.A.; Speakman, K.; Taylor, H.F.W. (1966), "Jennite, a new mineral" (PDF), American Mineralogist, vol. 51, pp. 56–74, retrieved 2009-02-04
Bibliography

Further reading

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