Jennite

Jennite
Crystal structure of jennite: elementary unit cell viewed in 3D
General
Category	Silicate mineral
Formula (repeating unit)	Ca9Si6O18(OH)6·8H2O
IMA symbol	Jnn[1]
Strunz classification	9.DG.20
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 10.56, b = 7.25 c = 10.81 [Å]; α = 99.7° β = 97.67°, γ = 110.07°; Z = 1
Identification
Formula mass	1,063 g/mol
Color	White
Crystal habit	Blade shaped crystals, fibrous aggregates, platy – sheet forms
Cleavage	Distinct on [001]
Mohs scale hardness	3.5
Luster	Vitreous (glassy)
Streak	White
Diaphaneity	Transparent to translucent
Density	2.32–2.33
Optical properties	Biaxial (−)
Refractive index	nα = 1.548 – 1.552 nβ = 1.562 – 1.564 nγ = 1.570 – 1.571
Birefringence	δ = 0.022
2V angle	Measured: 74°
Ultraviolet fluorescence	Weak 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

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

• Other calcium silicate hydrate (C-S-H) minerals:
  • Afwillite
  • Gyrolite
  • Thaumasite
  • Tobermorite
• Other calcium aluminium silicate hydrate (C-A-S-H) minerals:
  • Tacharanite
  • Hydrogarnet
  • Hydrogrossular
  • Hydrotalcite
  • Katoite

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. Jennite on Mindat
4. Jennite in the American Mineralogist Crystal Structure Database
5. Handbook of Mineralogy
6. 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

• Abdul-Jaber, Q.H.; Khoury, H. (1998), "Unusual mineralisation in the Maqarin Area (North Jordan) and the occurrence of some rare minerals in the marbles and the weathered rocks", Neues Jahrb. Geol. Paläontol. Abh., vol. 208, pp. 603–629

• Bonaccorsi, E.; Merlino, S.; Taylor, H.F.W. (2004), "The crystal structure of jennite, Ca₉Si₆O₁₈(OH)₆ · 8 H₂O, Locality: Fuka, Japan", Cement and Concrete Research, vol. 34, no. 9, pp. 1481–1488, doi:10.1016/j.cemconres.2003.12.033, retrieved 2009-02-04

Further reading

• Chen, Jeffrey J.; Jeffrey J. Thomas; Hal F.W. Taylor; Hamlin M. Jennings (2004), "Solubility and structure of calcium silicate hydrate", Cement and Concrete Research, 34 (9): 1499–1519, CiteSeerX 10.1.1.568.4216, doi:10.1016/j.cemconres.2004.04.034, ISSN 0008-8846
• Eakle, Arthur S. (1927), "Famous mineral localities: Crestmore, Riverside County, California", American Mineralogist, 12: 319–321, retrieved 2009-11-01
• Naomichi, Hara (2000), "Formation of jennite and tobermorite from amorphous silica", J. Soc. Inorg. Mater. Japan, 7 (285): 133–142, ISSN 1345-3769, archived from the original on 2012-02-17, retrieved 2009-02-04
• Merlino, S.; Bonaccorsi E.; Armbruster T. (2001), "The real structure of tobermorite 11A: normal and anomalous forms, OD character and polytypic modifications (Note: MDO2 – synchrotron radiation source. Locality: Bascenov, Urals, Russia)", European Journal of Mineralogy, 13 (3): 577–590, Bibcode:2001EJMin..13..577M, doi:10.1127/0935-1221/2001/0013-0577