Jennite

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

Jennite
Crystal structure of jennite: elementary unit cell viewed in 3D
General
Category	Silicate mineral
Formula	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 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