Tobermorite

Not to be confused with Torbernite.

Tobermorite is a calcium silicate hydrate mineral with chemical formula: Ca₅Si₆O₁₆(OH)₂·4H₂O or Ca₅Si₆(O,OH)₁₈·5H₂O.

Tobermorite
Crystalline mass of tobermorite
General
Category	Silicate mineral, Calcium silicate hydrate
Formula	Ca5Si6O16(OH)2·4H2O, or; Ca5Si6(O,OH)18·5H2O
IMA symbol	Tbm[1]
Strunz classification	9.DG.10
Crystal system	Orthorhombic
Crystal class	Disphenoidal (222) H-M symbol: (2 2 2)
Space group	C2221 (no. 20)
Unit cell	a = 11.17 Å, b = 7.38 Å c = 22.94 Å; β = 90°; Z = 4
Identification
Formula mass	702.36 g/mol
Color	Pale pinkish white, white, brown
Crystal habit	As minute laths; fibrous bundles, rosettes or sheaves, radiating or plumose, fine granular, massive.
Cleavage	{001} Perfect, {100} Imperfect
Mohs scale hardness	2.5
Luster	Vitreous, silky in fibrous aggregates
Streak	White
Diaphaneity	Translucent to translucent
Specific gravity	2.423 – 2.458
Optical properties	Biaxial (+)
Refractive index	nα = 1.570 nβ = 1.571 nγ = 1.575
Birefringence	δ = 0.005
Ultraviolet fluorescence	Fluorescent, Short UV:weak white to yellow, Long UV:weak white to yellow
References	[2][3][4]

Two structural varieties are distinguished: tobermorite-11 Å and tobermorite-14 Å. Tobermorite occurs in hydrated cement paste and can be found in nature as an alteration mineral in metamorphosed limestone and in skarn. It has been reported to occur in the Maqarin Area of north Jordan and in the Crestmore Quarry near Crestmore Heights, Riverside County, California.

Tobermorite was first described in 1880 for an occurrence in Scotland, on the Isle of Mull, around the locality of Tobermory.^[3][5]

Use in Roman concrete

Aluminum-substituted tobermorite is understood to be a key ingredient responsible for the longevity of ancient undersea Roman concrete. The volcanic ash that Romans used for construction of sea walls contained phillipsite, and an interaction with sea water caused the crystalline structures in the concrete to expand and strengthen, making that material substantially more durable than modern concrete when exposed to sea water.^[6][7][8]

Crystal structure of tobermorite: elementary unit cell.

Cement chemistry

Tobermorite is often used in thermodynamical calculations to represent the pole of the most evolved calcium silicate hydrate (C-S-H). According to its chemical formula, its atomic Ca/Si or molar CaO/SiO₂ (C/S) ratio is 5/6 (0.83). Jennite represents the less evolved pole with a C/S ratio of 1.50 (9/6).

See also

• Other calcium silicate hydrate (C-S-H) minerals:
  • Afwillite – Nesosilicate alteration mineral also sometimes found in hydrated cement paste
  • Gyrolite – Rare phyllosilicate mineral crystallizing in small spheres
  • Jaffeite – Sorosilicate mineral
  • Jennite – Inosilicate alteration mineral in metamorphosed limestone and in skarn
  • Okenite – Delicate mineral with a cotton like appearance
  • Thaumasite – Complex calcium silicate hydrate mineral
  • Xonotlite – Inosilicate mineral
• Other calcium aluminium silicate hydrate, (C-A-S-H) minerals:
  • Hydrogarnet
  • Hydrogrossular
  • Hydrotalcite
  • Katoite
  • Tacharanite (Ca₁₂Al₂Si₁₈O₃₃(OH)₃₆)