Tobermorite

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Tobermorite
Crystalline mass of tobermorite
General
Category	Silicate mineral, Calcium silicate hydrate
Formula (repeating unit)	Ca5Si6O16(OH)2·4H2O, or; Ca5Si6(O,OH)18·5H2O
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	[1][2][3]

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

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 alteration mineral in metamorphosed limestone and in skarn. It has been reported from the 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.^[2]

Use in Roman concrete[]

Aluminum substituted tobermorite is understood to be a key ingredient in the longevity of ancient undersea Roman concrete. The volcanic ash that Romans used for construction of sea walls contained phillipsite, and that an interaction with sea water actually caused the crystalline structures in the mortar to expand and strengthen, making that material substantially more durable than modern concrete when exposed to sea water. ^{[4] [5][6]}

Crystal structure of tobermorite: elementary unit cell.

Cement chemistry[]

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Tobermorite is often used in thermodynamical calculations to represent the pole of the most evolved calcium silicate hydrate (C-S-H). The value of its atomic Ca/Si or molar CaO/SiO₂ (C/S) ratio is 0.83 (5/6) as directly calculated from its elementary composition formula. 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 spherules
  • Jennite – Inosilicate alteration mineral in metamorphosed limestone and in skarn
  • Thaumasite – Unusual calcium silicate mineral with carbonate, sulfate and hexacoordinated hydroxysilicate anions. Responsible of a harmful concrete sulfate attack
• Other calcium aluminium silicate hydrate, (C-A-S-H) minerals:
  • Hydrogarnet
  • Hydrotalcite

References[]

External links[]

• Tobermorite in the American Mineralogist Crystal Structure Database

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