Thermal expansion of hydrated six-coordinate silicon in thaumasite, Ca 3 Si(OH) 6 (CO 3 )(SO 4 )·12H 2 O

S. D. Jacobsen*, J. R. Smyth, R. J. Swope

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

27 Scopus citations


Thaumasite , Ca 3 Si(OH) 6 (CO 3 (SO 4 )12H 2 O, occurs as a low-temperature secondary alteration phase in mafic igneous and metamorphic rocks, and is recognized as a product and indicator of sulfate attack in Portland cement. It is also the only mineral known to contain silicon in six-coordination with hydroxyl (OH) - that is stable at ambient P-T conditions. Thermal expansion of the various components of this unusual structure has been determined from single-crystal X-ray structure refinements of natural thaumasite at 130 and 298 K. No phase transitions were observed over this temperature range. Cell parameters at room temperature are: a = 11.0538(6) Å, c = 10.4111(8) Å and V = 1101.67(10) Å 3 , and were measured at intervals of about 50 K between 130 and 298 K, resulting in mean axial and volumetric coefficients of thermal expansion (x10 -5 K -1 ); α v = 1.7(1), α c = 2.1(2), and α v = 5.6(2). Although the unit cell and VIII CaO 8 polyhedra show significant positive thermal expansion over this temperature range, the silicate octahedron, sulfate tetrahedron, and carbonate group show zero or negative thermal expansion, with α v ( VIS iO 6 ) = -0.6 ± 1.1, α v ( IV SO 4 ) = -5.8 ± 1.4, and α R (C-O) = 0.0 ± 1.8 (x10 -5 K -1 ). Most of the thermal expansion is accommodated by lengthening of the R(O...O) hydrogen bond distances by on average 5σ, although the hydrogen bonds involving hydroxyl sites on VI Si expand twice as much as those on molecular water, causing the [Ca 3 Si(OH) 6 (H 2 O ) 12 ] 4+ columns to expand in diameter more than they move apart over this temperature range. The average Si-OH bond length of the six-coordinated Si atom {R( VI Si-OH)} in thaumasite is 1.783(1) Å, being about 0.02 Å (∼20σ) shorter than VI Si-OH in the densehydrous magnesium silicate, phase D, MgSi 2 H 2 O 6 .

Original languageEnglish (US)
Pages (from-to)321-329
Number of pages9
JournalPhysics and Chemistry of Minerals
Issue number6
StatePublished - Jul 1 2003


  • Hydrogen bonding
  • Octahedrally coordinated silicon
  • Si-OH
  • Thaumasite
  • Thermal expansion

ASJC Scopus subject areas

  • Materials Science(all)
  • Geochemistry and Petrology


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