Hydromagnesite

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Hydromagnesite
Specific gravity
2.16–2.2
Optical propertiesBiaxial (+)
Refractive indexnα = 1.523 nβ = 1.527 nγ = 1.545
Birefringenceδ = 0.022
Ultraviolet fluorescenceFluorescent, short UV=green, long UV=bluish white.
References[2][3][4]

Hydromagnesite is a hydrated magnesium carbonate mineral with the formula Mg5(CO3)4(OH)2·4H2O.

It generally occurs associated with the weathering products of magnesium containing minerals such as

hydrothermally altered dolomite and marble. Hydromagnesite commonly appears in caves as speleothems and "moonmilk", deposited from water that has seeped through magnesium rich rocks. It is the most common cave carbonate after calcite and aragonite.[2] The mineral thermally decomposes,[5][6]
over a temperature range of approximately 220 °C to 550 °C, releasing water and carbon dioxide leaving a magnesium oxide residue.

Hydromagnesite was first described in 1836 for an occurrence in Hoboken, New Jersey.[3]

alkaline (pH greater than 9) freshwater lake (Salda Gölü) in southern Turkey are made of hydromagnesite precipitated by diatoms and cyanobacteria.[7]

SEM micrograph of hydromagnesite showing platy crystal morphology. Sample was collected from the hydromagnesite-magnesite playas near Atlin, British Columbia, Canada.[8]

Microbial deposition of hydromagnesite is also reported from

CO2 sequestration.[8]

One of the largest deposits of hydromagnesite exists in Greece.[10] It consists of a natural mixture with huntite. Local people have used the white mineral as a source of material for whitewashing buildings for centuries. In the mid 20th century the minerals, ground to a fine powder, found use as a filler for rubber shoe soles. The locals used the granite mills designed for grinding wheat. Commercial exploitation of the minerals began in the late 70s and early 80s with the mineral being exported worldwide. The Greek deposit is still operated commercially, although the world's largest commercially operated reserves are in Turkey.

Uses

Its most common industrial use is as a mixture with

polymers.[11][12][13] Hydromagnesite decomposes endothermically,[5][6] giving off water and carbon dioxide, leaving a magnesium oxide residue. The initial decomposition begins at about 220 °C making it ideal for use as a filler in polymers and giving it certain advantages over the most commonly used fire retardant, aluminium hydroxide.[14]

Thermal decomposition

Hydromagnesite thermally decomposes in three stages releasing water and carbon dioxide.[5][6]

The first stage starting at about 220 °C, is the release of the four molecules of water of crystallisation. This is followed at about 330 °C by the decomposition of the hydroxide ion to a further molecule of water. Finally, at about 350 °C carbon dioxide begins to be released. The release of the carbon dioxide can be further broken down into two stages depending on the rate of heating.[6]

References

  1. S2CID 235729616
    .
  2. ^ a b Handbook of Mineralogy
  3. ^ a b Webmineral data
  4. ^ Mindat
  5. ^
    doi:10.1016/j.tca.2010.06.012
    .
  6. ^
    doi:10.1016/j.tca.2011.11.002
    .
  7. doi:10.1016/S0037-0738(96)00073-5
    .
  8. ^
    S2CID 128900805
    .
  9. ^ R. W. Renaut, Recent Mamgnesite-Hydromagnesite sedimesntation in Playa Basins of the Caribou Plateau, "Archived copy" (PDF). Archived from the original (PDF) on 2004-11-22. Retrieved 2009-08-13.{{cite web}}: CS1 maint: archived copy as title (link) British Columbia Geologic Survey
  10. ^ Georgiades, GN (1996). "Huntite-hydromagnesite production and applications". Proceedings of the 12th Industrial Minerals Congress: 57–60.
  11. doi:10.1016/j.polymdegradstab.2010.08.019
    .
  12. doi:10.1016/j.polymdegradstab.2012.01.024
    .
  13. S2CID 96208830
    .
  14. ^ Rothon. R., Particulate-Filled Polymer Composites, 2nd Edition, 2003
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