Hydromagnesite

Hydromagnesite
Hydromagnesite balloon in Jewel Cave
General
Category	Carbonate mineral
Formula (repeating unit)	Mg5(CO3)4(OH)2·4H2O
Strunz classification	05.DA.05
Dana classification	16b.07.01.01
Crystal system	Monoclinic - Prismatic (pseudo-orthorhombic)
Identification
Formula mass	467.64
Color	Colorless, white
Crystal habit	Acicular and as encrustations
Twinning	Polysynthetic lamellar on {100}
Cleavage	{010} Perfect, {100} Distinct
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	3.5
Luster	Vitreous, silky, pearly, earthy
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.16 - 2.2
Optical properties	Biaxial (+)
Refractive index	nα = 1.523 nβ = 1.527 nγ = 1.545
Birefringence	δ = 0.022
Ultraviolet fluorescence	Fluorescent, short UV=green, long UV=bluish white.
References	[1][2][3]

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 serpentine or brucite. It occurs as incrustations and vein or fracture fillings in ultramafic rocks and serpentinites. It occurs in hydrothermally altered dolomite and marble. It 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.^[1] It thermally decomposes,^[4][5] over a temperature range of approximately 220 °C to 550 °C, releasing water and carbon dioxide leaving a magnesium oxide residue.

It was first described in 1836 for an occurrence in Hoboken, New Jersey.^[2]

Stromatolites in an alkaline (pH greater than 9) freshwater lake (Salda Gölü) in southern Turkey are made of hydromagnesite precipitated by diatoms and cyanobacteria.^[6]

SEM micrograph of hydromagnesite showing platy crystal morphology. Sample was collected from the hydromagnesite-magnesite playas near Atlin, British Columbia, Canada.^[7]

Microbial deposition of hydromagnesite is also reported from playas in British Columbia.^[8] The hydromagnesite-magnesite playas near Atlin, British Columbia are some of the most studied deposits of hydromagnesite. These deposits have been characterized in the context of a biogeochemical model for CO₂ sequestration.^[7]

One of the largest deposits of hydromagnesite exists in Greece.^[9] 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 huntite as a flame retardant or fire retardant additive for polymers.^[10][11][12] Hydromagnesite decomposes endothermically,^[4][5] 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.^[13]

Thermal decomposition

Hydromagnesite thermally decomposes in three stages releasing water and carbon dioxide.^[4][5]

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.^[5]

References

1. Handbook of Mineralogy
2. Webmineral data
3. Mindat
4. Hollingbery, LA; Hull TR (2010). "The Thermal Decomposition of Huntite and Hydromagnesite - A Review". Thermochimica Acta 509 (1-2): 1–11. doi:10.1016/j.tca.2010.06.012.
5. Hollingbery, LA; Hull TR (2012). "The Thermal Decomposition of Natural Mixtures of Huntite and Hydromagnesite". Thermochimica Acta 528: 45–52. doi:10.1016/j.tca.2011.11.002.
6. Braithwaite, C.; Zedef, Veysel (1996). "Living hydromagnesite stromatolites from Turkey". Sedimentary Geology 106 (3-4): 309. Bibcode:1996SedG..106..309B. doi:10.1016/S0037-0738(96)00073-5.
7. Power, I.M.; Wilson, S.A.; Thom, J.M.; Dipple, G.M.; Gabites, J.E.; Southam, G. "The hydromagnesite playas of Atlin, British Columbia, Canada: A biogeochemical model for CO₂ sequestration". Chemical Geology 206: 302–316. doi:10.1016/j.chemgeo.2009.01.012.
8. R. W. Renaut, Recent Mamgnesite-Hydromagnesite sedimesntation in Playa Basins of the Caribou Plateau, http://www.em.gov.bc.ca/DL/GSBPubs/GeoFldWk/1990/279-288-renaut.pdf British Columbia Geologic Survey
9. Georgiades, GN (1996). "Huntite-hydromagnesite production and applications". Proceedings of the 12th Industrial Minerals Congress: 57–60.
10. Hollingbery, LA; Hull TR (2010). "The Fire Retardant Behaviour of Huntite and Hydromagnesite - A Review". Polymer Degradation and Stability 95 (12): 2213–2225. doi:10.1016/j.polymdegradstab.2010.08.019.
11. Hollingbery, LA; Hull TR (2012). "The Fire Retardant Effects Huntite in Natural Mixtures with Hydromagnesite". Polymer Degradation and Stability 97 (4): 504–512. doi:10.1016/j.polymdegradstab.2012.01.024.
12. Hull, TR; Witlowski A; Hollingbery LA (2011). "Fire Retardant Action of Mineral Fillers". Polymer Degradation and Stability 96 (8): 1462–1469. doi:10.1016/j.polymdegradstab.2011.05.006.
13. Rothon. R., Particulate-Filled Polymer Composites, 2nd Edition, 2003

Radial sprays of glassy hydromagnesite needles are scattered on pastel-green magnesite (coloured by nickel impurities) with an unusual knobby/bubbly/drusy form. The specimen is from the Cedar Hill Quarry, Fulton Township, Lancaster County, Pennsylvania. Size: 7.3 x 5.5 x 3.1 cm.