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Dominant species
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Hydromagnesite
Hydromagnesite
Chemical
Formula
Mg5(CO3)4(OH)2·4H2O
Species
Carbonates
Crystal
System
Monoclinic
Mohs
Scale
3
Specific
Gravity
2.16-2.2
Color
Colorless, white
Streak
White
Luster
Vitreous, Silky, Pearly, Earthy
Refractive
Index
n = 1.523 n = 1.527 n = 1.545
Diaphaneity
Transparent
Cleavage
PerfectOn .
Fracture
Uneven
Crystal Habit:Acicular and as encrustations
Geological Setting:Low-temperature, hydrothermal environments in serpentine and altered magnesium-rich igneous rocks.
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. It thermally decomposes, 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, Hudson County, New Jersey.

Stromatolites in an alkaline (pH greater than 9) freshwater lake (Salda G?lü) in southern Turkey are made of hydromagnesite precipitated by diatoms and cyanobacteria. Microbial deposition of hydromagnesite is also reported from playas in British Columbia.

One of the largest deposits of hydromagnesite exists in Greece. 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. Hydromagnesite decomposes endothermically, 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.

Thermal decomposition

Hydromagnesite thermally decomposes in three stages releasing water and carbon dioxide.

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.


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