Magnesite is a magnesium carbonate mineral that is the main source of magnesium. After the solution containing magnesium acts on calcite, it will turn calcite into magnesite, so magnesite also belongs to the calcite family. Magnesium-rich rocks also change into magnesite. Magnesite often contains iron, which is the result of iron or manganese replacing magnesium. Magnesite is white or off-white with a glassy luster, and iron-containing magnesite will appear yellow to brown. If the crystal is grainy, it is massive if it does not show crystals. In addition to magnesium refining, magnesite can also be used as a refractory material and a compound for the preparation of magnesium.
The Crystal chemistry of Magnesite:
Theoretical composition (wB%): MgO 47.81, CO2 52.19. The complete isomorphism can be formed between MgCO3 and FeCO3, and the content of FeO in natural magnesite is generally <8%. About 9% of FeO is called iron magnesite; the more abundant is called Lingmei magnesium. Sometimes, it contains a mixture of Mn, Ca, Ni, Si, and the like. Dense block often contains impurities such as opal and serpentine.
Structure and form:
Trigonal system, rhombohedral unit cell: arh = 0.566 nm, α = 48. 10'; Z = 2; hexagonal unit cell: ah = 0.462 nm, ch = 1.499 nm; Z = 6. Calcite structure. Complex three-sided triangular facet crystal, D3d-3m (L33L23PC).
rhombohedron r, f, hexagonal column m, a, parallel double-sided c, complex three-sided triangular facet v. Often a grainy or cryptocrystalline dense block. In the weathering zone, it is often cryptocrystalline.
Physical and chemical properties:
White or light yellow white, grayish white, sometimes with a reddish hue, iron-bearing yellow to brown, brown; ceramics are mostly snow white. Glass luster. With complete cleavage. The porcelain is a shell-like fracture. Hardness 4~4.5. Crisp. The relative density is 2.9~3.1. Both the iron-bearing density and the refractive index increase. The cryptocrystalline magnesite is densely packed and looks like an unglazed porcelain, so it is also called porcelain magnesite.
Under the polarizer:
the one-axis crystal (-), the refractive index and the birefringence ratio become larger as the iron content increases, and have a significant double reflection.
magnesite with pinkish gray reflection color, light pink internal reflection color, common shape crystal, does not have polycrystalline twin crystal, easier to distinguish from other carbonate minerals.