MAGNESITE

Magnesite – MgCO3 – is a relatively common anhydrous carbonate, which constitutes an important industrial mineral when it occurs in large volumes and with high purity, used in the manufacture of refractories.

Magnesite forms a series with gaspeite (carbonate of Ni, Mg, and Fe), another series with siderite (carbonate of Fe), and an incomplete solid solution with rhodochrosite (carbonate of Mn). It has 10 varieties, depending on the habit, impurities (Fe, Li, Ni, etc.) and color. It may contain Fe, Mn, Co, Ca, Ni and organic material.

Sometimes it exhibits pale green to pale blue fluorescence under UV light. It may be phosphorescent under UV light. It is triboluminescent. It dissolves readily with effervescence in hot hydrochloric acid. Ground magnesite (powder) effervesces with cold dilute HCl.

It is difficult to impossible to differentiate magnesite from other common carbonates such as calcite and dolomite, both macroscopically and under a microscope. Other analytical techniques are necessary.

1. Characteristics

Crystal system: Trigonal scalenohedral          

Color: Colorless, white, pale yellow, pale brown, somewhat pink, lilac-pink.     

Habit: Magnesite typically occurs as an earthy to compact, chalky white, porcelain-like substance, in these cases with extremely fine granulation, formed by submicroscopic crystals. It can be fibrous, fine-grained, or coarse-grained, in the latter case resembling marble. Cryptocrystalline magnesites contain silica such as opal or chert.  

Well-formed magnesite crystals are rare. They occur tabular or as rhombohedral or hexagonal prisms and can reach 30 cm in length.

Cleavage: {10-11} perfect (3 directions of cleavage forming rhombohedra, like calcite.)

Tenacity: Brittle.        

Twinning: Rare, lamellar.

Fracture: Conchoidal. 

Mohs Hardness: 3.5 – 4.5

Parting: No.  

Streak: White.

Lustre: Vitreous.   

Diaphaneity: Transparent.           

Density (g/cm³): 3.0 – 3.2  

 

2. Geology and Deposits

Magnesite occurs mainly as a product of metamorphism or alteration on magnesian igneous rocks. In contact or regional metamorphic terrains, it forms veins as a product of alteration of Mg-rich rocks such as serpentinites. It can be found in regolith formed by weathering of ultramafic rocks (pyroxenites, dunites, peridotites), where it is deposited as a consequence of the dissolution of minerals with Mn by CO2 from groundwater.

It can form as a primary mineral in igneous and sedimentary rocks. It occurs in skarns formed by metasomatism in dolomitic limestones or by metasomatism of peridotites. Occasionally it occurs in hydrothermal veins, evaporites, carbonatites and meteorites.

It is found as idiomorphic porphyroblasts in talc schists, mica schists and chlorite schists, from the alteration of calcite. It forms pseudomorphs on olivine. It occurs as gel-magnesite along with opal.

 

3. Mineral Associations

With quartz, uvite, florencite (Ce), hematite, svanbergite, fluorite, and serpentine. Furthermore, in metamorphic rocks, it is associated with calcite, dolomite, talc, antigorite, and chlorite. In skarns, it is associated with wollastonite, periclase, and talc.

 

4. Transmitted Light Microscopy

Refraction indices:  nα:  1.700           nγ: 1.509

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, gray, or pale; if Fe-rich, brownish. It does not have pleochroism.

Relief: The relief varies from low to moderate to high every 90º of rotation of the stage in crystals with well-defined cleavage. This phenomenon has been dubbed “relief pleochroism” and is typical of carbonates (calcite, dolomite, aragonite, siderite, rhodochrosite, and magnesite). When in submicroscopic crystals, these carbonates do not exhibit “relief pleochroism”. 

Cleavage: Perfect rhombohedral cleavage {10-11}. Two cleavages are observed, forming angles of 60º and 120º between them.

Habits:  Hypidiomorphic (subhedral) to xenomorphic (anhedral) aggregates; rarely as idiomorphic (euhedral) crystals. Also granular, fibrous, in compact porcelain masses or as gels.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.191 to 0.219: extremely high, resulting in pearly colors of high order (creams), difficult to classify. 

Extinction: Symmetrical with respect to the cleavage planes.

Elongation sign: No information available. 

Twins: rare, lamellar pressure twins, in {01-12}         

Zoning:  No information available.             

CONVERGENT LIGHT

Character: U(-)          

2V angle: No.         

Alterations: A very robust mineral, difficult to alter, but it can be partially altered to brucite. When it contains iron, it may show orange to red spots along and around its cleavages and fractures. 

May be confused with: It is not possible to differentiate trigonal carbonates from each other under a petrographic microscope. This is only possible with the use of a universal stage. Staining and acid solubility techniques are indicated for the identification of carbonates. The rarity or absence of twinning differentiates magnesite and dolomite from calcite (calcite generally exhibits many twins).

Magnesite 01: In PPL, coarse-grained magnesite from the Magnesita S/A deposit in Brumado, Bahia state (Brazil), observed with the lowest magnification objective, 2.5x. It is colorless and rhombohedral cleavage can be observed in some grains.
Magnesite 01: The same section as of the previous image, now in XPL. It shows the high-order interference colors typical of carbonates. No twinning is observed, which is characteristic of magnesite.
In PPL, white, massive, cryptocrystalline magnesite observed with the lowest magnification objective (2.5x). The structure of globular aggregates is probably due to microbial action. Cementation with larger magnesite crystals occurs between the massive globular aggregates. In XPL the image is practically the same, only slightly darker.
In PPL, magnesite from the same cryptocristalline sample from the previous image.

5. Reflected Light Microscopy

It is not possible to identify magnesite under a reflected light microscope because it exhibits the same characteristics as other carbonates such as calcite and dolomite.

Sample preparation: Magnesite acquires a good polish easily.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray.       

Pleochroism: Strong in shades of gray, easy to spot.      

Reflectivity: Very low (~4%)       

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy, partially masked by internal reflections.        

Internal reflections: Generally clear, white to milky. They may appear in duplicate due to the double refraction of magnesite.     

May be confused with: Other carbonates include calcite and dolomite.

Calcite exhibits twinning more frequently.       

General Characteristics: 

Cleavage is easily observed in aggregates formed by large grains.

Polishing pits are frequent due to the perfect rhombohedral cleavage of magnesite.

Magnesite 01: In PPL, coarse-grained magnesite. Because the crystals are large and the polishing was not done perfectly, there are many polishing pits due to the cleavage. Pleochroism is quite evident (lighter gray/darker gray).
Magnesite 01: The same section as of the previous image, now in XPL. Despite the colorless internal reflections, the strong gray anisotropy of magnesite is clearly visible.