PERICLASE

Periclase – MgO – is a rare oxide, typical of some dolomitic marbles. It is not an ore.

Periclase generally has a stoichiometric composition, but up to 10% Fe can replace Mg. When high in Fe, it constitutes the ferro-periclase variety which, when altered, forms brucite and iron oxides separately. Some sources consider the existence of a solid solution between periclase and wüstite (FeO). Synthetic periclase is called lavanite.

It is slightly soluble in water and in dilute hydrochloric acid after pulverization. It exhibits yellow fluorescence under long-wave UV light.

1. Characteristics

Crystal system: Cubic hexaoctahedral.          

Color: Colorless, grayish-white, may be yellow, brownish-yellow, or black due to inclusions. 

Habit: Granular, massive, also octahedrons, more rarely cuboctahedrons or dodecahedrons.

Cleavage: {001} perfect, {111} good.

Tenacity: Brittle.        

Twinning: In synthetic crystals it exhibits spinel twinning, by {111}.

Fracture: Conchoidal to irregular.       

Mohs Hardness: 5.5

Parting: on {011}.         

Streak: White.         

Lustre: Subvitreous.          

Diaphaneity: Transparent.           

Density (g/cm³):  3.56 – 3.68

          

2. Geology and Deposits

Periclase forms in siliceous dolomitic marbles or dolomites from the dissociation of dolomite during high-temperature contact metamorphism. Thus, it is found in contact aureoles around intrusive magmatic rocks. In the type locality (Monte Somma, Italy) it was found in dolomitic limestone xenoliths in the lava.

In the sequence of minerals formed by progressive metamorphism of siliceous dolomites, periclase forms after wollastonite, but at lower temperatures than monticelite.
Supergene alteration forms brucite (Mg(OH)2) or hydromagnesite (Mg5(CO3)4(OH)2.4H2O).

It is found in chondritic meteorites and as an inclusion in kimberlite diamonds. It can form by pyrometamorphism, by fires in coal mines.

Periclase is the main constituent of basic refractory bricks, manufactured with magnesite.

 

3. Mineral Associations

It occurs associated with:

– carbonates such as calcite, dolomite, magnesite, hydromagnesite, spurrite and thaumasite,

– hydroxides such as brucite and portlandite,

– silicates such as olivine (forsterite), serpentine, humite group minerals and cuspidine,

– other oxides such as corundum, spinel, perovskite, cassiterite, hematite, anatase, rutile, srebrodolskite and hibonite,

– sulfates such as anhydrite and fluoro-ellestadite and

– sulfides such as oldhamite. It also occurs with fluorite.

 

4. Transmitted Light Microscopy

Refraction indices:  n: 1.735 – 1.756, increases with increasing Fe content.

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, it may show pale yellow or brown tones if it has a high iron content, but it never shows pleochroism.

Relief: High.           

Cleavage: {001} perfect, {111} good, can show partition in {011}.

Habits: Massive, granular, it may show an envelope (crust) of fibrous brucite. It can form octahedrons, cuboctahedrons, and dodecahedrons, but these idiomorphic forms are rarer.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Isotropic.           

Extinction: Isotropic.           

Elongation sign: Isotropic.            

Twins: Isotropic.         

Zoning: Isotropic.             

CONVERGENT LIGHT

Character: Isotropic.          

2V angle: Isotropic.         

Alterations: It easily alters (hydrates) to fibrous brucite. Brucite is very similar to serpentine, but brucite has a medium relief (serpentine (antigorite) = low relief) and more intense interference colors (maximum birefringence of 0.020; serpentine (antigorite) with a maximum birefringence of 0.006).

The ferro-periclase variety, when it alters, forms brucite and iron oxides separately.          

May be confused with: Isotropy and cubic cleavage, as well as paragenesis, are very typical.

Garnet, is similar, but garnet does not have cleavage, is much more common and far more typical of other parageneses.

Spinel is more colorful and does not exhibit cleavage.

Caption

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying periclase. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with periclase, such as hematite.

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption