PEROVSKITE

Perovskite – CaTiO3 – is a rarer oxide; it usually occurs as an accessory mineral in some types of plutonic rocks. It has economic interest due to its content of Rare Earth Elements such as Ce, La, Nd, and others. These REEs have given rise to several special names for perovskite, such as knopite, dysanalyte (perovskite with up to 5-10% Nb), uhligite, and loparite. Perovskite is also a titanium ore.

It is classified in the Perovskite Group and may contain Na, Fe, Ce, Sr, Nb, and REEs. It forms pseudo-cubic and pseudo-hexagonal crystals up to 12 cm, typically with polysynthetic twinning along {111}. Simple twinning along {110} and {100}, lamellar, and sectorized twinning is rarely found. The twinning is visible only in larger crystals. The crystals may be highly distorted and resemble dark galena cubes.

Perovskite gives its name to a class of substances that possess the same type of crystalline structure and is interesting for its properties of superconductivity, magnetoresistance, ionic conductivity, and dielectric properties, which are important in microelectronics and telecommunications.

1. Characteristics

Crystal system: Orthorhombic bipyramidal.          

Color: Dark brown, black, reddish-brown, shades of yellow.     

Habit: Pseudocubic crystals, Granular, massive, reniform, skeletal, dendritic.       

Cleavage: {001} poor. Striations // at {001} on the faces of the cube.

Tenacity: Brittle.        

Twinning: Complex twinning, see above.      

Fracture: Irregular, subconchoidal.       

Mohs Hardness: 5.5

Parting: No.         

Streak: Colorless, grayish-white.         

Lustre: Adamantine, metallic to submetallic, dull. 

Diaphaneity: Transparent. 

Density (g/cm³): 3.98 to 4.26

 

2. Geology and Deposits

Perovskite is found as an accessory mineral in mafic, ultramafic, and alkaline igneous rocks, such as nepheline syenites, pyroxenites, mellilitholites, and lamprophyres. Varieties containing REEs are concentrated in alkaline plutonic rocks.

It also occurs in kimberlites, as reaction boundaries around titaniferous ilmenite grains and as discrete crystals in the matrix.

It occurs in calcareous skarns in contact with basic or alkaline igneous rocks, frequently in varieties containing Ce and Nb.

It is rare in carbonatites, where it occurs idiomorphically in cubic or octahedral crystals. It can occur in chlorite schists, talc schists, and chondritic meteorites.

In magmatic titanomagnetite deposits, perovskite contains the highest Ti content, making the magnetic concentrate poorer in Ti.

 

3. Mineral Associations

In skarns it occurs with calcite, chlorite (clinochlorine), pyrite, pyrrhotite, garnet (andradite), monticellite and many other minerals.

In igneous rocks it occurs with feldspars (plagioclase), feldspathoids (nepheline, leucite), clinopyroxenes (augite), melilite, magnetite, melanite garnet, titanite, apatite and fluoroapatite.

 

4. Transmitted Light Microscopy

Refraction indices: nα: 2,300     nβ:  2,340     nγ: 2,380

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Variable, pale to deep violet-brown, amber-yellow, intense yellow, rarely green to colorless, with subtle pleochroism, when present.

Concentric color distribution is possible.

Very small crystals appear opaque because they reflect all light.

Relief: Extra-high (one of the highest known reliefs).           

Cleavage: {001} poor.  

Habits: Typically cube-shaped, more rarely octahedron-shaped (triangular sections). It can be pseudohexagonal, with sections having 6 sides. It rarely occurs with a skeletal habit.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Small crystals can simulate isotropy, but in slightly larger crystals the perovskite presents a weak birefringence.

Birefringence varies up to 0.080, with colors up to 4th order, difficult to classify. Birefringence is greater in perovskites containing REEs.           

Extinction: It never becomes completely extinct.

Elongation sign: Not applicable.

Twins: Abundant and widespread, polysynthetic along {111}, rarely simple twins along {110} and {100}, lamellar and sectorized, visible only in larger crystals.

As it is paramorphic, all larger crystals show polysynthetic twins like leucite, in addition to anomalous interference colors.

Due to the low birefringence of perovskite, it is useful to insert the condenser at NC to observe the polysynthetic twins.

Zoning: Frequently zoned.  

CONVERGENT LIGHT

Character: B(+), normally isotropic.

2V angle: ~90º 

Alterations: It is usually unaltered. 

May be confused with: Its restricted occurrence facilitates its identification. It is never associated with quartz.

Spinel (picotite) and melanite garnet have a much lower relief.

Rutile occurs in different parageneses.

Pyrochlore has a similar relief and color.

Perovskite 01: In PPL, perowskite grain with typical strong color and high relief. With biotite (brow/yellow) and clinopyroxene(?).
Perovskite 01: The same section as of the previous image, now in XPL. Perovskite exhibits dark (anomalous) interference colors and polysynthetic twins that, in this grain, resemble plagioclase, but are generally irregular, resembling leucite twins.
In XPL, perovskite grain. Due to the dark interference colors, polysynthetic twins are sometimes difficult to perceive. Care must be taken to ensure they are not overlooked.
In XPL, perovskite grain. Due to the dark interference colors, polysynthetic twins are sometimes difficult to perceive. Care must be taken to ensure they are not overlooked.
In XPL, perovskite grain. Due to the dark interference colors, polysynthetic twins are sometimes difficult to perceive. Care must be taken to ensure they are not overlooked.

5. Reflected Light Microscopy

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

Sample preparation: Perovskite slowly acquires an excellent polish. 

PLANE POLARIZED LIGHT – PPL

Reflection color: Medium bluish gray, varies with variations in chemical composition.

Compared to the color of magnetite and ilmenite, the color of perovskite is darker, with a more defined dark blue tone.       

Pleochroism: No.      

Reflectivity: 16,31% . If with Nb, 15,6%        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Isotropic, without anisotropy.        

Internal reflections: Abundant and strong, always present: colorless, brown in various shades, pink, and orange.

Sometimes internal reflections allows us to recognize the complicated twinning.

May be confused with: Several other minerals, it is very difficult to identify perovskite under reflected light.

Chromite has much darker internal reflections.

Magnetite is more pink and less bluish and has no internal reflections.

Sphalerite and greenockite, which are very similar, occur in completely distinct parageneses.

General Characteristics:

Grain shape: Perovskite generally forms isolated grains or friable aggregates of grains. The rule, however, is small, isolated crystals in extremely calcium-rich basaltic rocks, where they easily go unnoticed.

Cleavage is not visible in very small crystals.

Lamellar twinning in complex patterns may become visible due to internal reflections.

Dissolution was not observed.

Substitutions 1: Perovskite can be replaced by rutile and anatase.

Substitutions 2: Perovskite can replace ilmenite.

Intergrowths with magnetite and ilmenite may occur.

Inclusions of heazlewoodite, hematite, and ilmenite may occur in perovskite. 

Caption.