SPINEL

Spinel –  MgAl2O4 – is a common oxide that occurs mainly as an accessory mineral of igneous rocks. It has economic importance only as a gemstone, imitating ruby (red corundum) and sapphire (blue corundum). Synthetic spinel in sheets resembles glass, but has much greater strength.

The spinel belongs to the Spinel Group and forms three series: the spinel-hercynite FeAl2O4 series, the spinel-gahnite ZnAl2O4 series and the spinel-magnesiochromite Mg(Cr,Al,Fe)2O4 series. The individual crystals reach up to 30 cm. There are 16 varieties of spinel, based on colors or higher levels of certain impurities.

It may contain Fe, Zn, Al, Ca, Mn and Ti. The hardness, refractive indices and density vary greatly with the chemical composition: Mg-Al spinels have hardness 8, Al spinels, more frequent in deposits of ores, have hardness 5. 

1. Characteristics

Crystal system:  Cubic hexaoctahedral.         

Color: Red, blue, pink, dark green, brown, black, colorless, orange, yellow, indigo, violet.     

Habit: It forms octahedrons, usually twinned. Massive, rounded, coarse granular, compact.       

Cleavage: No.       

Tenacity: Brittle.        

Twinning: Spinel can be twinned by the “Spinel Law”, on {111}, in sixlings.       

Fracture: Conchoidal, irregular.       

Mohs Hardness: 7,5 – 8

Parting: On {111}.         

Streak: Grayish white.        

Lustre: Vitreous, dull.          

Diaphaneity: Transparent.           

Density (g/cm³):  3.6 – 4.1, increases with Fe and Zn content.

 

2. Geology and Deposits

Spinel is a common mineral, formed at high temperatures as an accessory mineral in ultramafic plutonic igneous rocks, such as peridotites, dunites, harzburgites, lherzolites and pyroxenites (websterites).

It occurs in kimberlites, xenoliths and in exotic rocks such as spinellites.

In metamorphic rocks, it occurs in cornubianites (hornfels) and carbonate rocks that have undergone contact metamorphism (marble and skarn). Also in buchites, which are glassy rocks produced by contact metamorphism.

It occurs in metapelites (schists) formed by regional metamorphism and can be found in pegmatites. Because it is very resistant, it is detrital, present in sediments and clastic sedimentary rocks.

 

3. Mineral Associations

In basic rocks (peridotites) it is associated with chromite, olivine (forsterite), clinopyroxenes, pargasite and scapolite.

In carbonate rocks (marble, skarn) it occurs with carbonates (calcite, dolomite), olivine (forsterite), clinohumite, chondrodite, graphite, serpentine, scapolite, pargasite and phlogopite.

In metamorphic rocks (schists) it is found with garnet (almandine), minerals from the Al2SiO5 Group (sillimanite, andalusite), cordierite, corundum, pargasite and quartz.

Also with galena, chalcopyrite, sphalerite, pyrite, graphite, rutile and hematite.

 

4. Transmitted Light Microscopy

Refraction indices:  n: 1,719

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Highly variable: colorless, light green or dark green, red, blue, yellow, brown, black.

Anomalous pleochroism only in some blue zinc-containing varieties.          

Relief: Extra high, very high.           

Cleavage: Does not have, only a partition in (111) of poor quality (indistinct).

Exception: spinel in marbles to forsterite (olivine) shows good cleavage according to {100}.           

Habits: Normally anhedral grains, may form octahedra or show cube or dodecahedron faces. The sections are rhombic.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Isotropic.

Extinction: Isotropic.           

Elongation sign:  Isotropic.           

Twins: Are not visible under the microscope, but are common according to {111} (Spinel Law), interpenetration or contact twins, can form radial groups of 6 individuals (sixlings).         

Zoning: Frequently zoned.             

CONVERGENT LIGHT

Character: Isotropic.          

2V angle: Isotropic.         

Alterations: does not alters, is detrital and may occur concentrated in sands.         

May be confused with:  spinel is the only common isotropic mineral with intense colors.

Garnet can show pink color, but much fainter than spinel colors. 

Green spinel in PPL. In the center of the image is a large grain; around it several small grains. The color is deep green, very dark, may look opaque (black) at first glance. The very high relief is characteristic, as well as the absence of cleavage and anhedral shapes. In XPL, the spinel is isotropic (black at 360° rotation of the stage).
Deep brown spinels in PPL. In XPL, they are isotropic.
In PPL, deep green spinels. In XPL, they are isotropic (black).
Yellow-greenish spinel, in PPL. Idiomorphic forms are very rare in spinel.
Yellow spinel in PPL.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of spinel. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with spinel.

Sample preparation: Spinel polishing takes a little time due to its high hardness, but it is possible to obtain a good polish.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray.       

Pleochroism: No.      

Reflectivity:  ~6 – 9%.       

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Isotropic, may appear completely black, with no internal reflections.        

Internal reflections: Many internal reflections, colorless, intense green, more rarely brown or brown.      

May be confused with: spinel identification is difficult. Considering the paragenesis, the possibilities of mistakes decrease. Very typical are the lamellae for demixing spinel in magnetite, chromite and ilmenite.

Garnets can be similar.

Fluorite usually has color zonation and occurs in other paragenesis.

General Characteristics: 

Grain shape: it is often idiomorphic (euhedral) or idioblastic, forming octahedra. Very common, however, are xenomorphic (anhedral) grains.

Cleavage was not observed.

Unmixing (exsolutions) OF spinel occur in many magnetites.

Unmixing (exsolutions) IN spinel may be of magnetite, which may be decomposed into magnetite + ilmenite ± ulvospinel. Rarely, unmixing bodies are made of ilmenite.

Lammelar twins on (111) can be recognized at times.

Edge decomposition occurs in some spinels of titanomagnetite differentiates, forming crusts of idioblastic corundum (+hogböhmite) that extend into the spinel.

Spinel 01: In PPL, at left, spinel. At right, orthopyroxene. The polish of the spinel is much better than the polish of ortopyroxene. Holes in black.
Spinel 01: The same section as of the previous image, now in XPL. Spinel is isotropic and orthopyroxene shows brownish internal reflections.
Spinel 02: In PPL, aligned spinel crystals (diagonally, almost black, low reflectivity) as exsolution in magnetite (brown-pink) partially martitized to hematite (white).
Spinel 02: The same section as of the previous image, now in XPL. Spinel with yellowish internal reflections, magnetite isotropic (black), hematite in thin white paralell lamellae.
Spinel 02: From the same sample as the two previous images, now in detail the idiomorphic spinel unmixings in magnetite. XPL.