HEDENBERGITE

Hedenbergite – CaFe2+Si2O6 – is an inosilicate, a rarer monoclinic pyroxene (clinopyroxene), characteristic of very specific paragenesis. It has no economic importance.

It is part of the diopside – augite – hedenbergite solid solution series and may contain, as impurities, Ti, Mg, Mn, Na, K, Al and Zn. It forms epitaxy with babingtonite and has 3 varieties.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Red brown, black.     

Habit: Short prismatic, columnar, acicular, granular, lamellar, massive.       

Cleavage: {110} good, like all pyroxenes.       

Tenacity: Brittle.        

Twinning: It can have simple and polysynthetic twins according to {100} and {100}       

Fracture: Irregular, conchoidal.       

Mohs Hardness: 5.5 – 6.5

Parting: On {100} e {010}         

Streak: White, gray.         

Lustre: Vitreous, dull.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.56

          

2. Geology and Deposits

Hedenbergite is a clinopyroxene typical of syenites and alkaline granites.

It also occurs in metamorphosed iron formations and in skarns (Fe-rich siliceous carbonates) of amphibolite facies.

It was found in xenoliths in kimberlites.

 

3. Mineral Associations

Hedenbergite occurs, in magmatic rocks, with augite and pigeonite (usually in the matrix), plagioclase, olivine and feldspathoids (nepheline and leucite).

In contact metamorphic rocks, it is associated with calcite, diopside, garnets (andradite, grossular), vesuvianite, olivine (forsterite), wollastonite and clinozoisite.

It is also associated with ilvaite, prasium, hematite, arsenopyrite, titanite and fluorite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.699 – 1.739    nβ: 1.705 – 1.745    nγ: 1.728 – 1.757

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Weakly pleochroic in green-brown:

X = pale green or bluish green;

Y = green, bluish green;

Z = green, yellowish green.

Relief: High.           

Cleavage: Cleavage {110} perfect, like all pyroxenes:

– in the longitudinal sections only one cleavage direction,

– in the basal sections two cleavages that intersect at angles of 87 and 93º.           

Habits: Short prismatic (typical of pyroxenes), long columnar, granular to acicular. Also massive. Base sections with 8 sides.

May show folding and lamellar exsolutions of orthopyroxenes. 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.005 to 0.029: intense colors up to 2nd order green.

Extinction: Oblique, with increasing obliquity with increasing Fe content.

Elongation sign: No information available.   

Twins:  Simple and polysynthetic on {100} and {010}, forming pyroxene lamellae that are oriented obliquely to the cleavages. 

Zoning: Frequently zoned.             

CONVERGENT LIGHT

Character: B(+)          

2V angle:  58 – 63o        

Alterations: can alter in three ways.

High temperature pneumatolytic to hydrothermal alteration leads to uralitization: fibrous aggregates (brush texture) of amphiboles (tremolite or actinolite).

Low temperature hydrothermal alteration leads to serpentinization.

Weathering produces carbonates, hematite and quartz.          

May be confused with: several other minerals.

Augite has no colors in PPL with blue tones, and its interference colors are higher.

Olivine has higher birefringence, parallel extinction and shows no cleavage.

Omphacite and jadeite occur in different paragenesis.

Amphiboles such as green hornblende are more strongly pleochroic, have a different cleavage pattern, and tend to be B(-).

Orthopyroxenes have parallel extinction and are not green in PPL.

Fe-rich members of the Epidote Group have anomalous interference colors and different cleavages.

Arfvedsonite has much darker colors, amphibole cleavage (in the basal sections two cleavage directions intersect at angles of 56 and 124º) and lower interference colors.         

In PPL, dense aggregate of prismatic crystals of hedenbergite. Observing the larger crystals, they clearly shows the green color with blue tones, an important diagnostic feature.
In XPL, hedenbergite aggregate (colorful) with some quartz (white). In XPL, hedenbergite presents strong and colorful interference colors, from gray to 2nd order blue and green.
Hedenbergite 01: Hedenbergite crystal showing, in this position, its first pleochroism color. The pleochroism is very weak, almost imperceptible. The next image shows the second pleochroism color.
Hedenbergite 01: The same crystal of the previous image, again in PPL but now rotated 90 degrees to show the second pleochroism color opf hedenbergite. It is necessary to observe the thin section carefully to perceive this diagnostic pleochroism.
In PPL, basal section with 8 sides of hedenbergite. These sections have lower interference colors.
Twinned hedenbergite in XPL. The grain was fractured and the two portions shifted relative to each other.
Hedenbergite 02: In PPL, in a skarn, hedenbergite with calcite, ilvaite (not shown in this image) and arsenopyrite (opaque = black). The green color of the hedenbergites is visible, but pleochroism is weak and not easy to observe.
Hedenbergite 02: The same section as of the previous image, now in XPL. Stronger interference colors of hedenbergite are better seen in larger crystals.
Hedenbergite 03: In PPL, roughly radial aggregate of hedenbergite in skarn, with calcite, ilvaite and arsenopyrite. As the hedenbergite crystals are fine fine, the green color is almost not visible.
Hedenbergite 03: The same section as of the previous image, now in XPL. Strong interference colors, oblique extinction, habit and paragenesis are diagnostic for hedenbergite.
Hedenbergite 04: In PPL, zoned clinopyroxene in phonolite, with plagioclase (colorless) and melanite garnet (brown). Under the microscope, the optical characteristics indicate that clinopyroxene may be an aegirine-augite, a diopside or a hedenbergite, but in the electron microprobe it was found that in the phenocrysts the nucleus is of diopside and the edge of hedenbergite. In the matrix the clinopyroxenes are hedenbergite.
Hedenbergite 04: The same section as of the previous image, now in XPL. Plagioclase in gray, melanite is isotropic (black) and this pyroxene shows yellow interference colors.

5. Reflected Light Microscopy

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

Sample preparation: like all pyroxenes, hedenbergite acquires only a reasonable polish because of the cleavage it presents. Depending on the orientation of the cleavage in relation to the plane of the polished section, the polish can be good (cleavage parallel to the section plane) or very poor (cleavage intersecting the section plane). Polishing is usually not of good quality.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray tones.       

Pleochroism: No.      

Reflectivity: Low (4-8%)        

Bireflectance: Distinct.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Grayscale anisotropy, often masked by internal reflections.        

Internal reflections: Intense, colorless to green and brown, usually very dark. Green tones are hard to see.      

May be confused with: it is not possible to identify hedenbergite by Reflected Light. Knowing of its existence in the section, it is easy to identify. 

Hedenbergite 01: In PPL, hedenbergite (lighter gray) and carbonate (darker gray). Hedenbergite shows many polishing imperfections, including lined up polishing pits.
Hedenbergite 01: The same section as of the previous image, now in XPL. Hedenbergite is very dark, but with bright lighting and a little goodwill, it is possible to identify green tones in the internal reflections. Carbonate with yellowish reflections and twins, perhaps it is a siderite.