GREEN HORNBLENDE

“Green Hornblende” is not a mineral, but just a term referring to very common rock-forming silicates of the Amphibole Supergroup – (Na,K)0-1Ca2(Mg,Fe,Al)5(Si,Al)8O22(OH)2. Hornblendes are not an ore.

Thus, “green hornblende” is a generic term used for dark green to black amphiboles of complex chemical compositions, which form 3 series: the Barroisite Series, the Hastingsite Series and the Tschermakite Series. The classification of amphiboles is complex and is constantly being updated. Check the latest literature on this. It is not possible to differentiate the minerals of the minerals under the microscope.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Dark green to black.

Habit: Prismatic, granular, massive, fibrous.

Cleavage: {110} perfect, with angles of 56º and 124º in the basal sections.

Tenacity: Brittle.

Twinning: Simple and lamellar parallel to {100}.

Fracture: Irregular.

Mohs Hardness: 5 – 6

Parting: On {100} e {010}.

Streak: Pale green to gray and colorless.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 3.0 – 3.4

 

2. Geology and Deposits

Green hornblende is the most common amphibole in intrusive igneous rocks and the most abundant in acidic and intermediate rocks, occurring in granites, granodiorites, tonalites, monzonites, quartz-diorites and diorites. Also in syenites, diorites, anorthosites and others. In volcanic rocks it occurs in dacites, rhyodacites, andesites, phonolites, dolerites and others.

It is less common in basic and ultrabasic rocks, but occurs in some gabbros, norites, peridotites and pyroxenites where other amphiboles can also be found.

In metamorphic rocks it is more abundant in high-grade regional metamorphic rocks, such as schists, gneisses, granulites and amphibolites. It can occur in cornubianites.

It can be found in some immature sediments such as graywacke in the form of clastic grains.

 

3. Mineral Associations

Green hornblende occur with many other rock-forming minerals: quartz, potassic feldspar (orthoclase, microcline, sanidine), plagioclase, mica (biotite, muscovite), titanite, apatite, epidote, garnets (grossular and melanite), rutile, other amphiboles ( actinolite), magnetite, chamosite and many others.

It also occurs in rocks with feldspathoids, associated with leucite, cancrinite, analcime, haüyna, sodalite and nepheline.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.687-1.694     nβ: 1.700 – 1.707     nγ: 1.701 – 1.712    (Fe-hornblende)

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Usually strongly colored in green, yellow-green, blue-green and brown. Strong pleochroism in these colors, mainly in various shades of green and brown. Green varieties are typically

X = light yellow-green,  

Y = green or gray-green, 

Z = dark green.

Brown varieties have X = greenish yellow-brown, Y = yellowish to reddish brown, and Z = gray to dark brown.

Relief: Moderate to high.

Cleavage: {110} perfect, resulting in two cleavages that intersect at 56º and 124º angles (diamond shape) in the basal sections. In the prismatic sections, there is only one cleavage. In addition, there are {100} and {001} partitions, which are difficult to see under a microscope.

Habits: Columnar, fibrous, fine to coarse granular, euhedral to anhedral. Short to long prismatic crystals. In the basal sections they are often pseudohexagonal or rhombic (diamonds). Longitudinal sections are prismatic.

Pleochroic (dark) halos around radioactive inclusions such as zircon and monazite are common, as in biotite.

Large numbers of inclusions can generate sieve texture. Alteration of pyroxenes generate very fine-grained masses of green hornblende.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.014 to 0.023 – interference colors normally between the end of the 1st order end and the beginning of the 2nd order: orange, red, blue, green, various shades of pink and violet.

Extinction: Oblique with angles from 14º to 22º (most common between 14º and 25º) in the longitudinal sections. Symmetrical extinction in the basal sections.

Elongation sign: ES(+). Important diagnostic feature!

Twins: Single or lamellar twins at {100} are common.

Zoning: Common.

CONVERGENT LIGHT

Character: B(-), more rarely B(+).

2V angle: 35 – 90o, common between 65 and 85o.

Alterations: often alters (uralitization) to a mixture of tremolite and actinolite, sometimes with epidote. It also alters to biotite (as a border around the grain), chlorite, epidote and calcite.

In volcanic rocks, green hornblende may be replaced by brown hornblende due to oxidation of Fe2+ to Fe3+; the process may be accompanied by partial or total opacification; this process starts along cleavages and fractures.

Retrograde metamorphism leads to the formation of actinolite and then chlorite or antigorite.

Weathering transforms hornblende into carbonate, limonite and quartz.

May be confused with: few other minerals. The colors, the pleochroism and the cleavages are very characteristic.

Biotite has parallel and mottled extinction.

Sodic amphibole tend to be bluish.

Actinolite has pleochroism in much weaker green colors and lower interference colors.

Pyroxenes are generally colorless and have 2 cleavages that cross at ~90° in the basal sections.

Tourmaline has parallel extinction and no cleavage.

Hornblende 01: In PPL, in the center of the image, two longitudinal sections of green hornblende with one cleavage direction and, on their right, a basal (rounded) section with two cleavage directions. Its strong pleochroism in green is clear. The hue of the green can change, but the colors are stronger than the green colors of actinolite and weaker than the green colors of arfvedsonite.
Hornblende 01: The same section as in the previous figure, now in XPL, showing the strong interference colors. Around the hornblendes, quartz and feldspars.
Basal section of green hornblende in PPL, showing the two cleavage directions that intersect in a diamond pattern (56º and 124º).
Single contact twin in green hornblende in XPL. The straight line that divides the grain into two portions with different interference colors is the twin plane.
Hornblende 02: In PPL, several grains of green hornblende with its pleochroism in various shades of green.
Hornblende 02: The same section as of the previous figure, now in XPL. Strong interference colors are typical for green hornblende.
Hornblende 03: In PPL, green hornblendes in an amphibolite, forming a very dense aggregate of small crystals with some feldspars among them. The field of view is of 7 mm. The strong green colors are easily seen, but the pleochroism needs to be observed with higher magnification.
Hornblende 03: The same section as of the previous image, now in XPL. The strong interference colors of the green hornblendes stand out. The rock is crossed by a quartz vein.

5. Reflected Light Microscopy

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

Sample preparation: the polishing of green hornblende, despite of its cleavage, is not difficult and is similar to the polishing of other silicates that accompany it, such as quartz and feldspars.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, a little lighter than quartz and feldspars and much lighter than biotite.

Pleochroism: No.

Reflectivity: Low (<10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: If there is anisotropy, it is masked by internal reflections.

Internal reflections: Generalized in shades of dirty green, ranging from very light to almost black. The shade of green depends on the thickness of the grain observed: the finer the grain, the lighter the green. At the edges of the grains, lighter green colors are observed.

May be confused with: pyroxenes, which have even darker internal reflections and have habits that tend to be granular (short prismatic), while hornblendes usually have more or less well-developed prismatic habits.

General Characteristics: 

Polishing pits are usually present, forming isosceles triangles.

Cleavage is often visible, further highlighted by the polishing pits.

Hornblende 01: In the center of the image and left of it, longitudinal sections. of green hornblende with cleavage.
Hornblende 01: The same section as of the previous image, now in XPL. Very dark internal reflections, with a definite greenish hue, are typical for green hornblende.
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