BROWN HORNBLENDE

Brown hornblende – Na(2)Ca(2)(Mg,Fe2+)4(Fe3+,Ti,Al)(OH)2|AlSi7O22 – is an inosilicate of the Amphibole Supergroup. It is a rarer mineral, characteristic of some types of igneous rocks. It has no economic importance.

“Brown Hornblende” is actually not a mineral, but just an informal name, a nickname, for brown to black amphiboles of complex compositions such as kaersutite, katophorite and others. Classification of amphiboles is complicated and constantly changing. The old names “basaltic hornblende”, “oxyhornblende”, “lamprobolite” and “hornblende” have been discredited by the IMA (International Mineralogical Association). Identification of the specific mineral species among the “brown hornblendes” must be performed with analytical methods other than optical microscopy.

Macroscopically, the recognition of brown hornblende is very difficult, almost impossible, as they are very dark to black minerals like other amphiboles too and, incidentally, like many pyroxenes too.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color:  Dark brown to black, with undertones of green, blue or yellow, depending on the mineral.

Habit: Prismatic short to long, massive, granular, fibrous, skeletal. 

Cleavage: {110} perfect, typical for amphiboles.      

Tenacity: Brittle.        

Twinning: Common.       

Fracture: No information.       

Mohs Hardness: 5 – 6

Parting: No.         

Streak: Pale gray-brown.         

Lustre:  Vitreous.         

Diaphaneity: Transparent.           

Density (g/cm³): ~3.2

 

2. Geology and Deposits

Brown hornblendes, in general, occur in intermediate to acidic igneous rocks, such as biotite-hornblende dacites, fourth-latite andesites, ignimbrites and other types of pyroclastic rocks.

Kaersutite is relatively rare and occurs in basic plutonic, volcanic and subvolcanic rocks (dykes) such as essexites, theralites, tephrites, basanites, camptonites, monchiquites, etc. It is rare in alkaline basalts where it occurs in gabbroic or peridotite nodules. Syenites, monzonites, carbonatitic tuffs and alkaline gabbros may occur.

Katophorite occurs in alkaline (volcanic and plutonic) magmatic rocks such as nepheline-syenites and in subvolcanic Na-shonquinites such as phono-nephelinites. Also in jadeitites of blueschist facies.

 

3. Mineral Associations

The minerals that make up the “brown hornblendes” are associated with titanoaugite, olivine, calc-sodium feldspars (albite), potassium feldspars (microclinium), feldspathoids (nepheline), fluorapatite, biotite, zircon and eudialite.

Katophorite is found with arfvedsonite, aegirine, chromite, and eckermannite, as well as a number of rare and very rare minerals such as vlasovite, gittingsite, miserite, agrellite, and mosandrite-(Ce).

Kaersutite is associated with hematite, ilmenite, spinel, Ti-pargasite, titanite and garnet (pyrope), in addition to rare minerals such as rhönite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.640 – 1.681     nβ: 1.658 – 1.688      nγ: 1.660 – 1.692 (Katophorite)

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Deep brown and deep reddish-brown, with intense pleochroism.

Also black-brown, walnut-brown, brown-red, pale yellowish-red, yellow-orange and other colors.

The colors have an uneven distribution in spots, due to the uneven distribution of oxidation in the crystal.

Kaersutite and kathophorite have brownish colors (similar to biotite); are often sharply zoned or banded or hourglass in color.    

Relief: Moderate to high.           

Cleavage: {110} perfect. In the longitudinal sections there is only one cleavage. In the basal sections there are two cleavages that intersect at angles of 56 and 124º, forming the typical rhombuses of the basal sections of amphiboles. 

Habits: Euhedral to subhedral crystals with short to long prismatic habits, often with eroded margins.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Birefringence between 0.009 and 0.094: katophorite has low interference colors (0.009 to 0.021), kaersutite has 1st and 2nd order colors (0.026 – 0.047), similar to green hornblende. Strongly colored amphiboles usually have the interference colors masked by their own color.          

Extinction: Oblique. Kaersutite 3-19º, katophorite 8-16º. Low extinction angles are very diagnostic!          

Elongation sign: Kaersutite ES(+), katophorite ES(-). In strong colored minerals the elongation sign is difficult to determine. 

Twins: Lamellar twins in (100) are common.         

Zoning: Zonation is common, with lighter colored cores and dark edges. Color changes at multiple isomorphic levels occur.            

CONVERGENT LIGHT

Character: B(-)          

2V angle:  2V angle: kaersutite 74-82º (usually 80º); katophorite 0–57º (usually 35º)        

Alterations: by magma imbalance, alteration to magnetite + hematite + iron-poor clinopyroxenes and others. This process is called opacitization and can affect biotite as well, initially at the margins and then throughout the entire crystal. Pressure drop also leads to corroded margins. Only in ignimbrites and in lava flows that have cooled rapidly is opacification minimal or absent.

Weathering leads to the formation of carbonate, limonite and quartz.   

May be confused with: diagnostic are cleavage, strong colors and intense pleochroism.

Biotite has parallel and mottled extinction.

Stilpnomelan, in the longitudinal sections, is similar (and similar to biotite!), but the extinction is parallel. In basal sections it does not show the two amphibole cleavages. Occurs in other paragenesis.

Oxyhornblende from volcanic rocks is oxidized green hastingsite, optically very different.

Brown hornblende 01: In PPL, brown hornblende showing, in this position, the first pleochroism color. The next image show the second pleochroism color.
Brown hornblende 01: The same section as of the previous image, now rotated 90 degrees to show the second pleochroism color. The next image show the same crystal in XPL.
Brown hornblende 01: The same crystal as of the two previous images, now in XPL. Interference colors, of 1st and 2nd order, are intense and varied.
In XPL, twinned brown hornblende.
In XPL, zoned brown hornblende. The transition from brown to green hornblende is sometimes observed.
Basal section of brown hornblende in PPL, showing the two cleavage directions that intersect at 56º and 124º, typical of amphiboles.
In PPL, brown hornblende in a matrix of volcanic rock. Top right, pyroxene phenocryst.
Pseudohexagonal basal section of brown hornblende in PPL.
Brown hornblende 02: In PPL, basal section of brown hornblende in a volcanic rock (andesite or latite/trachyte). The hornblendes and associated feldspars are profoundly altered. Feldspar was almost completely replaced by carbonate and the hornblendes have the typical opaque edge, the result of a probably igneous process, in which there is a rebalancing of mafic minerals (hornblende) to Fe oxides (magnetite) as the magma crystallizes, generating a coronitic texture (crown of opaques around the hornblende).
Brown hornblende 02: The same section as of the previous image, now in XPL. Hornblende was totally replaced by some metamorphism/hydrothermal process, possibly to tremolite/talc(?). The opaque spots within the crystal indicate the segregation of Fe that did not enter the magnesian structure of the alteration. And it is possible to discern the two directions of cleavage in these basal sections.
Brown hornblende 03: In PPL, from the same rock pictured before, two other basal sections of brown hornblende.
Brown hornblende 03: The same section as of the previous image, now in XPL. Nothing is left from the original brown hornblende crystal besides form and cleavage traces.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of brown hornblende. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with brown hornblende, like magnetite, ilmenite, cromite, pyrrhotite and others.

Sample preparation: the polishing of brown hornblende offers no major problems other than those resulting from the excellent cleavage of amphiboles. There will always be some imperfections like polishing pits.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, but lighter than quartz and feldspars. 

Pleochroism:  No.     

Reflectivity: Low (<<10%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections: Widespread, very dark, at first glance they appear black, but at the edges of the crystals, where the thickness is less, it is usually possible to distinguish some greenish tones.      

May be confused with: green hornblende and with pyroxenes, which are very similar, with very dark to black internal reflections in XPL.       

General Characteristics: 

Cleavage is not visible.

Polishing scratches may occur.

Brown hornblende 01: In the center of the image, diagonally, brown hornblende phenocryst with two inclusions (a small yellowish pyrrhotite at the bottom and a larger whitish magnetite at the top) and a reaction rim, in lamprophyre. In PPL, the reflection color is slightly lighter than the feldspars in the rock matrix.
Brown hornblende 01: The same section as of the previous image, now in XPL. Internal reflections are very dark, almost black.
In PPL, brown hornblende phenocrystal with many triangular polishing pits lined up. Depending on the position of the cleavage in relation to the plane of the polished section, the figures are numerous, infrequent or absent.
In PPL, brown hornblende phenocrystal with a well-developed reaction rim (eroded margins), visible due to the alteration and small crystal size of this edge, which results in poor polishing with many small (black) holes.
In XPL, brown hornblende phenocryst with green to multicolored internal reflections, generated by an especially favorable position of the cleavage in relation to the plane of the polished section.
In XPL, brown hornblende phenocryst showing a single twin. Twins are relatively simple to observe by uncrossing the nicols in XPL by a few degrees, which the literature calls “crossed polarizers +2°”.