AUGITE

Augite – (Ca,Na)(Mg,Fe,Al,Ti)Si2O6 – is an inosilicate of the Pyroxene Supergroup. It is a very common clinopyroxene (monoclinic pyroxene), an important rock forming mineral of basic igneous rocks (basalts, etc.), but it has no economic importance.

It is part of a solid solution formed by diopside (CaMgSi2O6), augite and hedenbergite (CaFe2+Si2O6). There are four variants of chemical element substitutions (Ti, Cr, Na, Mn, K) in the formula; for details see Morimoto et al (1988). Seven varieties of augite were defined based on habit or chemical composition.

1. Characteristics

Crystal system:  Monoclinic prismatic.         

Color: Black, black-green, brown, brown-green, brown-violet.     

Habit: Short prismatic with square or 8-sided basal sections. Acicular, skeletal, tabular, dendritic.       

Cleavage: {110} good, typical for pyroxenes.       

Tenacity:  Brittle.       

Twinning: Common on {100}, simple or multiple.

Fracture: Irregular to conchoidal.       

Mohs Hardness: 5.5 – 6

Parting: On {100} and {010}.         

Streak: Greenish gray, light/dark brown.         

Lustre:  Vitreous.         

Diaphaneity: Transparent.           

Density (g/cm³): 3.19 – 3.56

 

2. Geology and Deposits

Augite is an essential mineral in mafic and ultramafic igneous rocks (gabbros, basalts, olivine-gabbros, lamburgites and peridotites) and in their metamorphic equivalents. It occurs in syenites, monzonites, diorites, pyroxenites, hornblendites, dacites, rhyodacites, trachytoids, andesites, basalts, phonolites, tephrites, foidites, dolerites, porphyries and lamprophyres.

It sometimes occurs in high-grade metamorphic rocks such as gneisses and granulites. Also in metamorphic iron formations, cornubianites and impactites. Titanoaugite is restricted to basic to intermediate igneous rocks.

Augite is also found in meteorites (acondrites) and occasionally occurs in immature sandstones such as graywackes.

 

3. Mineral Associations

In mafic rocks, it is associated with plagioclase (labradorite), olivine, sanidine, leucite, amphibole (hornblende), other pyroxenes (pigeonite, etc.), garnet (melanite), titanite, fluor-apatite, oxides (magnetite, ilmenite, hematite), sulfides (pyrite, chalcopyrite), native copper and a huge variety of secondary minerals, both from alteration and those formed by other processes after the formation of the host rock.

Among these secondary minerals are calcite, epidote, prehnite, many different zeolites and clay minerals.

Also associated with scapolite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.671 – 1.735  nβ: 1.672 – 1.741     nγ: 1.703 – 1.774

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Normally colorless, tending to gray due to the higher relief. It can be very pale green, gray, pale brown or brownish green.

Pleochroism is usually non-existent or weak. Stronger colors may mask pleochroism (x = faint green or bluish green; y = pale green, brown, green, or bluish green; z = pale brownish green, green, or yellow-green).

If with Ti (>3% TiO2 = titanoaugite) it has distinct to strong pleochroism from pale brown, brownish violet or violet.

If with Fe, it is darker, with weak pleochroism.  

Relief: Medium to high.           

Cleavage: {110} good to distinct. Basal sections show two cleavages that form 87º and 93º angles to each other. Prismatic sections has only one cleavage.           

Habits: Prismatic, equidimensional, columnar shapes. Anhedral crystals and masses are common. Idiomorphic basal sections show 4 or 8 sides.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.018 – 0.034: high 1st order to high 2nd order colors: intense colors, yellow, red, etc., gray only in sections perpendicular to one of the optical axes (isotropy sections).           

Extinction: Oblique, with an angle between 35 and 48º.

Parallel in sections (010); may present hourglass extinction.          

Elongation sign: Does not apply; the crystals may be elongated in several directions.            

Twins:  Frequent, simple to lamellar on {100} and {001}. Can occur combined to form a herringbone pattern. Orthopyroxene exsolution lamellae may occur.        

Zoning: Often zoned, it can be sectorized or oscillatory.

Titanoaugite may show hourglass zoning.             

CONVERGENT LIGHT

Character:  B(+)         

2V angle: 25o to 61o.        

Alterations: due to uralitization, augite alters during crystallization of the magma to green, sometimes brown, amphiboles (hornblende, actinolite). This may occur too in low- to medium grade metamorphic rocks and in contact-metamorphosed aureoles. In magmatic rocks, augite can change to chlorite and, in some cases, with the addition of K, into celadonite, forming pseudomorphs. Under low-temperature metamorphism, augite suffers chloritization, forming to chlorite, epidote and talc. Weathering forms carbonate (calcite), hematite and quartz.

May be confused with:some authors consider it impossible to distinguish the various clinopyroxenes (augite, diopside, pigeonite) from each other under a petrographic microscope. They state that the identification under the microscope is only possible with the aid of an universal stage. The identity of these minerals is usually confirmed with other analytical techniques such as microprobe. What may facilitate the identification is the knowledge of the paragenesis (type of rock, associated minerals).

Diopside is extremely similar and can be recognized only with the aid of an universal stage.

Pigeonite is also extremely similar; has a 2V angle < 30°. Hedenbergite has a slightly higher relief.

Titanoaugite has much stronger pleochroism.

Orthopyroxenes (enstatite, etc.) have parallel extinction and lower birefringence.

Omphacite and jadeite occur in other paragenesis.

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

Epidotes rich in Fe of the Epidote Group show anomalous interference colors and a differente cleavage.         

 

Augite 01: In PPL, augite crystals in a basaltic rock. Magnetite is opaque, plagioclase is colorless with low relief and augite is colorless, slightly brownish, with moderate relief.
Augite 01: The same section as of the previous image, now in XPL. Plagioclases in white/gray and augite very colorful.
In PPL, in a volcanic rock, 8-sided basal section of augite with low interference colors.
In XPL, 8-sided basal section of augite in volcanic rock. Cleavages are well-visible.
In XPL, 8-sided basal section of augite with a simple twin. There are basic rocks in which no augite is twinned; others have some twinned augites and in some cases, all the augites are twinned.
In XPL, twinned 8-sided basal section of augite in volcanic rock, with a very fine matrix.
In XPL, longitudinal section of augite with a simple twin. One direction of cleavage is easily seen. The rock is volcanic, with a very fine-grained matrix.
In XPL, twinned and zoned phenocryst of augite in volcanic rock.
In XPL, augite phenocryst in volcanic rock. Twinned, but not zoned.
In XPL, twinned and zoned augite phenocryst in volcanic rock.
In XPL, augite phenocryst with hourglass zoning.
In XPL, augite phenocryst with a ciclic twin, formed by several individual interpenetrating crystals. The twin, in section, takes the form of a wheel or star.
In XPL, augite phenocryst with a ciclic twin, formed by several individual interpenetrating crystals. The twin, in section, takes the form of a wheel or star.
In XPL, zoned idiomorphic augite phenocryst in volcanic rock.
In XPL, idiomorphic augite phenocryst with hourglass zoning.
Augite 02: In some volcanic rocks, there are two generations of clinopyroxenes, probably augites. The first generation is heavily altered and/or dark, with the alteration advancing through the cleavages and making them visible (dark grains). The other generation is unaltered, shows little or no cleavage (grain above the dark one). PPL.
Augite 02: The same section as of the previous image, now in XPL. The altered augite grains are still dark, and the second generation of augites are clear and twinned.
In volcanic rocks with two generations of clinopyroxenes, the second generation of clinopyroxene can form a corona, an envelope, around the first generation clinopyroxene, as here.
In volcanic rocks with two generations of clinopyroxenes, the second generation of clinopyroxene can form a corona, an envelope, around the first generation clinopyroxene, as here.
Augite 03: Early clinopyroxene phenocryst, probably augite, in acidic volcanic rock. The crystal is intensely uralitized, now formed by green hornblende and/or actinolite. Macroscopically it resembles a green tourmaline. PPL.
Augite 03: The same section as of the previous image, now in XPL. The uralitized augite shows strong colors and nearly paralell extinction.
Augite 04: A quartz grain may be placed accidentally in a rock subsaturated in silica, as here in a basaltic rock. The quartz grain could be a xenolith carried during the magma rise or could be a sand grain thrown into the lava during the volcanic event. In this case, a crown (“corona”) of minerals such as clinopyroxene (augite), olivine and chlorite develops around the quartz grain. PPL.
Augite 04: The same section as of the previous image, now in XPL. The envelope of augite, chlorite and epidote is very fine-grained and colorful.
Augite 04: Detail of the situation shown in the two previous images. Basaltic rock is below, quartz in above and, in the middle, detail of the corona with augite, chlorite and epidote.
Augite 05: In the matrix of a peridotite whose olivines are completely altered to serpentine, the augite in PPL, as here, forms chaotically scattered colorless rods with medium relief. Its shape resembles that of some apatites.
Augite 05: The same section as of the previous image, now in XPL. Intense interference colors and oblique extinction characterize clinopyroxene. Small dark (actually green) minerals, isotropic in XPL are a mineral of the Spinel Group, perhaps perovskite.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of augite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with augite, like magnetite, ilmenite, hematite, pyrite, chalcopyrite, native copper and others.

Sample preparation: polishing the augite is relatively simple and with a little care it shows good quality despite the cleavage.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, but a lighter color than feldspars and much lighter than mica.       

Pleochroism: No.      

Reflectivity: Very low (<8%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Distinct and strong anisotropy between light gray and dark gray. In some cases this anisotropy is very evident. In others, internal reflections make it difficult to see.        

Internal reflections: Generalized in dark shades.      

May be confused with: other dark colored silicates. Considering the paragenesis, the possibilities of mistakes are greatly reduced.

Augite 01: Augite, plagioclase, magnetite and ilmenite in a pegmatoid diabase. In PPL, ilmenite with magnetite (pink brown, high reflectivity), augite (light gray, better polish), plagioclase (dark gray).
Augite 01: The same section as of the previous image, now in XPL. Imenite with magnetite shows anisotropy of ilmenite, augite shows a twin and its strong anisotropy in gray, and plagioclases show dark generalized internal reflections with some multicolored ones.
In pegmatoid diabase, in PPL, chalcopyrite (golden), magnetite with ilmenite (pink gray, high reflectivity, almost white), skeletal augite (light gray) and plagioclase (dark gray).
In a gabbro, in PPL, ilmenite (pink gray, high reflectivity, almost white), olivine (high relief, many fractures filled with dark green gray alteration material), augite (light gray, excellent polish), and plagioclase (dark gray).