OLIVINE

Olivine – (Fe,Mg)SiO4 – is a very common nesosilicate, occurring mainly in basic igneous rocks. It is an important industrial mineral with a number of uses.              

“Olivine” is actually not a mineral, but just a generic term used for nesosilicates of the isomorphic series between forsterite (Mg2SiO4) and fayalite (Fe2SiO4), a complete solid solution. An old terminology, now obsolete, divided the series into six members: forsterite (Fo100-90), chrysolite (Fo90-70), hyalosiderite (Fo70-50), hortonolite (Fo50-30), ferrohortonolite (Fo30-10) and fayalite. (Fo10-0). It is not possible to differentiate the members from each other through their optical properties. However, several physical and chemical properties vary systematically with composition and a detailed analysis of these can allow an estimation of composition. Most olivines, however, tend to be rich in forsterite. Compositional zonation can occur, with the core richer in Mg.          

In addition to the forsterite-fayalite series there is the forsterite-tephroite (Mn2SiO4) series and the fayalite-tephroite series. Olivine may contain Fe, Ni and PO4.

1. Characteristics

The data below refer to forsterite; fayalite is very similar.

Crystal system: Orthorrombic, bipiramidal.

Color: Green, pale yellow, brown or white. Darker with higher Fe content.

Habit: Usually rounded crystals, can be biterminated prismatic. Massive, granular.

Cleavage: {010} imperfect, {100} imperfect. Striations paralell to elongation.

Tenacity: Brittle.

Twinning: Twins are very rare, they can occur after {100}, {011} and {012}.    

Fracture: Conchoidal.

Mohs Hardness: 7

Parting: No.

Streak: White.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 3.275

 

2. Geology and Deposits

Olivine is typical of mafic to ultramafic intrusive and extrusive igneous rocks, as well as their metamorphic counterparts. Thus, it occurs in gabbros, in syenites with feldspathoids (shonquinites), in gabbros with feldspathoids (theralites) and in peridotites and pyroxenites (sagvandites and websterites). Dunite is a rock formed predominantly by olivine.

Olivine also occurs in trachytoids (shoshonites), alkali and subalkaline basalts (tholeiitics), and foidites (e.g., sodalitites). It is found in rocks with melilite, such as thurjaites and melilitites. Also in lamproites, kimberlites and some platinum ores. Fayalite can occur in granitic pegmatites, in felsic plutonic rocks and in metamorphosed Fe-rich sediments.

Olivine also occurs in chalcosilicate rocks formed by contact metamorphism (skarns) on dolomitic limestones. Some types of meteorites have olivines. Accumulates in the sands of beaches on oceanic islands.

 

3. Mineral Associations

In gabbros, basalts, picrites and basanites it is associated with plagioclases, clinopyroxenes, magnetite, ilmenite and analcime. It also occurs with feldspathoids (leucite, nosean, sodalite, nepheline, etc.).

Other minerals that can occur in association are chromite, spinel, orthopyroxenes (enstatite-ferrosilite), clinopyroxenes (augite, pigeonite, diopside), dolomite, calcite and phlogopite (in marble), ludwigite, kamacite, quartz, taenite, serpentine (antigorite), brucite, corundum, amphibole (arfvedsonite, hedenbergite), etc.

 

4. Transmitted Light Microscopy

Refraction indices:  nα:  1.636 – 1.730     nβ: 1.650 – 1.739     nγ: 1.669 – 1.772

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. It is rarely pale greenish-yellow with very diagnostic pale yellow pleochroism, due to higher Fe contents.

Relief: Moderate to extremely high.

Cleavage: {010} and {001} poor cleavages, normally not visible under a microscope. Fe-rich members (tending to fayalite) may show moderate {010} cleavage.

Habits: Anhedral crystals, more rarely subhedral, always tending to roundish. In volcanic rocks it can form euhedral prismatic phenocrysts, with hexagonal or octagonal sections. In glassy volcanic rocks it can be skeletal. In komatiites it can be acicular (“spinifex” texture). Fractures are usually abundant, very irregular, containing alteration materials (“iddingsite” and/or various others).

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringences of 0.035 (forsterite) and 0.052 (fayalite): strong, intense and variable colors of up to 3rd order. Sections parallel to (001) have 2x higher birefringence than sections parallel to (100).

Extinction: Parallel to the form, which is rarely possible due to the tendency of olivine to occur anhedral. In metamorphic rocks the extinction can be undulating.

Elongation sign: SE (-) or SE (+), depending on the orientation, it is not diagnostic.

Twins: Rare and simple, according to various plans. They are not diagnostic.

Zoning: Frequently zoned, with Mg-rich cores and Fe-rich rims.

CONVERGENT LIGHT

Character: B(+) or B(-), depending on the composition.

2V angle: 46 – 98º. Rich in Mg: >72º

Alterations: olivine can undergo at least six alteration processes.

1) Iddingsite: olivine can alter to iddingsite, which is not a mineral, but a yellow-brown to red-brown mixture, composed of clay minerals from the Chlorite Group and the Smectite Group, also iron oxides (goethite and hematite), ferrohybrids and others. Iddingsite is weakly pleochroic and can form pseudomorphs on olivine.

2) Bowlingite: alteration composed of green minerals such as smectite, chlorite, serpentine, talc and mica.

3) Serpentine: olivine is progressively replaced by antigorite (green schist facies), chrysotile and lizardite from the fractures and intergranular limits, with the fibers oriented perpendicularly to the fracture walls. In this way, perfect pseudomorphs of serpentine on olivine are common.

4) Reaction rims occur when olivines are in contact with anorthite-rich plagioclases in gabbroic rocks. These reaction rims (queliphytic textures) are composed of fibrous, radiated green hornblende, also pyroxenes, spinels, and garnets. On the other hand, when olivines are in contact with silica-rich fluids, reaction rims composed of orthopyroxenes are generated.

5) Calcite: complete replacement of olivine by calcite (pseudomorphosis) may occur.

6) Iron oxides and hydroxides can develop along fractures and in grains.

May be confused with: other minerals of high relief and strong colors in CPL.

Clinopyroxenes always show their typical cleavage and have oblique extinction.

Andalusite may be similar, but does not occur in the same rocks.

Minerals of the Humite Group, such as chondrodite, have lower relief and lower birefringence.

 

Olivine 01: Unaltered olivine in marble (calcite), in PPL. Olivine is colorless, without cleavage and its relief is higher than the calcite relief.
Olivine 01: The same section as in the previous figure, now in XPL. Calcite is cream and olivine shows various and intense interference colors (blue, yellow, orange, etc). On top there is an idiomorphic grain in the parallel extinction position (larger black grain, horizontally).
Olivine 02: Olivine in gabbro, in PP, with a little iron (hydr)oxides in its fractures. In PPL, the relief contrasts are clear: plagioclase (colorless + low relief), clinopyroxene (colorless + medium relief) and olivine (colorless + high relief).
Olivine 02: The same section as in the previous image, now in XPL. Plagioclase is gray (twins), augite is colored with cleavage and olivine also is colorful, but without cleavage.
In XPL, idiomorphic olivines in peridotite, short prismatic with pointed ends (orthorhombic bipyramids).
In XPL, skeletal olivine in volcanic rock.
Olivine 03: In PPL, partially altered olivne. Most of the grain is already altered to serpentine, but in the center, there are some portions of higher relief and intense interference colors (in XPL) that represent the original characteristics of this olivine grain.
Olivine 03: now in XPL, the same section as in the previous image. Colored portions are unaltered olivine. The outer part of the grain is altered to serpentine.
Olivine 04: in PPL, olivine grain completely altered to serpentine. It is now a pseudomorphosis. In the next image, the same in XPL.
Olivine 04: now in XPL, the same crystal of the previous image. You see only serpentine, with the typical gray colors. It is a very typical pattern of alteration. Combined with the shape and paragenesis, turns the identification of olivine easy.
Olivine fully altered to serpentine. The rock is a marble, composed of calcite. XPL.
Olivine completely altered (pseudomorphosis) to iddingsite (orange). It is a volcanic rock, composed of opaque minerals (black), plagioclase (colorless + low relief) and rare clinopyroxenes. PPL.
Image of an entire thin section of komatiite in PPL, with acicular olivines, altered to serpentine, forming a network. The spaces between the olivines are filled with serpentine.
Euhedral olivine crystal in volcanic rock in PPL, fully replaced by calcite, thus constituting a pseudomorphosis.
In a komatiite rock, olivines with acicular habit and totally altered to serpentine, forming the spinifex texture. XPL.
Olivine 05: In skarn, calcite and, in the center of the image, olivine grain altered to serpentine and bowlingite. In PPL, bowlingite is greenish-yellow and serpentine is colorless.
Olivine 05: Now is XPL, the same section as in the previous image. Serpentine is gray to white and the bowlingite has bright colors between yellow and green. Calcite in brown/cream.
In XPL, olivine altered to bowlingite, of complex structure. In PPL, the color of bowlingite is green, tending to yellowish or reddish.
Olivine 06: In PPL, olivine crystal altered to chlorite (light green), serpentine (colorless, difficult to see), bowlingite (at the edges) and iron (hydr)oxides (in the fractures).
Olivine 06: The same crystal of the previous image, now in XPL.
Olivine 07: In the center of the image, subhedral (hypidiomorphic) crystal of olivine with iron oxides and hydroxides (red) along the edges and fractures. The grain itself is unaltered, the alteration is concentrated along crystal borders and fractures. PPL.
Olivine 07: The same section as in the previous image, now in XPL. Olivines show strong colors.
Olivine 08: In peridotite, olivines as phenocrysts and in the matrix; completely altered to serpentine. In PPL, the grain shape is characteristic for olivine, as is the pattern of alteration, with some less altered portions within the grains.
Olivine 08: The same section as in the previous image, now in XPL. Gray interference colors characterize serpentine, a common alteration of olivine.

5. Reflected Light Microscopy

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

Sample preparation: olivine acquires a good polish with some ease. When altered, on the other hand, which is very common, the polish is of poor quality and persistent grooves are difficult to eliminate.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, with no associated color tone.

Pleochroism: No.

Reflectivity: Very low (~7%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not observed.

Internal reflections: Generalized in the color of the mineral in hand specimen, which can be in yellow, brownish or greenish tones, always with low luminosity.

May be confused with: garnet may be similar, but occurs in other paragenesis. Considering the paragenesis, the rounded shape, the fractures with alteration materials and the colors of the internal reflections are relatively diagnostic for olivine. Miscellaneous alteration products can change the color of the internal reflections.

General Characteristics: 

Grain shape usually tends to be rounded or elliptical, in volcanic rocks it can be rectangular or biterminated.    

Fractures are always present, filled with alteration materials (iddingsite or serpentines) that have a darker reflection color, many polishing grooves due to low hardness, and clearer and more luminous internal reflections (yellow to white).           

Relief: high relief may occur due to hardness differences between olivine and altering materials.

Inclusions of magnetite and ilmenite can occur as inclusions in olivine.

Pyrrhotite can be concentrated, in the form of lamellae, along the edges of the olivine grains.

Olivine 01: In PPL, the color is similar to the color of other silicates.
Olivine 01: the same section as in the previous image, now in XPL. The internal reflections are always in these light colors; can be greenish or brown.
Olivine in XPL, with internal reflections of different colors from previous images. Alteration materials can modify these colors.
Olivine in XPL, with internal reflections of different colors from previous images. Alteration materials can modify these colors.
Olivine 02: In skarn, olivine altered to bowlingite, with calcite at the top of the image and some pyrrhotite lamellae (very clear, yellowish tone, high reflectivity, at the top and on the left). In PPL, you can clearly see the lighter serpentine developing in the fractures of the olivine grain and the greenish gray color of the bowlingite that occupies most of the grain.
Olivine 02: The same section as in the previous image, now in XPL. Calcite has clear to white internal reflections and anisotropy. Bowlingite and serpentine show reflections in various shades of green, it can be very dark, almost black.
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