DIOPSIDE

Diopside – CaMgSi2O6 – is an inosilicate of the Pyroxene Supergroup. It is a rarer mineral, typical of some types of alkaline igneous rocks and marbles. It has no economic importance.   

Integrates the solid solution series between diopside, augite and hedenbergite. The color variation that it presents macroscopically is due to variable levels of impurities such as Fe, Mn, Ti, V and Cr. In addition to these, it can contain Zn, Al, Na and K.

It has a dozen varieties, established according to different colors or higher contents of certain elements.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Light to dark green, can be brown, blue, white, gray, pale violet to colorless.

Habit: Prismatic with almost square basal sections. Granular, columnar, massive.

Cleavage: {110} good, typical of pyroxenes.

Tenacity: Brittle.

Twinning: Common, single or multiple, on {100} and {010}.

Fracture: Irregular.

Mohs Hardness: 5.5 – 6.5

Parting: On {100}.

Streak: White.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 3.22 – 3.38

2. Geology and Deposits

Diopside-rich pyroxenes are common in Na- and K-rich (alkaline) magmatic rocks such as alkaline basalts, alkaline gabbros and related rocks (hawaiites, mugearites, latites, trachytes, monzonites, etc.). They are not so common in tephrites, basanites, phonotephrites and nephelinites, as augite-(Ti) is the most typical pyroxene in these types of rocks.

Diopside occurs in certain lamprophyres and ultrabasic rocks such as dunites and kimberlites (eclogites) (in the latter as emerald-green chromo-diopsides). It also occurs, more rarely, as phenocrysts in tholeiitic basalts and andesites.

Diopside is often found in Mg-rich metamorphic limestone rocks (marbles) and in associated contact metamorphic rocks such as skarns, where it is very common.

It is also found in rocks with melilite, lamproites, lamprophyres, lapis lazuli, granulites and gneisses. Some achondrites meteorites also have diopside.

3. Mineral Associations

In magmatic rocks it occurs with augite and pigeonite (generally in the matrix), plagioclase, olivine and with or without feldspathoids (nepheline and leucite).

In limestone rocks that have undergone contact metamorphism, it is typically associated with calcite, quartz, garnet (grossular), wollastonite, scapolite, vesuvianite, olivine (forsterite), phlogopite, minerals from the Humite Group, spinel, hessonite, tremolite and clinozoisite.

It is also associated with clinochlore, epidote, titanite and albite.

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.663 – 1.699    nβ: 1.671 – 1.705    nγ: 1.693 – 1.728

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless or pale green with very weak pleochroism in very light green tones. It can be gray or brown.

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 87º and 93º, which is characteristic for pyroxenes.

Habits: Typically granular or short prismatic subhedral crystals. Rarely acicular or columnar. Octagonal basal sections (8 sides). Lamellar exsolutions of orthopyroxenes may occur.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: high birefringence, 2nd higher order, ranging from 0.029 to 0.031: yellow, blue, red colors.

Extinction: tends to oblique, with increasing angle with increasing Fe2+ content. Angle between 38º and 48º.

Elongation sign: Can not be used.

Twins: In volcanic rocks, crystal aggregates are typical, interwoven in various ways. Single twins predominate according to (100), forming pyroxene lamellae oriented obliquely to the cleavage. 

Zoning: Sometimes is zoned.

CONVERGENT LIGHT

Character: B(+)

2V angle: 58 – 62º, increases with increasing Fe2+ content.

Alterations: by pneumatolytic and hydrothermal alteration of high temperatures, the diopside undergoes uralitization and changes to fibrous/capillary aggregates of amphibole (tremolite or actinolite).

By hydrothermal alteration at low temperatures, it undergoes serpentinization (alters to serpentine).

Weathering changes diopside to carbonate, hematite and quartz.

May be confused with: distinguishing between the various clinopyroxenes (augite, diopside and pigeonite) can be very difficult to impossible under the petrographic microscope. Some authors report that the distinction between clinopyroxenes can only be performed with the aid of an universal stage. What facilitates identification is knowledge of paragenesis (rock type, associated minerals). In some cases the description only uses the term “clinopyroxene” for these minerals; their precise identification must be carried out with other analytical methods.

Pigeonite has an angle of 2V < 30°.

Hedenbergite has a slightly higher relief.

Augite-(Ti) shows strong pleochroism.

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

Omphacite and jadeite occur in other paragenesis.

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

Chondrodite may be similar.

 

Diopside 01: in PPL, diopside grain in longitudinal section. Almost imperceptible in the image, the diopside presents a discreet pale green color in contrast to the neighboring mineral, which is colorless. Pleochroism is not observed.
Diopside 01: same grain of the former image, this time in XPL. Strong colors are typical, with oblique extinction. Some authors argue that it is impossible to distinguish from augite and pigeonite under the microscope.
Diopside 02: in PPL, basal section of diopside showing the typical pyroxene cleavage, with two cleavage directions intersecting almost on 90 degrees.
Diopside 02: same basal section of the former image, this time in XPL. Strong interference colors can be seen.
Diopside grains in XPL. Strong colors and high-angle oblique extinction are characteristic.
Clinopyroxene in XPL undergoing uralitization: the green spots refer to amphiboles that are developing and that can occupy the entire grain.

5. Reflected Light Microscopy

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

Sample preparation: polishing diopside offers no difficulties and is equivalent to polishing quartz, feldspars and amphiboles.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, lighter than quartz and feldspar and much lighter than micas.

Pleochroism: No.

Reflectivity: Low (<<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Weak, but noticeable, anisotropy.

Internal reflections: Generalized, usually very dark. At the grain edges and in very fine grains, reflections are dark green to light yellowish green.

May be confused with: amphiboles and other pyroxenes, whose behavior is very similar. It is not possible to recognize diopside under a Reflected Light microscope. At most it is possible to recognize the mineral as mafic, most likely pyroxene.

Diopside 01: : in granulite, in PPL, magnetite (very light, pinkish), feldspar and quartz (very dark gray) and diopside (lighter gray). In black, holes in the polished section.
Diopside 01: the same section of the previous image, now in XPL. Magnetite is isotropic, quartz and feldspars have clear to white internal reflections and diopside shows internal reflections in various shades of green, usually lighter along the grain edges and very dark, almost black, in the center of the grains and in thick grains
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