CHONDRODITE

Chondrodite – Mg5(SiO4)2F2 – is a fairly rare nesosilicate, characteristic of skarns and carbonatites. It has no economic importance.
It is classified in the Humite Group, is dimorphic to ribbeite, and may contain Ti, Al, and Mn. There is a variety containing a higher Ti content, titanochondrodite.

It can be macroscopically confused with clinohumite.

1. Characteristics

Crystal system: Monoclinic prismatic          

Color: Yellow, red, or brown.     

Habit: Rounded grains, massive, may show somewhat flattened grains.       

Cleavage: {100} poor.      

Tenacity: Brittle.        

Twinning: No.       

Fracture: No information available.      

Mohs Hardness: 6 – 6.5

Parting: On {001}.         

Streak: Gray-white, yellow.          

Lustre: Vitreous, resinous.          

Diaphaneity: Transparent.          

Density (g/cm³): 3,16 – 3,26

          

2. Geology and Deposits

Chondrodite is a mineral that occurs in limestones and dolomites that have undergone contact metamorphism or metasomatism from granite intrusions, especially if there is Fe-B-F metasomatism. Therefore, it can be found in skarn and marble, both calcitic and dolomitic.

It also occurs in metamorphosed ultramafic rocks (talc-schists, serpentinites, peridotites). Furthermore, in carbonatites and kimberlites.

 

3. Mineral Associations

It is associated with the common rock-forming minerals of these parageneses, such as calcite, diopside, olivine (forsterite), tremolite, magnetite, and phlogopite.

In this context, spinel, garnet (grossular), clinochlore, graphite, aragonite, lizardite, wollastonite, monticellite, molybdenite, and cuspidine also occur.

Carbonatites are rocks that can show a very diverse mineralogy, with around 80 different minerals. Therefore, the list of minerals that can occur associated with chondrodite can be quite extensive.

The above-cited parageneses are not exclusive to chondrodite; the other members of the Humite Group can also occur there.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1,592 – 1,643      nβ: 1,602 – 1,655      nγ: 1,619 – 1,675

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Pleochroic between colorless and pale yellow, greenish, or brownish tones:

X = colorless, very pale yellow, brownish yellow
Y = colorless, yellowish-green
Z = colorless, pale green
Pleochroism is sometimes very weak and may go unnoticed. In some cases, chondrodite does not exhibit pleochroism.

Relief:  Moderate, similar to clinopyroxenes and carbonates that occur in the same paragenesis!

Cleavage: indistinct in {100}, not visible under a microscope. Partition in {001}.

Habits: Typically granular: isolated anhedral rounded grains or in clusters. Massive. Rare crystals, flattened parallel to {010}.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.027 – 0.032: colors ranging from first-order yellow to the middle of the second order. Strong, intense colors, combining yellow and blue. 

Extinction: Oblique, 22 to 31º in relation to the cleavage. It is difficult or impossible to ascertain due to its granular habit and because it rarely shows cleavage.

Elongation sign: ES(+) or ES(-), generally difficult to determine (nor is it a diagnosic). 

Twins: They are relatively common, simple or lamellar. They may resemble the polysynthetic twins of plagioclase.

Zoning: No information available.             

CONVERGENT LIGHT

Character: B(+)          

2V angle: 64 – 90º         

Alterations: Chondrodite alters to chlorite or serpentine.          

May be confused with: The most characteristic features are the rounded grain habit, the pale yellow to greenish colors, the twinning it frequently exhibits, and the association with marbles/carbonatites/skarns.

Diopside may be similar, but shows cleavage. When granular, it is very similar; it may be impossible to distinguish small grains of diopside from small grains of colorless chondrodite.

Olivine may be similar, but it is usually colorless, has a different alteration pattern, parallel extinction, and is usually B(-). Colorless and anhedral chondrodite grains, however, are practically indistinguishable from anhedral olivines when the olivine extinction cannot be determined.

The other members of the Humite Group exhibit similar optical characteristics: norbergite, humite, and clinohumite. The differences are:

– norbergite and humite exhibit parallel extinction,

– clinohumite shows oblique extinction, but at a smaller angle, of 9-15º.

Norbergite exhibits a smaller 2V angle, of 44-50º.

Staurolite shows similar pleochroism, but has lower interference colors and occurs in other geological contexts.

Chondrodite 01: In PPL, chondrodite grain in marble (or carbonatite), showing the first of its pleochroism colors, which is colorless. Its moderate relief in this situation is masked by the equally moderate relief of the calcite. The habit is typically granular, very rarely idiomorphic.
Chondrodite 01: The same section as of the previous image, now rotated 90 degrees to show the second pleochroism color of chondrodite, a yellow-green.
Chondrodite 01: The same section as of the two previous images, now in XPL. Chondrodite displays strong, intense colors, combining yellow, red, green, and blue.
Chondrodite 02: In PPL, chondrodite grains between magnetites (opaque). This is the first pleochroism color.
Chondrodite 02: The same section as of the previous image, now rotated 90 degrees to show the second pleochroism color, a light brown.
Chondrodite 02: The same section as of the two previous images, now in XPL. Strong interference colors are typical for chondrodite.
Chondrodite grain in marble (or carbonatite). Poorly defined shapes, intense pleochroism ranging from colorless to greenish-yellow, and poorly developed cleavage are typical. Calcite is colorless. In XPL, the intense own color of chondrodite masks its interference colors.
In XPL, chondrodite with twinning, very easy to identify. In black, magnetite.
In XPL, detail of the twinning of chondrodite.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying chondrodite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with chondrodite, such as magnetite, molybdenite, and graphite.

Sample preparation: Polishing chondrodite is relatively simple despite its high hardness. However, the fact that it occurs in granular aggregates causes holes in the polished section when some smaller grains or grain fragments break off.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray, the same shade as the lightest color of calcite pleochroism. 

Pleochroism: No.      

Reflectivity: Very low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Subtle anisotropy in shades of gray, depending on the section and masked by internal reflections.

Internal reflections: Intense, widespread, orange-yellow in various shades.

May be confused with: Few other minerals. The identification of chondrodite is relatively simple by both Transmitted Light and Reflected Light due to its optical properties, which are quite peculiar, with the usual exceptions.

Olivine and garnet sometimes exhibit internal reflections of the same color.       

General Characteristics: 

Grain shape: generally aggregates of rounded grains, rarely idiomorphic.

High relief is possible when it occurs with calcite or dolomite, which have much lower hardness. Mainly after intense polishing on softer cloths. Less soft cloths are recommended.

Chondrodite 01: In PPL, chondrodite (center, higher relief) with calcite (lower relief, pleochroism in gray).
Chondrodite 01: The same sections as of the previous image, now in XPL. Chondrodite shows intense red-orange internal reflections. Calcite with milky reflections.
In XPL, chondrodite in marble, showing yellow-brownish internal reflections.
In XPL, chondrodite (center) in marble. Sometimes the internal reflections show weaker colors, but always very diagnostic.
Chondrodite 02: In PPL, magnetite (whitish), chondrodite (rounded grains) and calcite (between chondrodite grains). The color of chondrodite is almost the same as the color of calcite.
Chondrodite 02: The same section as of the previous image, now in XPL. Magnetite is isotropic (black), calcite shows witish internal refletions and chondrodite has the diagnostic yellow-orange-red internal reflections.