NONTRONITE

Nontronite – Na0.3Fe2((Si,Al)4O10)(OH)2 · nH2O – is a phyllosilicate, a clay mineral that is part of the 17-member Smectite Group. It is quite common and has no economic importance. It can contain Mg, Ca and Ti and has two varieties (Al, Cr).

Conclusive identification of nontronite by optical microscopy is not possible. Other analytical techniques should be used, such as X-Ray Diffractometry. The microscope can only offer a suggestion of identification, considering the optical properties of the material and the paragenesis in question.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Yellow colors (yellow, yellow-green, orange) and green colors (green, olive), can be brown.

Habit: Usually cryptocrystalline, massive, rarely small crystals in radial or spherulitic aggregates.

Cleavage: {001} perfect.

Tenacity: No information available.

Twinning: No information available.

Fracture: Conchoidal, splintery.

Mohs Hardness: 1.5 – 2

Parting: No information available.

Streak: No information available.

Lustre: Resinous, dull.

Diaphaneity: Transparent.

Density (g/cm³): 2.2 – 2.3

2. Geology and Deposits

Nontronite occurs as an alteration product of basalts, kimberlites and ultramafic igneous rocks. It also develops in poorly drained volcanic soils and in some hydrothermally altered mineral deposits.

It also occurs in basalts of mid-ocean ridges and in carbonate rocks that have undergone contact metamorphism (scarnites).

In recent marine sediments it forms as an autigenic mineral.

3. Mineral Associations

It ocurrs with silica (quartz, opal, chalcedony), “hornblendes”, pyroxenes, olivines, micas and kaolinite.

In volcanic rocks, it is usually associated in cavities (vesicles), with zeolites (chabazite, analcime, thomsonite, mesolite), barite, hollandite, pyrite and calcite.

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.530 – 1.580   nβ: 1.555 – 1.612       nγ: 1.560 – 1.615

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Yellow, without pleochroism.

Relief: Moderate.

Cleavage: Not visible due to the very small size of the crystals.

Habits: Micaceous flakes, can form radial, fan-shaped aggregates that can evolve into spherulites. Massive.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence from 0.030 to 0.035: colors up to half second order. Gray, yellow, red, blue, orange. The strong colors of nontronite, however, can completely mask these interference colors.

Extinction: No information available.

Elongation sign: It is not possible to determine.

Twins: It is not possible to determine.

Zoning: It is not possible to determine.

CONVERGENT LIGHT

Character: B(-)

2V angle: 5 – 66º

Alterations: Nontronite is an alteration product.

May be confused with: jarosite, which is also cryptocrystalline and show strong yellow color.

 

In acid volcanic rock (rhyodacite), with plagioclase phenocrysts, in PPL, nontronite fills small vesicles, with a very conspicuous yellow color, quite unusual. It is easy to recognize.
Nontronite 01: in PPL, in acid volcanic rock (rhyodacite), vesicle filled with nontronite and a bundle of zeolites. The yellow color of nontronite is very conspicuous.
Nontronite 01: the same section as in the previous image, now in XPL. Nontronite is very dark, almost isotropic, while zeolites have their normal white and gray interference colors.

5. Reflected Light Microscopy

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

Sample preparation: polishing of clay minerals is not possible due to the very small size of the individual crystals. Therefore, in PPL clay shows a dark surface because it does not reflect, but in XPL the internal reflections are the same as the color of the mineral in hand specimen and allow a suggestion of identification.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, darker than the color of feldspars, can be black because it does not reflect light.

Pleochroism: No.

Reflectivity: Very low (<<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not observed.

Internal reflections: Widespread in bright yellow in various shades. These are very bright colors that stand out in the image and are unparalleled in common rock-forming minerals.

May be confused with: jarosite, which also has these bright yellow colors.

Nontronite 01: In acid volcanic rock (rhyodacite), nontronite-filled vesicle at approximately center of images. In PPL, it is impossible to recognize nontronite.
Nontronite 01: the same section as of the previous image, now in XPL. Nontronite stands out due to its yellow internal reflections. At its side, with colorless to white internal reflections, plagioclase phenocrysts. In dark colors, partially devitrified volcanic glass.
Nontronite 02: in PPL, vesicle filled with nontronite. in acid volcanic rock (rhyodacite).
Nontronite 02: the same section as of the previous image, now in XPL. Strong yellow internal reflections are typical for nontronite.
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