ZINCITE

Zincite – ZnO – is a very rare oxide that was Zn ore in only one occurrence, in the Franklin Mining District (New Jersey, USA). New Jersey is a particular case, unique in the world, where a pre-Cambrian weathered Zn-Mn-Fe oxide and silicate deposit, hosted in marble, underwent metamorphism. Franklin and Sterling Hill mines, deactivated for decades and currently open to visitors, are famous for their large amount of fluorescent minerals.

It is isostructural with bromellite. It rarely fluoresces under long UV waves.

Pure Zincite is colorless, but almost always contains small amounts of Mn and Fe. Its red color is due to the Mn levels. Artificial zincite is a by-product of zinc ore processing and is therefore available on the market. Natural and artificial zincite, along with galena, were important as crystalline detectors in the early era of radio, before the invention of vacuum tubes.

1. Characteristics

Crystal system: Hexagonal piramidal dihexagonal.

Color: Dark red to yellow-orange, rarely yellow, green or colorless. Small fragments are yellow.

Habit: Massive, compact, foliate, granular, spread throughout the matrix. Very rare hemimorphic crystals, up to 2.5 cm.

Cleavage: {10-10} perfect, but difficult.

Tenacity: Brittle.

Twinning: On {0001}

Fracture: Conchoidal, irregular.

Mohs Hardness: 4

Parting: Basal, at {000-1}, normally distinct.

Streak: Orange-yellow 

Lustre: Adamantine to resinous.

Diaphaneity: Transparent.

Density (g/cm³): 5.64 – 5.68

 

2. Geology and Deposits

Zincite is a primary mineral in metamorphic stratiform zinc mineral deposits, such as those found in the Franklin Mining District (Sussex Co., New Jersey, USA).

It also occurs as a secondary mineral derived from other zinc minerals in the oxidation zone of Zn-rich ores deposits.

It can occur as a product of volcanism.

 

3. Mineral Associations

In the type locality (Franklin Mining District, Sussex Co, New Jersey, USA) it is associated with franklinite, magnetite, willemite, hausmannite and calcite.

Also with other Zn minerals such as sphalerite, hydrozincite, smithsonite, hemimorphite and chalcophanite.

With other Mn minerals such as jacobsite, sussexite, hodgkinsonite, leucophoenicite, tephroite, hetaerolite and chlorophoenicite.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 2.013 – 2.032      nε: 2.029 – 2.048

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Deep red to yellow, without pleochroism.

Relief: Very high.

Cleavage: {10-10} perfect.

Habits: Generally rounded to anhedral grains. Massive, compact granular.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.016, corresponding to colors from gray and white to 1st order orange.

Extinction: No information available, probably paralell. 

Elongation sign: No information available.

Twins: On {0001}.

Zoning: No information available.

CONVERGENT LIGHT

Character: U(+)

2V angle: No.

Alterations: weathering or interaction with carbonated or hydrated solutions form secondary zinc minerals. The most frequent alteration products include smithsonite, hydrozincite, and goslarite.

May be confused with: other yellow minerals in a thin section, but these occur in other paragenesis.

Clinohumite is less red and more yellow.

Cuprite is a deep red color and much less orange.

Cinnabar is less orange.

Realgar can be similar.

Image of a thin section with zincite. The red-orange color is easily visible. Franklinite is black; willemite is colorless.
Zincite 01: In PPL, franklinite (opaque), willemite (colorless) and zincite, with various shades of yellow and orange. There is no pleochroism.
Zincite 01: now in XPL, the same section as in the previous image. The intense color of zincite masks possible interference colors. Willemite presents strong, colorful colors, with gray colors in the sections close to the isotropy section.

5. Reflected Light Microscopy

Sample preparation: after careful grinding, zincite acquires a good polish. The polishing hardness of zincite is moderate, much lower than the hardness of franklinite and hausmannite, with which it occurs in association.

PLANE POLARIZED LIGHT – PPL

Reflection color: Medium gray with a very light pink or pink brown tone.

Pleochroism: Very weak.

Reflectivity: 11.4 – 11.5%.

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy, probably low, completely masked by strong and vivid internal reflections.

Internal reflections: Strong and vivid inner reflections. The color of the internal reflections varies with the observed grain thickness: very fine grains show yellow, almost yellowish-white reflections; slightly thicker grains show orange reflections and very thick grains show intense red reflections in various shades (crimson, tile, etc.).

May be confused with: zincite’s characteristics are so distinct that its recognition is easy, considering its paragenesis.

General Characteristics: 

Grain shape: The grains are usually rounded or anhedral and their sizes vary widely.

Cleavage is often visible.

Partition becomes visible if the polished section is of low quality.

Twins are not visible in polished section.

Hausmannite exsolution lamellae are frequent, in the form of lamellae of varying thickness arranged parallel to (0001); in other directions shows grains with 4, 6 or 8 sides.

Partition becomes visible through a set of lines that form an almost checkered pattern of red and yellow tones

Zincite 01: In PPL, franklinite (white-yellow, left and right), zincite (light gray) and calcite (dark gray) in Franklin ore (New Jersey, USA). Franklinite closely resembles magnetite and zincite does not show diagnostic characteristics in PPL.
Zincite 01: the same section as in the previous image, now in XPL. Franklinite is isotropic (black), calcite has clear yellowish internal reflections and zincite shows many internal reflections in intense red, resembling cinnabar.
In XPL, zincite (red), calcite (yellowish) and franklinite (black – isotropic). Zincite has no characteristic habit, it is usually massive.
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