HYDROZINCITE

Hydrozincite – Zn5(CO3)2(OH)6 – is a common carbonate mineral that occurs primarily in the oxidation zones of zinc ores. It constitutes a minor zinc ore. “Calamine” is a mining term applied to a mixture of hydrozincite, smithsonite, and hemimorphite.

It may contain Fe and Cu and forms pseudomorphs after dolomite. It exhibits intense blue fluorescence—sometimes with associated white or lilac hues—under short-wave ultraviolet light.

Literature on hydrozincite is very scarce. Traditional mineralogy textbooks often fail to even mention it, let alone describe its microscopic characteristics.

1. Characteristics

Crystal system: Monoclinic prismatic.        

Color: White to gray; it can be pink, pale yellow, or brown.  

Habit: Hydrozincite generally occurs as massive aggregates and crusts—ranging from earthy and compact to porous or even pulverulent—formed by very small crystals; these can resemble chalk. The structure of these masses tends to be radially fibrous and may be reniform. The density of these massive aggregates is high, reminiscent of agate. It can also occur with stalactitic and pisolitic habits. Tabular crystals are rare, reaching up to 6 mm in size.

Cleavage: {100} perfect.

Tenacity: Very brittle.    

Twinning: Contact twins on {100}.

Fracture: Irregular.

Mohs Hardness: 2 – 2.5

Parting: No. 

Streak: White.

Lustre: Silky, pearly, earthy matte.

Diaphaneity: Transparent.   

Density (g/cm³): 3.5 – 4

 

2. Geology and Deposits

Hydrozincite forms in the oxidation zone of Zn ores, primarily from sphalerite in sulfide ores, but also from smithsonite or hemimorphite in oxidized Zn ores.

It commonly occurs as encrustations on the walls of abandoned Zn mines.
 

3. Mineral Associations

It is associated with typical gangue minerals such as quartz, calcite, barite, and fluorite.

In oxidation zones, it occurs with:

– secondary Zn minerals (smithsonite, hemimorphite, willemite, franklinite, zincite, and ashoverite),

– secondary Cu minerals, with or without Zn (aurichalcite, rosasite, malachite, agardite-(La)),

– secondary Fe minerals (limonite), and

– Pb minerals, both primary (galena) and secondary (cerussite, wulfenite, boulangerite, meneghinite, plattnerite).

Naturally, it occurs with sphalerite, from which hydrozincite formed.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.630     nβ: 1.642     nγ: 1.750

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. 

Relief: Moderate.   

Cleavage: {100} perfect.

Habits: Massive aggregates with a radial fibrous structure. It can be earthy or resemble chalk. There are various other habits.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Very high maximum birefringence of 0.120, resulting in high-order interference colors (up to the 6th order) – specifically cream and brown hues – that are difficult to classify.    

Extinction: No information available.  

Elongation sign: No information available.  

Twins: Contact twins on {100}.     

Zoning: No information available.      

CONVERGENT LIGHT

Character: B(-)   

2V angle: 40º (calculado)

Alterations: No information available.        

May be confused with: No information available. 

Caption.

5. Reflected Light Microscopy

Reflected Light Microscopy is obviously not the recommended analytical method for identifying hydrozincite. However, it is important to create a slide or polished section to identify the opaque minerals that occur associated with hydrozincite, such as galena and boulangerite.

Sample preparation: xx

PLANE POLARIZED LIGHT – PPL

Reflection color: xx

Pleochroism: xx

Reflectivity: xx

Bireflectance:  xx

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: xx

Internal reflections:  xx

May be confused with: xx

General Characteristics:

Caption.