MIMETITE

Mimetite – Pb5(AsO4)3Cl – is a rarer arsenate, typically a secondary mineral in Pb deposits. It is part of oxidized Pb ores, but is never the main ore mineral in the deposit. Aesthetic pieces fetch high prices in the collector mineral market.

It is classified in the Apatite Group, a group of isomorphic hexagonal minerals. It forms an isomorphic series with pyromorphite, another series with vanadinite, and a third series with hedyphane. In many cases, the minerals are so similar to each other that they can only be differentiated through chemical analysis.

Mimetite is a polymorph of Mimetite-2M and Mimetite-M. It may contain F, Cr, Ca, and V. There are at least 6 varieties. It may exhibit reddish-yellow fluorescence under shortwave and longwave ultraviolet light. It is piezoelectric.

Since mimetite is neither a transparent rock-forming mineral nor an opaque ore mineral, its microscopic characteristics, both in transmitted and reflected light, are not addressed in either of the two groups into which mineralogy textbooks and other sources are divided. Furthermore, its occurrence in oxidized ores, typically cavernous/porous, makes the preparation of thin sections and polished sections difficult, requiring impregnation. As a result, there is little information available about the characteristics of mimetite under a microscope.

1. Characteristics

Crystal system: Hexagonal bipyramidal.          

Color: Pale yellow, yellowish brown, orange-yellow, orange-red, red, green, white, colorless.     

Habit: Generally hexagonal prisms, up to 12 cm. Barrel shape possible. Stalactitic, globular, reniform, granular, tabular, acicular.

Cleavage: {10-11} in traces.      

Tenacity: Brittle.        

Twinning: On {11-22}, very rare.

Fracture: Irregular, subconchoidal.       

Mohs Hardness: 3.5 – 4

Parting: No.         

Streak: White.         

Lustre: Sub-adamantine, resinous.          

Diaphaneity: Transparent.           

Density (g/cm³): 7.24

           

2. Geology and Deposits

Mimetite is a typical secondary mineral that occurs in oxidized zones of Pb ores, usually formed from galena and arsenopyrite.
It also occurs in other environments containing Pb and As.

 

3. Mineral Associations

It occurs associated with common gangue minerals such as quartz, barite, fluorite and carbonates (calcite, dolomite).

It evidently occurs with several other secondary Pb minerals such as anglesite, leadhillite, mottramite, vanadinite, pyromorphite, bayldonite, caledonite, cerussite, descloizite, wulfenite and beudantite.

It is associated with secondary Zn minerals (hemimorphite, smithsonite, willemite) and secondary Cu minerals (malachite).

Also with rare minerals like behoite.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 2.147      nε: 2.128                 

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless or pale yellow. Weak pleochroism if macroscopically colored.

Relief: Very high.           

Cleavage: Not visible.           

Habits: Hexagonal prisms are very typical.
It can show a barrel shape. Stalactitic, globular, reniform, granular, acicular. Forms subparallel groups of prismatic crystals.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.019, corresponding to colors up to the end of the 1st order: gray, yellow, orange, red, and blue. Basal sections are isotropic.

Extinction: Parallel. Isotropic in basal sections.

Elongation sign: No information available.

Twins: On {11-22}, very rare.         

Zoning: No information available.             

CONVERGENT LIGHT

Character: U(-), may be anomalous B(-).          

2V angle: No.         

Alterations: Mimetite alters to bayldonite.          

May be confused with: Other secondary lead minerals, especially pyromorphite and vanadinite.         

Caption.

5. Reflected Light Microscopy

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

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption.