SENARMONTITE

Senarmontite – Sb2O3 – is a rare oxide, typically a product of alteration of Sb minerals. It is found in Sb ores, where it can even occur frequently, but always in small quantities, never as the main ore mineral.

The compound (Sb2O3) is commercially known as “antimony trioxide” and is obtained by melting stibnite, which is oxidized in furnaces at temperatures between 850 and 1,000°C. It is mainly used as a flame retardant in non-flammable materials, combined with other materials. It is toxic when volatile; probably carcinogenic.

Senarmontite is isostructural with arsenolite and dimorphic with valentinite (orthorhombic), which also forms under the same conditions as senarmontite.

In the literature, there is practically no information about the characteristics of senarmontite under a microscope. There are no images at all. Even classic books and websites either ignore it or present only some fragmentary data.

1. Characteristics

Crystal system:  Cubic hexaoctahedral.         

Color: Colorless, grayish white, white.     

Habit: Octahedrons, modified by cube and dodecahedron, up to 3 cm. Granular, crusty to massive.       

Cleavage: {111}, in traces.       

Tenacity: Brittle.        

Twinning: No.       

Fracture: No information available.       

Mohs Hardness: 2 – 3

Parting: No.         

Streak:  White.        

Lustre: Resinous, greasy to adamantine.          

Diaphaneity: Transparent.           

Density (g/cm³):  5.5

 

2. Geology and Deposits

Senarmontite is formed from the alteration of stibnite, native antimony, and other antimony minerals in the oxidation zone of hydrothermal deposits containing Sb.

 

3. Mineral Associations

It occurs associated with some common gangue minerals, such as quartz (including chert), dolomite, and barite.

It also occurs with a number of Sb minerals, such as native antimony, Sb sulfides (stibnite, metastibnite, berthierite), Sb oxysulfides (cetineite), Sb oxides (kermesite, valentinite, stibioconite, and tripuhyite), and Sb hydroxides (mopungite).

Also with celestine and native sulfur.

 

4. Transmitted Light Microscopy

Refraction indices:  n:: 2.087  

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.

Relief: Very high.           

Cleavage: {111} in traces, which is not visible under a microscope.

Habits: Granular, crusty, massive. Crystals typically as octahedrons, combined with cubes and dodecahedrons. 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: It is isotropic, but it can show strong anomalous anisotropy; it can be sectorized.

Extinction:  Isotropic.           

Elongation sign:  Isotropic. 

Twins: No.

Zoning: It can occur, and it can also be sectorized. 

CONVERGENT LIGHT

Character: Isotropic.

2V angle:  Isotropic.        

Alterations: No information available.          

May be confused with: No information available.          

Caption.

5. Reflected Light Microscopy

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

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.