KOSMOCHLOR

Kosmochlor – NaCr(Si2O6) – is a very rare inosilicate, from the Clinopyroxene Subgroup (Pyroxene Group). It is important as an ore because it is a constituent of the ornamental rock “Maw Sit Sit”.

It may contain, as impurities, Mn, Mg, Fe, Ti, Ca, K, P and Al.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color:  Dark/intense emerald green. In thin flakes it is green or yellow.    

Habit: Short, fibrous, granular prismatic structure. Crystals up to 2 mm.        

Cleavage:  {110} good.      

Tenacity: Brittle.        

Twinning: Simple and lamellar by {100} and {001}.

Fracture: Irregular.       

Mohs Hardness: 6

Parting: on {001}.          

Streak:  White  .      

Lustre: Vitreous.          

Diaphaneity: Transparent.           

Density (g/cm³):  3.51 – 3.60 

 

2. Geology and Deposits

Kosmochloride is found in high-grade metamorphic rocks rich in Cr, such as pyroxenites (“kosmochlorites”). It is the main rock-forming mineral in some jadeites.

Kosmochloride also occurs in iron meteorites of the hexahedrite and octahedrite types, so much so that the type locality is the Toluca meteorite (Mexico).

It is responsible for the color of the ornamental rock with the (commercial) name “Maw Sit Sit”.

Occurrences of kosmochloride are divided between terrestrial rocks (9 occurrences) and meteorites (9 occurrences). 

 

3. Mineral Associations

It is associated with chromite, jadeite, Cr-jadeite, Cr-magnesium arfvedsonite, Cr-rich jadeite pyroxenes (diopsides), graphite (“cliftonite” – pseudocubic graphite), and sodic amphiboles (e.g., eckermannite).

In meteorites, it is associated with troilite.

It is also associated with daubréelite, krinovite, roedderite, Cr-albite, native tin, richterite, and chlorite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1,740 – 1,766     nβ: 1,756 – 1,778      nγ: 1,742 – 1,781

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Green or yellow, with strong pleochroism between:

X = yellowish green
Y = grass green
Z = emerald green.

Relief: High.            

Cleavage: {110} good, in the basal sections the two cleavages are exposed which show the characteristic angle of pyroxenes, of 87º and 93º.

Habits: short prismatic (2 mm), fibrous, granular.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.015 to 0.022, corresponding to colors ranging from white, yellow, orange, blue, red, and green up to half of the 2nd order.      

Extinction: oblique, with angles between 8 and 22º.           

Elongation sign:  No information available.           

Twins: Simple and lamellar twinning occurs for {100} and {001}.       

Zoning:  No.            

CONVERGENT LIGHT

Character:  B(-)         

2V angle: 53º           

Alterations: No information available.          

May be confused with: other monoclinic pyroxenes.

Other common clinopyroxenes are B(+).

Aegirine also exhibits green pleochroism, but the pleochroism and birefringence of kosmochlor are more intense.         

Caption

5. Reflected Light Microscopy

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

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
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