DAVYNE

Davyne – (Na,K)6Ca2(Al6Si6O24)(Cl2,SO4) – is an extremely rare tectosilicate, typical of volcanic lavas and ejecta containing leucite. It is not an ore.

It is classified in the Feldspathoid Group, within the Cancrinite Group. It can be considered a Cl-rich cancrinite. There is a CO2-free variety.

It is extremely similar to quadridavyna; differentiation requires analytical methods other than microscopy.

1. Characteristics

Crystal system: Hexagonal bipyramidal.          

Color: Colorless, white.     

Habit: Prismatic. Crystals up to 4 cm.       

Cleavage: {10-10} perfect, {0001} indistinct. Striations parallel to the length.

Tenacity: Brittle.        

Twinning: No.       

Fracture: Irregular, conchoidal.       

Mohs Hardness: 5.5 – 6

Parting: No.         

Streak: White. 

Lustre: Vitreous. 

Diaphaneity: Transparent.

Density (g/cm³): 2.42 – 2.53

       

2. Geology and Deposits

Davyna is typical of leucite-rich lavas and associated volcanic ejecta, in lithic blocks in fall deposits from phreatomagmatic eruptions.

It can occur in veins running through metabasalts.

The best-known occurrences are in Germany and Italy; the type locality is located in Italy (Monte Somma, Vesuvius, Naples). Other occurrences are on an island in the Red Sea and on the Kola Peninsula (Russia).

 

3. Mineral Associations

It occurs associated with minerals typical of basic and ultrabasic volcanic rocks, such as augite, olivine, and biotite.

It occurs with other feldspathoids (leucite, nepheline, and sodalite), with feldspars (sanidine), and amphiboles (hornblende).

Also with spinel, wollastonite, kaliophilite, humite, garnet, and vesuvianite.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 1,515 – 1,519    nε: 1,519 – 1,522

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:Colorless.

Relief: Low.

Cleavage: {10-10} perfect and {0001} indistinct.

Habits: Short hexagonal prisms, modified with bipyramids and pinacoids.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.004, extremely low, corresponding to interference colors between black and dark gray.

Extinction: Probably paralell.           

Elongation sign: No information available.

Twins: No.  

Zoning: No information available.            

CONVERGENT LIGHT

Character: U(+)          

2V angle: No.         

Alterations: No information available.           

May be confused with: Probably similar to analcime and chabazite.

Caption.

5. Reflected Light Microscopy

Reflected light microscopy is not the correct analytical technique for identifying davyna.

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.