PRISMATINE

Prismatine – (◻,Fe,Mg)(Mg,Al,Fe)5Al4Si2(Si,Al)2(B,Si,Al)(O,OH,F)22 – is an extremely rare sorosilicate, found in only a few locations on the planet. It has no economic importance.

For many years, prismatine crystals were confused with kornerupine, with which it forms a solid solution. There is recent literature on the subject, which should be consulted for details.

1. Characteristics

Crystal system: Orthorhombic bipyramidal.        

Color: Gray-green.

Habit: Long prismatic shape, which gave rise to the name.

Cleavage: {110} perfect.

Tenacity: Brittle.      

Twinning: No information available. 

Fracture: No information available.

Mohs Hardness: 6.5 – 7

Parting: No information available.   

Streak: No information available.

Lustre:  Vitreous.

Diaphaneity: Transparent.     

Density (g/cm³): 3.34

 

2. Geology and Deposits

The mineral was discovered in an outcrop of hydrothermally altered granulite, located behind the railway station of Waldheim, a small town in Middle Saxony, Germany.

It represents the granulite facies metamorphism of boron-rich pelitic (clayey) sediments.

 

3. Mineral Associations

It occurs associated with quartz, plagioclase (oligoclase),
micas (biotite, muscovite (sericite) and chlorite),
minerals of the Al2SiO5 Group (kyanite, sillimanite and andalusite),
garnet (almandine), rutile, corundum (ruby), tourmaline, dumortierite, sapphirine.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.669 – 1.671         nβ: 1.681 – 1.683         nγ: 1.682 – 1.684

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. 

Relief: High.    

Cleavage: {110} perfect.

Habits: Long prismatic.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.013, corresponding to colors up to first-order yellow, orange, and red.

Extinction: Parallel, symmetrical in the basal sections.   

Elongation sign: ES(-)    

Twins:  No.     

Zoning: No.     

CONVERGENT LIGHT

Character: B(-)  

2V angle: 28-38º

Alterations: No information available.        

May be confused with: In the paragenesis of the type locality, there are five different minerals that are similar to each other.

Corundum has no cleavage and is uniaxial.

Sillimanite has more intense interference colors and is ES(+).

Andalusite has different habits.

Kyanite has oblique extinction and is ES(+).

Kornerupine is extremely similar, but may exhibit pleochroism.

Prismatine 01: In PPL, longitudinal section of prismatine, accompanied by tourmaline, plagioclase, quartz, corundum, sillimanite, sericite and rutile. The thin section is from the granulite of the type locality, the small town of Waldheim in Saxony (Germany), in the outcrop behind the town's railway station.
Prismatine 01: The same section as of the previous image, now in XPL. Low interference colors are characteristic for prismatine.
Prismatine 02: In PPL, other longitudinal section of prismatine from the type-locality. Excellent cleavage and high relief are typical.
Prismatine 02: The same section as of the previous image, now in XPL. Low interference colors and negative elongation sign are diagnostic, as well as paragenesis with tourmaline.
Prismatine 03: In PPL, approximately basal section of prismatine. The section tends to have 6 sides. The section resembles a garnet (but which, in XPL, is isotropic, which is not the case for prismatine).
Prismatine 03: The same basal section as of the previous image, now in XPL. It is similar to the basal section of corundum, which occurs in the same paragenesis, but is uniaxial, while these basal sections of prismatine exhibit a biaxial figure with a high 2V angle.
Prismatine 03: The same basal section as of the two previous images, now in the extinction position. It is possible to see that extinction is symmetrical.

5. Reflected Light Microscopy

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: xx

Caption.