ARFVEDSONITE

Arfvedsonite – [Na] [Na2] [Fe2+4Fe3+] Si8O22(OH)2 –  is an inosilicate, a sodium amphibole, quite rare, typical of rare alkaline rocks. It has no economic use; it is found on the market only as a mineral for collectors.

“Arfvedsonite” is not actually a mineral, but just a generic term used to designate the Arfvedsonite Group’s minerals. This group is composed of fluoro-arfvedsonite (extremely rare), magnesio-arfvedsonite (rare, but the most common), magnesio-fluoro-arfvedsonite (very rare), potassic arfvedsonite (extremely rare), potassic-magnesio-arfvedsonite (extremely rare) and potassic-magnesio-fluoro-arfvedsonite (extremely rare). Differentiation of the members of the Group under the microscope is not possible.

Chemically, arfvedsonite is similar to riebeckite and forms a solid solution with eckermannite. The largest crystals reached 60 cm in length. May contain Ca, Al, Mn, Ti, K and F.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Bluish black to black.     

Habit: Generally elongated prisms, may be in radial aggregates. Tabular, granular.       

Cleavage: {110} perfect, typical of amphiboles. Striations paralell to the z axis. 

Tenacity: Brittle.        

Twinning: Simple and lamellar parallel to{100}.       

Fracture: Irregular.      

Mohs Hardness: 5 – 6

Parting:  On {010}.        

Streak: Dark bluish gray, gray-green.         

Lustre: Vitreous.       

Diaphaneity: Transparent.           

Density (g/cm³): 3.3 – 3.5

          

2. Geology and Deposits

Arfvedsonite occurs in rare alkaline igneous rocks such as alkaline granites (brandbergites and ekerites), pegmatitic peralkaline syenites (bowralites), nepheline-syenites (lakarpite, kakortokite and lujavrite), sodic syenites (naujaites) and associated pegmatites, phonoliths, and trachytes.

It also occurs in eudyalithites. In volcanic rocks it occurs in pantellerites.

 

3. Mineral Associations

Arfvedsonite is generally associated with riebeckite, aegirine, aegirine-augite, hastingsite, kataphorite, alkali feldspar (microcline, orthoclase, in volcanic rocks to anortoclase), albite, biotite, sodalite and nepheline. Also with quartz, aegirine, zircon, zektzerite and calcite.

In brandbergites, it is associated with potassic feldspar and biotite.

In ekerites, it occurs with anorthoclase, micropertites and aegirin.

In bowralites, it occurs with potassic feldspar and aegirine-augite.

In larkarpites, it is associated with potassium feldspar, nepheline, aegirine and pectolite.

In kakortokites, it occurs with potassium feldspar, nepheline and eudyalite.

In lujavrites with alkali feldspars (orthoclase or microcline), nepheline, eudyalite, aegirine and albite.

In naujaites with alkali feldspar, sodalite and nepheline.

In pantellerites with anortoclase (or sanidin), aegirin, fayalite, ilmenite, aenigmatite and ferrorichterite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα:   1.652 – 1.699     nβ:  1.660 – 1.705     nγ: 1.666 – 1.708

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Strong, deep colors dominating the green: bluish green, yellowish-green, brownish-green, grayish-green, violet tones are possible.

Moderate to strong pleochroism in these colors:

X = blue-green to deep blue-green;

Y = pale bluish-green to blue-gray;

Z = pale yellowish-green, dark green, greenish-blue grey.

The color can be so dark that arfvedsonite can even be mistaken for an opaque mineral. It is possible to identify it by crossing the nicols, inserting the compensator and rotating the stage: Opaque minerals remain completely black; transparent isotropic minerals turn uniform pink in color and arfvedsonite shows luminosity variations in dark colors.          

Relief: High.           

Cleavage: {110} perfect, as in all amphiboles: in the basal sections two perfect cleavages forming an angle of 124 and 56º between each other, in the longitudinal sections only one cleavage. Partition on {010}.           

Habits: Thick prismatic crystals elongated along the z axis, without perfect terminations. Tabular according to {010}, acicular, granular, also poikilitic (“sieve texture”).            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.010 to 0.014, colors from gray to yellow of superior 1st order, strongly anomalous, often masked by the intense colors of the mineral.

With increasing Fe content, refraction increases and interference colors decrease.           

Extinction: Tends to oblique, from 0 to 30º.           

Elongation sign:  ES(-). The strong colors of arfvedsonite make it extremely difficult to determine the Elongation Sign.           

Twins: Simple on (100) and lamellar.         

Zoning: Frequently zoned.             

CONVERGENT LIGHT

Character: B(-) or B(+), very difficult to determine due to the own strong colors of the mineral.          

2V angle:  normally 70-80° (may range from 0 to 100° depending on chemical composition).        

Alterations: through uralitization it alters to pyroxenes (tremolite, actinolite).          

May be confused with:  few other minerals.

Glaucophane is bluer, less green, lighter and occurs in different paragenesis.

Tourmaline may be similar, but has color in patches and zones and is uniaxial.

Aegirine also has strong colors, but is lighter and its pleochroism is much more intense (between green and yellow). Furthermore, it is a pyroxene and has basal sections with two cleavages crossing at angles of almost 90º.         

In PPL, arfvedsonite with the bluish green color that it can show. Sometimes the blue is more concentrated in spots. Sometimes it is absent. Around arfvedsonite, quartz (top) and orthoclase (right).
Arfvedsonite 01: In PPL, idiomorphic arfvedsonite. It shows strong colors and strong pleochroism in green and blue(!). The relief is high and the cleavage a little bit difficult to see. Around it, feldspars altered to clay minerals (gray) and quartz (colorless).
Arfvedsonite 01: The same section as of the previous image, now in XPL. Strong original colors mask the interference colors.
Arfvedsonite 02: In PPL, longitudinal section of arfvedsonite with its first pleochroism color, almost black. The next image shows the second pleochroism color.
Arfverdsonite 02: The same section as of the previous image, rotated 90 degrees clockwise. In this position, arfvedsonite shows the second pleochroism color, a yellowish green.
Arfvedsonite 03: The same section as of the two previous images, now in XPL. The interference colors of arfvedsonite are masked by the strong clors of the mineral.
Arfvedsonite 03: In PPL, several arfvedsonite crystals side by side.
Arfvedsonite 03: The same crystals as of the previous image, also in PPL, now rotated 90 degrees counterclockwise to show the second pleochroism color.
Arfvedsonite 03: The same crystals as of the two previous images, now in XPL. Its strong colors masks the interference colors.
In PPL, arfvedsonite with well-defined zonation.
In XPL, arfvedsonite with intense anomalous colors. Around it, orthoclase in gray.
In this arfvedsonite crystal, observed in XPL and in the maximum illumination position, the blue color is very strong, dominant. Despite being quite dark, it is easy to see.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of arfvedsonite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with arfvedsonite.

Sample preparation: arfvedsonite is easily polished, it polishes slightly better than feldspars. The polished section makes it possible to observe in 3D the acicular arfvedsonite crystals embedded in feldspars.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray. A lighter color, however, than feldspar in general.      

Pleochroism: No.      

Reflectivity:  Very low (<<10%).       

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Faint anisotropy masked by internal reflections.       

Internal reflections: Widespread, very dark, almost black. At some points, especially at the edges of the crystals, the reflections are bright green.     

May be confused with: other dark minerals.

Green hornblende has better defined cleavage.

Pyroxenes (aegirine) have stronger anisotropy.

Biotite has well-defined straight extinction, darker reflection color, better cleavage, and much lower quality polish.       

General Characteristics: 

Grain shape: prisms typically lack the perfect termination.

Polishing pits, due to cleavage, are common and can evolve into holes without a defined shape and with several different sizes.

Cleavage was not observed.

Arfvedsonite 01: In PPL, in syenite, feldspar (left, darker gray), arfvedsonite (right, lighter gray) and hematite (white, top, from the transformation of a magnetite). In black, holes in the polished surface.
Arfvedsonite 01: The same section as of the previous image, now in XPL. Feldspar has white internal reflections, arfvedsonite has very dark internal reflections with some greenish spots, and hematite, due to the small grain size, is dark in the image.
In XPL, acicular crystals of arfvedsonite in feldspar of a syenite. Reflected Light allows you to visualize in 3D the small arfvedsonite crystals, when they are inside a transparent mineral of light colors.
In XPL, poor quality termination of a prismatic arfvedsonite crystal. Typically, it is not idiomorphic, but irregular, with several crystal growth points, aligned or not.