VESUVIANITE

Vesuvianite – Ca10(Mg, Fe)2Al4(SiO4)5(Si2O7)2(OH,F)4 –  is a sorosilicate of more restricted occurrence, typical of impure marbles that have undergone contact metamorphism. It has no economic importance beyond its occasional use as a gemstone.

An old name for vesuvianite, still widely used, is “idocrase”.

Vesuvianite is part of the Vesuvianite Group and is isostructural with fluor-vesuvianite, mangan-vesuvianite, and wiluite. Common impurities are many: Be, B, F, Cu, Li, Fe, Mn, Na, K, Cr, Zn, and H2O. It has nine varieties, generally based on higher contents of elements such as Be, Ce, Cr, Mg, and Mn.

When yellowish or brown in color, with a short to rounded prismatic shape, it is easily confused in hand samples with garnets (grossularite and andradite) that occur in the same paragenesis.

1. Characteristics

Crystal system: Tetragonal, ditetragonal bipyramidal.          

Color: Brown, caramel, yellow, dark brown, very light green, bluish green, white, red, purple, blue. Zoning is common.

Habit: Short to long prismatic, granular, massive. Crystals of great morphological complexity. 

Cleavage:  {110} bad, {100} e {001} very bad.      

Tenacity: Brittle.        

Twinning: Very thin.       

Fracture: Subconchoidal to irregular.       

Mohs Hardness: 6.5

Parting: No.         

Streak: White.         

Lustre: Vitreous to resinous.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.32 – 3.43

    

2. Geology and Deposits

It commonly occurs with impure marbles and limestones that have undergone contact metamorphism (skarns, calc-silicate rocks).

Sometimes it occurs as a primary phase in alkaline igneous rocks (nepheline syenites).

It also occurs in veins and cavities of hydrothermally altered basic and ultrabasic (mafic and ultramafic) rocks, such as gabbros, serpentinites, and peridotites.

 

3. Mineral Associations

It is associated with the characteristic and common minerals of skarns, such as calcite, garnet (grossular, andradite), diopside, tremolite, epidote, titanite, and wollastonite.

It also occurs with fassaite (a variety of augite), pectolite, hessonite, clinochlore, scapolite, spinel, and groutite.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 1.703 – 1.752        nε: 1.700 – 1.746

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, pale green, pale brown, pale yellow.

It usually does not have pleochroism. Very rarely it has very weak pleochroism between:

X = colorless to yellowish;

Y = yellowish, greenish, brownish.

Relief: High.           

Cleavage: {110} bad; {100} and {001} very bad, are generally not visible.           

Habits: Short and long prisms, but it is usually granular anhedral.

It also occurs massive. It is rarely acicular, and may form spherulitic aggregates.

It may show growth zones with different optical characteristics (sectorized).

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Birefringence of 0.003 – 0.006: corresponds to first-order mid-gray colors.

Very common, however, are intense anomalous colors: gray,lavender blue, indigo blue, leather brown, gray-green, or purple.

In the section parallel to the major axis, hourglass textures may bepresent.          

Extinction:  Tends to paralell.          

Elongation sign:  SE(-). The wiluita variety exhibits SE(+).           

Twins: Typically there are no twins, but the grains may show sectors.

Anomalous biaxial varieties may show four sectors in basal sections.

Zoning: typically zoned or with colored stripes (“patchy”)             

CONVERGENT LIGHT

Character: U(-), can be an anomalous B(-).

The wiluite variety (very rare) is U(+) and has SE(-) and SE(+).

2V angle: It does not present. If anomalous biaxial, 2V of 17 – 33º.

Alterations: It is a very robust mineral, but in some cases it alters to prehnite.          

May be confused with:  Several other minerals.

Vesuvianite is a more difficult mineral to identify. Diagnostic features include high relief, absence of cleavage, low birefringence (gray colors), and, when present, anomalous interference colors distributed by sectors. Paragenesis is important: the presence of calcite and clinopyroxenes (diopside) suggests the possibility of vesuvianite occurrence.

Andalusite has a larger 2V angle and its habit is generally prismatic.

Carbonatite apatite can be very similar, but it does not exhibit anomalous interference colors, sectorized grains, or concentric zoning, which is common in vesuvianite.

Melilite has lower refractive indices, a different habit, and a different paragenesis.

Zoisite and clinozoisite can be confused with the very rare variety wiluite.

 

Vesuvianite 01: in PPL, idiomorphic grain of vesuvianite in skarnite. It shows high relief and a very pale brown color with zoning (border with a stronger color).
Vesuvianite 01: The same crystal as of the previous image, now in XPL. It shows low interference colors (gray, white) with sectorization.
Vesuvianite 02: Vesuvianite crystals tending to be idiomorphic. In PPL they are typically colorless, with high relief and no cleavage.
Vesuvianite 02: The same section as of the previous image, now in XPL. Vesuvianites show anomalous interference colors in yellow-brown and violet, with spots. Extinction is parallel and the elongation sign is negative.
Vesuvianite 03: In PPL, colorless vesuvianite crystal with few diopsides (sligthly greenish). Sometimes, as here, vesuvianite is difficult to identify.
Vesuvianite 03: The same section as of the previous image, now in XPL. Vesuvianite is gray without other diagnostic features and diopsides are colorful.
In PPL, vesuvianites with anomalous interference colors, here with gray and brownish colors. These anomalous colors, their distribution in spots and eventual zonation are quite typical for vesuvianite.
In XPL, vesuvianite with yellow-brown, gray and violet interference colors.
In XPL, prismatic grains of vesuvianite with a subtle zoning in anomalous interference colors.

5. Reflected Light Microscopy

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

Sample preparation: Polishing vesuvianite requires no special care and results in very good quality polishing, equivalent to the quartz and calcite that usually accompany it in skarns.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, lighter than calcite, pyroxenes, and amphiboles, approximately the same color as quartz.       

Pleochroism: No.      

Reflectivity: Low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: No anisotropy is observed.        

Internal reflections: Widespread in the same colors of the mineral in hand specimen, usually brown, may be yellowish to caramel. The thickness of the mineral at the point in question influences the intensity of the color: the thicker the grain, the more intense the color, and vice versa.      

May be confused with: Considering its paragenesis, it can be confused with grossular garnet, which may have a similar color.       

Vesuvianite 01: In PPL, two vesuvianite grains surrounded by quartz and calcite. Vesuvianite show a better polish, but the reflection color is not helpful for its identification.
Vesuvianite 01: The same section as of the previous image, now in XPL. Vesuvianite, in this case(!), exhibits brownish-yellow reflections in various shades of intensity. Calcite and quartz show light/white reflections.
In XPL, vesuvianite in skarn with reflections in various shades of yellow-brown and some associated multicolored reflections.
In XPL, in the same sample as of the previous image, vesuvianite with intense reddish-brown reflections, showing the great variation in the shades of the internal reflections. Calcite and quartz with white reflections.
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