TOPAZ

Topaz – Al2[(F,OH)2SiO4)] – is a rare nesosilicate that constitutes an important gemstone of high value.

It has three varieties: “imperial topaz” (orange), “killiecrankie Diamond” (a Tasmanian variety) and “pyknite” (fine-grained topaz). It rarely fluoresces under ultraviolet light, exhibiting yellow, white, orange, or greenish-yellow fluorescence.

When colorless, even well formed and especially when granular, it is easily confused macroscopically with quartz if the observer does not pay attention to the crystalline form.

1. Characteristics

Crystal system:  Orthorhombic bipiramidal.         

Color:  Many colors: colorless, yellowish, pink, blue, red, orange, brown, green, blue violet, etc.    

Habit: Short to long, complex prismatic crystals are common. Columnar, compact, massive. 

Cleavage: {001} perfect. Vertical striations on the faces.      

Tenacity: Brittle.        

Twinning:  No.      

Fracture: Subconcoidal, irregular.       

Mohs Hardness: 8

Parting: No.         

Streak: White.         

Lustre: Vitreous.          

Diaphaneity: Transparent.           

Density (g/cm³):  3.5 – 3.6

 

2. Geology and Deposits

Topaz is characteristic of high temperature hydrothermal veins rich in volatiles related to granites, granitic pegmatites, greisens and rhyolites. In these rocks, topaz can occur as inclusions in other minerals or free in cavities, in the second case showing well-developed forms.

It also forms from the metamorphism of aluminous sediments rich in quartz and containing fluorine.

May occur as heavy mineral in placers.

 

3. Mineral Associations

In hydrothermal veins and pegmatites, it is associated with microcline, albite (clevelandite), tourmaline (schorlite, elbaite), beryl, fluorite, muscovite, quartz and bixbyite.

In greisens it is associated with muscovite, quartz, cassiterite and mica (“zinnwaldite”). In this type of deposit it can also occur disseminated in the rock or in cavities.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.606 – 1.634    nβ: 1.609 – 1.644   nγ: 1.616 – 1.644

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless without pleochroism.

In thick sections it is yellow, red or blue and is pleochroic: X = yellow; Y = yellow, violet or blue; Z = violet, bluish, yellow, pink.   

Relief: Moderate to high. Decreases with increasing fluoride content.           

Cleavage: {001} very good to perfect.          

Habits: Short columnar, but can form long prisms. It can be compact, massive, acicular or brush-shaped (pycnite). Typically in single grains, which may be euhedral. Tends to have many fluid inclusions.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.008 to 0.010, resulting in 1st order colors: white, gray in various shades to straw yellow, similar to quartz.           

Extinction:  Parallel in longitudinal sections and symmetrical in basal sections.          

Elongation sign: ES(-)            

Twins:  No.        

Zoning: No.             

CONVERGENT LIGHT

Character: B(+)          

2V angle: 48º (F, OH) up to 68º (F)         

Alterations: Can alter to kaolinite. But it’s tough, it’s a detrital mineral that concentrates in placers.

May be confused with: other minerals with low birefringence.

Quartz, which occurs associated, but quartz has low relief, has no cleavage and is U(+).

Andalusite is B(-) and has an irregular pink color.

Beryl is U(-).

Barite and celestite occur in different paragenesis.         

Topaz 01: In PPL, diagonally in the image, long prismatic crystal of topaz. It is colorless, with a medium-high relief and a goo cleavage perpendicular to elongation. Around it, muscovite and quartz.
Topaz 01: The same section as of the previous image, now in XPL. Topaz shows initial 1st order interference colors, parallel extinction and negative Elongation Sign.
Topaz 02: Quartz, topaz and mica in greisen from the famous town of Zinnwald (Cínovec), Czech Republic. In PPL, quartz and topaz are colorless and mica has a slight brown color. Topaz is granular and has high relief.
Topaz 02: The same section as of the previous image, now in XPL. Topaz and quartz show 1st order colors (gray, white) and micas are colored.
Topaz 03: In PPL, detail from the same thin section of the two previous images, from Zinnwald. In the center of the image, a mica (“zinnwaldite”) with a slight pleochroism in brown.
Topaz 03: The same section as of the previous image,, now in XPL. The mica ("zinnwaldite") is colorful, quartz and topaz are white/gray.
In PPL and observed with a 63x objective, topaz (high relief) and quartz (low relief). Topaz present a series of fluid inclusions, apparently all monophasic.
In PPL and observed with a 63x objective, topaz (high relief) and quartz (low relief). Topaz present a series of fluid inclusions, apparently all monophasic.

5. Reflected Light Microscopy

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

Sample preparation: despite the higher hardness, the polishing topaz is no problem and is of good quality.

PLANE POLARIZED LIGHT – PPL

Reflection color:  Dark gray, like quartz and feldspars.      

Pleochroism:  No.     

Reflectivity: Very low (~5%)        

Bireflectance:  No.      

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections:  Generalized in the color of the mineral in hadn specimen, usually in softer tones. Blue beryl shows discrete internal reflections in various shades of blue. Yellow beryl (orange) shows various shades between yellow and orange, etc.

Along the fractures (subconchoidal!) there are abundant multicolored reflections.     

May be confused with:  many other transparent minerals of several different colors.      

General Characteristics: 

Fluid inclusions, monophasic or biphasic (with gas bubbles (CO2)) can be observed in XPL, since the polished section allows visualization up to a certain depth of the sample. It is a diagnostic feature to a certain extent for topaz, considering its genesis, always remembering that quartz also has fluid inclusions very often.

Topaz 01: Blue topaz, with the edge of the polished section on the right. . Left, In PPL, it shows a gray color like common rock-forming silicates.
Topaz 01: The same section as of the previous image, now in XPL. Topaz shows soft and diffuse bluish tones. There is no diagnostic feature.
Topaz 02: Imperial topaz, with the edge of the polished section on the right. In PPL, it shows a gray color like common rock-forming silicates. There are some cavities and fractures (black).
Topaz 02: The same section as of the previous image, now in XPL. It shows internal reflections in the color of the hand specimen, which is a deep reddish orange.
In XPL, fluid biphasic inclusions (with gas bubbles) in blue topaz. These inclusions are easy to observe in XPL.
In XPL, fluid biphasic inclusions (with gas bubbles) in blue topaz.
In XPL, inclusions in Imperial Topaz.
In XPL, in imperial topaz, multicolored internal reflections on the fractures and orange internal reflections on the intact surface. At the top, the edge of the polished section. Multicolored internal reflections appear at various points in the topaz sections, regardless of the color of the mineral in hand specimen.