GLAUCODOT

Glaucodot – (CoFe)AsS – is a quite rare and unusual sulfide, important as a cobalt ore alongside cobaltite, alloclasite, and other cobalt minerals.

It is classified within the arsenopyrite group. It may contain nickel.

1. Characteristics

Crystal system: Orthorhombic pyramidal. 

Color: From grayish tin-white to reddish silver-white.     

Habit: Large, euhedral prismatic crystals. It can be granular or massive.

Cleavage: {010} perfect, {101} good. 

Tenacity: Very brittle.        

Twinning: On {101} and {012}, forming cross-shaped penetration twins. It also forms three-crystal twins, known as “trillings.”       

Fracture: Irregular.       

Mohs Hardness: 5

Parting:  No.        

Streak: Black.         

Lustre: Metallic.          

Diaphaneity: Opaque.           

Density (g/cm³): 6.0

 

2. Geology and Deposits

Glaucodot is characteristic of high-temperature hydrothermal deposits formed at great depths, generally occurring in complex polymetallic deposits.

 

3. Mineral Associations

It is associated with a large number of sulfides (of Zn, Fe, Cu, Pb, and Ag) and other minerals. Its association with arsenopyrite is particularly common.

It is found alongside:

Fe sulfides such as pyrite and pyrrhotite,

Cu sulfides such as chalcopyrite,

Co sulfides such as cobaltite,

arsenides such as rammelsbergite, safflorite, skutterudite, and niccolite.

It is also associated with uraninite (pitchblende) and native gold.

Highly important—and diagnostic—is its association with erythrite, a transparent mineral with an extremely characteristic magenta-red color known as “cobalt bloom,” which forms as a secondary mineral on Co-bearing minerals in general. Thus, erythrite “reveals” the presence of cobalt in the ore and indicates the presence of glaucodot, cobaltite, and/or other cobalt-bearing minerals.

The potential for association with radioactive minerals highlights the need to conduct a radioactivity analysis on cobaltite-bearing ore!

 

4. Transmitted Light Microscopy

Not applicable, as glaucodot is completely opaque.

5. Reflected Light Microscopy

Sample preparation: Its polishing hardness is relatively high, though slightly lower than that of arsenopyrite and significantly lower than that of cobaltite. Consequently, it takes an excellent polish. Its relief relative to chalcopyrite, sphalerite, and other medium-hardness ore minerals is lower than its relief compared to arsenopyrite.       

PLANE POLARIZED LIGHT – PPL

Reflection color: White to very pale cream.
Compared to the color of arsenopyrite, the color of glaucodot is more bluish-white rather than creamy-white. 

Pleochroism: Weak, difficult to perceive, in yellowish tones. Weaker than arsenopyrite pleochroism. 

Reflectivity: 50%        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Distinct anisotropy between blue and yellow (or brownish-gray), very similar to—but less sharp than—that of arsenopyrite.

Internal reflections: No.      

May be confused with: several other minerals that are white, with absent or weak pleochroism and anisotropy ranging from blue to another color, usually yellowish or brownish:

Arsenopyrite exhibits much stronger anisotropy than glaucodot, in similar colors.

Löllingite is very similar to glaucodot, and detailed analysis is required.

Marcasite shows stronger pleochroism and anisotropy in brown hues in addition to blue.

Safflorite exhibits anisotropy ranging from dark brown to blue; the crystals typically form rhombs.

Rammelsbergite shows stronger pleochroism and anisotropy ranging from dark brown to blue.

Stibnite displays much stronger pleochroism than glaucodot and less vivid colors.

General Characteristics: 

Grain shape: generally well-developed, large, idiomorphic crystals—polygonal with straight edges and sharp vertices—strongly resembling arsenopyrite.

Tarnishing may occur on glaucodot, unlike the associated arsenopyrite, which does not tarnish as readily. In this context, it is interesting to examine polished sections prior to repolishing!

Cleavage parallel to (001) and (110) may be well developed.

Inclusions IN glaucodot are frequent; they may consist of galena.

Skeletal inclusions OF glaucodot occur in cobaltite.

Glaucodot rims may form around pyrite and cobaltite.

Veinlets of cosalite and cobaltite may occur within glaucodot.

Exsolution lamellae of glaucodot may occur in Co-bearing arsenopyrite.

In PPL, tarnished glaucodot grains (left) alongside untarnished arsenopyrite (right) within a silicate matrix (dark gray). Pits and fractures in the polished section appear black.
In PPL, tarnished glaucodot grain (center) alongside untarnished arsenopyrite within a silicate matrix (dark gray). Pits and fractures in the polished section appear black. Due to the very characteristic and rapid tarnishing of glaucodot, it is advisable to polish the sections and then let them sit for two weeks in order to accurately assess the relative proportions of arsenopyrite (which does not tarnish as easily) and glaucodot, given that the other optical properties of these two minerals are very similar.
The tarnishing of glaucodot also makes it possible to distinguish arsenopyrite inclusions (white and untarnished) within glaucodot grains (tarnished). PPL.
Parallel white bands of arsenopyrite (untarnished) suggest a more complex pattern of coexistence with glaucodot (tarnished). Generally, these features are no longer visible after repolishing the section. Silicates in dark gray, holes in black. PPL.
In PPL, the center of the image, a glaucodot grain exhibits areas of varying tarnishing distributed in well-defined zones, suggesting it represents one of the common twins of glaucodot. Arsenopyrite appears white, silicates dark gray, and voids and fractures black.
In PPL, in the center of the image, a twinned glaucodot grain, revealed by variations in tarnishing distributed in well-defined zones. Arsenopyrite in white, silicates in dark gray, and voids and fractures in black.
In PPL, at the center of the image, a perfectly idiomorphic, complex glaucodot grain with arsenopyrite inclusions (white) and zones of varying intensity of tarnishing, suggesting the presence of twinning.
Glaucodot can exhibit exactly the same rhombic sections as arsenopyrite. Only the intensity of the anisotropy colors distinguishes the two minerals. PPL.
Glaucodot 01: Glaucodot and arsenopyrite. Their reflection colors, reflectivity, and habit are almost identical. In this case, the grains above the red line are glaucodot. The grains below the red line are arsenopyrite. PPL.
Glaucodot 01: The same section as of the previous image, now in XPL. The most intense anisotropy colors – blue and orange – are typical of arsenpyrite, while glaucodote shows similar, but much weaker, colors. Furthermore, the arsenopyrite appears more uniform while the glaucodote, at least in this sample, appears mottled, with inclusions that are probably glaucodote grains with a different orientation.
Glaucodot 01: The same section as of the two previous images, now also in XPL and rotated 90 degrees. The color differences are very evident.
In XPL+2 degrees, (with photoshop!), a roughly rhombohedral grain of glaucodot showing its orange anisotropy color and some inclusions of arsenopyrite (bluish-gray).