LÖLLINGITE

Löllingite – FeAs2 – is a relatively rare arsenide, usually found in small quantities in deposits containing arsenic minerals. It does not serve as a primary ore of arsenic, at most acts as a secondary one.

It is classified within the Löllingite Group and forms a series with safflorite. It may contain Bi, Sb, and S, and has four varieties (containing Ni, Co, S, or higher levels of Co).

When struck or heated, löllingite releases vapors with a strong garlic odor. These arsenic-bearing vapors are TOXIC! Furthermore, the ore can be radioactive due to the presence of pitchblende and uraninite.

1. Characteristics

Crystal system: Orthorhombic bipyramidal. 

Color: From steel gray to brilliant silver-white.

Habit: Prismatic, granular, massive. 

Cleavage:  {010} distinct, {101} distinct.      

Tenacity: Brittle.        

Twinning: Trillings by {001} and polysynthetics by {101}.       

Fracture: Subconchoidal.       

Mohs Hardness: 5 – 5.5

Parting: No.         

Streak: Grayish-black.         

Lustre: Metallic.          

Diaphaneity: Opaque.           

Density (g/cm³): 7.43

           

2. Geology and Deposits

Löllingite occurs in various types of hydrothermal deposits—ranging from high- to medium- and low-temperature environments—in association with other arsenic minerals. It may also occur in contact pneumatolytic deposits.

It is found in quartz-wolframite greisens and also in pegmatites alongside rose quartz.

Due to the difficulty of identifying it, löllingite was once considered a rare mineral; however, it occurs in small quantities much more frequently than previously thought.

 

3. Mineral Associations

It is associated with common gangue minerals (quartz, calcite, siderite), common sulfides (pyrite, chalcopyrite, galena, sphalerite), and common oxides (magnetite, hematite).

In pegmatites with scheelite, tourmaline, apatite, albite, wolframite, synchysite-(Ce), muscovite, and fluorite.

With other As minerals such as native arsenic, arsenopyrite, niccolite, safflorite, rammelsbergite, skutterudite, maucherite, stibarsen, pararammelsbergite, annabergite, karibibite, and scorodite.

Also with native bismuth, dyscrasite, cobaltite, analcime, sodalite, vesuvianite, serpentine, diopside, garnet (andradite), hedenbergite, olivine, willemite, ilvaite, vonsenite, pitticite, uraninite, and pitchblende.

 

4. Transmitted Light Microscopy

Not applicable, as löllingite is completely opaque.

5. Reflected Light Microscopy

Sample preparation: Polishing löllingite takes longer, but yields excellent results. Its polishing hardness is significantly higher than that of chalcopyrite, lower than that of magnetite, and much lower than that of arsenopyrite. It is slightly harder than pyrrhotite, safflorite, and rammelsbergite. Polishing hardness varies greatly across different crystal directions and even between individual zones within the grains. When drying the sample with a hair dryer after polishing, care must be taken to avoid overheating it, as the heat causes löllingite to release toxic arsenic vapors with a strong garlic odor.       

PLANE POLARIZED LIGHT – PPL

Reflection color: White, often with a faint yellowish tinge.       

Pleochroism: Faint, ranging from bluish-white to yellowish-white; there are two shades of yellowish-white depending on the direction of the section.

Reflectivity: ~54%        

Bireflectance: Weak.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Very strong anisotropy ranging from light yellow-orange to reddish-brown; can also appear brown, blue, pale blue, or green. 

Internal reflections: No.      

May be confused with: 

Arsenopyrite is very similar, especially if it has a higher Co content. In a direct comparison, arsenopyrite appears whiter and has a yellowish tint.

Glaucodot exhibits similar anisotropy, though much weaker than that of löllingite.

Rammelsbergite and safflorite are very similar; their habits vary slightly, but definitive identification via microscopy may be impossible. Their parageneses differ somewhat.

Marcasite displays similar anisotropy colors, but its pleochroism is stronger, and its anisotropy colors include turquoise—a hue not found in löllingite.

Safflorite can be very similar but generally exhibits distinct habits.

General Characteristics: 

Grain shape is generally idiomorphic—or idioblastic—because löllingite has a strong tendency to form idiomorphic grains. This tendency toward idiomorphism is absent only in certain high-temperature, pegmatitic-pneumatolytic deposits. Well-developed radiating aggregates or massive, very fine-grained aggregates may occur. Skeletal aggregates are occasionally found, as are thin crusts or grains intergrown with safflorite. Multiple generations of löllingite may occur in a given deposit, each with its own habit.

Crystal size is generally small—much smaller than that of arsenopyrite—though large crystals can occur, such as at the type locality (Lölling, Carinthia, Austria).

Cleavage is not observed.

Zoning may be present in large crystals but is not as well-developed as in safflorite.

Twinning is very common; twins may be simple or complex. Triplets (or trillings) similar to those found in safflorite also occur, as do polysynthetic twins with lamellae parallel to {101}.

Oriented intergrowths occur with niccolite, safflorite, rammelsbergite, and arsenopyrite.

Inclusions within löllingite may consist of dyscrasite, sphalerite, galena, chalcopyrite, and native arsenic.

Inclusions of löllingite occur in arsenopyrite (very frequent!), skutterudite, and maucherite.

Replacements 1: Löllingite can replace uraninite, sphalerite, arsenopyrite, and pitchblende.

Replacements 2: Löllingite is replaced by cobaltite, chalcopyrite, sphalerite, galena, arsenopyrite, stibarsen, and native antimony.

Veins of löllingite occur in skutterudite, rammelsbergite, and niccolite.

Crusts of löllingite crusts may envelop safflorite-rammelsbergite grains.

Cataclastic textures occur more frequently than in arsenopyrite.

Löllingite 01: In PPL, löllingite (white) and gangue minerals (dark gray). Pleochroism and bireflectance of löllingite are hard to see.
Löllingite 01: The same section as of the previous image, now in XPL. The anisotropy colors, very similar to those exhibited by other arsenates, allow for the recognition of the radial habit of the constituent crystals. Gangue minerals with white internal reflections.
Löllingite 02: In PPL, larger grains of löllingite (white) with gangue minerals (darker) and holes (black). It is not possible to distinguish individual grains in PPL.
Löllingite 02: The same section as of the previous image, now in XPL. The larger grains allow recognition of the two anisotropy colors of löllingite, which are very similar to the anisotropy colors of other aresenates.
In XPL, radial aggregate of löllingite crystals in gangue minerals (milky internal reflections).
In XPL, with photoshop to have a lighter image, radial aggregate of löllingite crystals with its anisotropy colors. Gangue with clear internal reflections.
In XPL, radial aggregates of löllingite crystals in gangue. Under the microscope, the image is usually much darker, but the spherulitic pattern is well-defined. The spheres are very small.
In XPL, with photoshop(!), group of löllingite spherulites in gnague. The original image is much darker; the spheres are very small.
In XPL, with photoshop(!), löllingite spherulites "floating" in gangue minerals. The radial structure is easy to see and the anisotropy colors are very similar to the anisotropy colors of safflorite.