Mineralogy of Sulfides and Sulfosalts

Introduction to the Sulfides and Sulfosalts Mineral Class

The sulfides and sulfosalts mineral class has a general formula of A_mX_n, where A represents a metallic element (Fe, Cu, Zn, Pb, As, Sb, Mo, etc.), and X is a sulfide - an anion of sulfur (S^2-). The letters m and n are simply integers. Sometimes, the A (metallic element) position can be shared between several elements, as in chalcopyrite (CuFeS₂).

Sulfides

Sulfides are crucial ore minerals. They are primary ores for cobalt, arsenic, nickel, molybdenum, lead, and copper. These elements have many applications in construction, medicine, chemical, electronics, and other technologies, had important roles in the development of civilization, and played an essential part in the nanotechnology revolution. (Pearce et al., 2006)

There are several hundred known species of sulfide minerals. Pyrite (FeS₂), pyrrhotite (Fe_1−xS (x = 0 to 0.17)), galena (PbS), sphalerite (ZnS), and chalcopyrite (CuFeS₂) are the most common and can be categorized as "rock-forming," with pyrite and pyrrhotite dominant. (Bowles et al., 2011)

Don't be afraid pyrrhotite's strange-looking formula. Certain sulfides exhibit a non-stoichiometric formula. For example, pyrrhotite is commonly given the general formula Fe_1−xS where 0 < x < 0.2. The varying compositions correspond to different concentrations of vacancies in iron atom sites, indicating fewer amounts than S. Some iron sites in the crystal lattice are empty (vacant).

Sulfosalts

Although far less abundant than sulfides, sulfosalts are usually studied together with sulfides. Mineralogy students can understand sulfosalts as more complex sulfides that contain metal, sulfur, and semi-metals (Sb, As). Some serve as important ores for Cu, Ag, and As.

To simplify sulfosalts, think of them as double sulfides. For example, think of a sulfosalt such as enargite (Cu₃AsS₄) as a double sulfide 3Cu₂S ∙ As₂S₅.(Berry et al., 1983)

Some examples of important sulfosalts are:

• Pyrargyrite Ag₃SbS₃
• Tetrahedrite Cu₁₂Sb₄S₁₃
• Enargite Cu₃AsS₄

Physical Properties of Sulfide and Sulfosalt class

Sulfides share some similar properties. Those we can observe macroscopically are color, opacity (except for sphalerite), metallic luster, and high density (and, therefore, significant heft). Generally, sulfides have relatively low hardness, from the 1 for molybdenite to 6.5 for pyrite.

Morphological features of sulfides and sulfosalts vary. Minerals can occur in large isometric crystals, such as cubic crystals of pyrite and galena, or elongated prismatic crystals, such as stibnite. They can also form granular crystalline aggregates, sometimes called incrustations (e.g., cinnabar, realgar, orpiment), or occur in the form of compacted tabular or foliated crystals (e.g., molybdenite).

Sulfide cleavage and fracture differ from other minerals. While native elements usually have hackly fractures, sulfide fractures are either conchoidal or uneven.

Geology of Sulfides and Sulfosalts

Sulfide minerals typically form under similar geological conditions. For example, iron is more likely to form a compound with oxygen. Therefore, the formation of sulfides requires oxygen-starving (reducing) environments. Most sulfides crystalize from hydrothermal solutions. They can also develop from more complex situations with magmatic and metasomatic origins or even in sedimentary rocks under reducing conditions in zones of secondary enrichment.

Various sulfide minerals often crystallize together in veins, creating massive polymetallic ore bodies of sulfide ores, for example, copper-zinc-lead sulfide ores. Additionally, different sulfides of the same metal often form complex deposits. For instance, chalcopyrite, cuprite, and bornite (all copper-bearing minerals) can occur together.

Many sulfides (with some exceptions) are very unstable and prone to oxidation at surface conditions (with abundant presence of oxygen). When exposed to surface conditions, sulfides (S^2-) change to sulfate (minerals with an (SO₄^2-) ion), oxide, halide, and carbonate mineral classes.

Sulfides and Sulfosalts as Ore Minerals

Sulfide and sulfosalt minerals are the primary ores of many crucial metal elements for industry and chemistry. These ores are important because breaking chemical bonds between sulfur and metal in sulfides is easier than in other mineral classes, such as complex silicates, carbonates, and oxides.

Another reason sulfides are so important economically is that they generally occur in close association, thus creating polymetallic ores and deposits. So, miners can extract several metals by working just one sulfide vein.

Here is a list of vital elements and their sulfide and sulfosalt ore minerals:

• Copper (Cu) — bornite (Cu₅FeS₄), chalcocite (Cu₂S), covellite (CuS), domeykite (Cu₃As), digenite (Cu₉S₅), enargite (Cu₃AsS₄), tetrahedrite ((Cu,Fe)₁₂Sb₄S₁₃).
• Nickel (Ni)— pentlandite ((Fe,Ni)₉S₈) and its alteration product, violarite (FeNi₂S₄), millerite (NiS), nickeline (NiAs), pararamnelsbergite (NiAs₂).
• Molybdenum (Mo) — molybdenite (MoS₂)
• Arsenic (As) — arsenopyrite (FeAsS), orpiment (As₂S₃) realgar (AsS).
• Lead (Pb) — galena (PbS).
• Zinc (Zn) — sphalerite (ZnS)
• Silver (Ag) — argentite (Ag₂S)
• Mercury (Hg) — cinnabar (HgS)
• Cobalt (Co) — cobaltite (CoAsS)

In geology, scientists are typically divided into two groups: ore geologists, who specialize in sulfide mineralogy, and silicate-mineral geologists and mineralogists, who work primarily with silicates.

Important Notice on Sulfides: Environmental Issues and Precautions

The breakdown of sulfides exposed by weathering at the Earth's surface generates sulfuric acid and releases potentially toxic metals into the water and soil. This pollution may arise from mine wastes or sulfide-containing natural rocks. (Vaughan & Corkhill, 2017)

Arsenopyrite is one of the most hazardous sulfides because of its abundance, significant arsenic content, and easy disintegration.

IMPORTANT NOTE: When I was a student, I had to wash my hands after each lab class on sulfides. Some of them, like stibnite, cinnabar, orpiment, and realgar, have toxic metals in their composition. You, too, must be careful around these minerals. Please wash your hands after handling them and store them safely in places not accessible to children and animals. Consult our guide to toxic and radioactive gems and minerals for more recommendations.

Diagnostic Characteristics of Sulfides

Below, you'll find diagnostic characteristics for identifying sulfide minerals quickly. Included here are the most common sulfides: pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, molybdenite, arsenopyrite, stibnite, cinnabar, realgar, and orpiment. We emphasize how to differentiate them from minerals with very similar appearances. The best diagnostic characteristics are highlighted in bold.

Remember: sulfides occur in close association, so having several sulfide minerals in one rock sample is normal.

Pyrite

Formula: FeS₂

Pyrite is the most common sulfide. Many people know it as "Fool's Gold" because of its brass-yellow color and metallic luster. A streak test is the best way to distinguish pyrite from gold. Pyrite has a greenish black to brownish black streak color, while gold has a yellow streak color, identical to its classic appearance. Also, the striations (parallel groves) on the crystal faces of a typical pyrite crystal will help you differentiate it from other sulfides easily. This is especially useful if you are examining well-formed pyrite crystals.

Chalcopyrite

Formula: CuFeS₂

Chalcopyrite commonly occurs in association with pyrite. However, chalcopyrite has a more orangey yellow color. Chalcopyrite usually has an iridescent surface that looks like gasoline in a puddle of water. This helps differentiate it from other sulfides. A greenish black streak also helps distinguish it from gold.

Pyrrhotite

Formula: Fe_1−xS

Pyrrhotite usually occurs in massive or granular form, as do many other sulfides. However, magnetism can help identify pyrrhotite easily. Interestingly, the lower the iron amount, the stronger pyrrhotite's magnetic properties.

Galena

Formula: PbS

Galena is a silver-colored sulfide. However, the color won't help with identification. On the other hand, the cubic morphology of galena crystals and their perfect cleavage are more helpful for identification. These properties distinguish it from other gray sulfides.

Sphalerite

Formula: ZnS

Sphalerite is the only sulfide that can be transparent to translucent. Other sulfides are typically opaque. However, transparent sphalerite is quite rare. Iron impurities (more than 6 wt.%) usually make sphalerite wholly black and opaque. (Cook et al., 2009) Thus, in most cases, you can identify opaque sphalerite due to its combination of dark hues and pale brown to pale yellow and white streak color.

Molybdenite

Formula: MoS₂

Molybdenite has a very light gray color with a shiny metallic luster. It's so soft you can identify it with a scratch test. A fingernail can scratch molybdenite. It occurs in foliated massive aggregates and has a greasy feel due to its low hardness, another distinguishing characteristic.

Arsenopyrite

Formula: FeAsS

Arsenopyrite is one of the most difficult sulfides to identify. It has a medium hardness, gray color, and non-distinctive crystal morphology. However, it has greater hardness than fellow gray sulfides stibnite and molybdenite. Arsenopyrite's black color streak also differs from sphalerite (brownish), stibnite (gray), and molybdenite (gray).

Stibnite

Formula: Sb₂S₃

Stibnite is easy to identify due to its crystal morphology. Stibnite crystals as angular rods, needles, or other slender shapes usually occur in radiating clusters. In cases where crystal morphology is not observable, stibnite's low hardness and gray streak can help distinguish it from arsenopyrite. A very unusual property of stibnite is its low melting point (550° C; 1,022° F; 823 K). This means it can melt in candle fire.

Cinnabar

Formula: HgS

Cinnabar is easily identified due to its vivid scarlet-red color. It can occur in crystals but is most commonly seen as massive red veins or fine incrustations. Crystal terminations (ends) can help differentiate cinnabar from realgar or other red mineral crystals. Cinnabar has a rhombohedral form, which makes crystals look like arrowheads. In contrast, realgar is prismatic and ends with more flattened surfaces.

Realgar

Formula: AsS

Realgar occurs in a range of red and orange colors. However, unlike cinnabar, it has a significant orangey hue. Realgar is also soft and can occur in fine-granular or massive forms. Unlike cinnabar, realgar has a prismatic crystal habit.

Orpiment

Formula: As₂S₃

Orpiment has a chemical composition is very similar to realgar, which explains their close association. In massive aggregates, realgar and orpiment always occur together. Unlike realgar, orpiment has less arsenic relative to sulfur. Realgar also shows orange colors lighter than those of realgar as well as intense yellow hues. Fibrous aggregates are also common.

Important Safety Advice for Cinnabar, Realgar, and Orpiment

You should try to study bright red and yellow-colored sulfides as one group. Start with vivid red cinnabar, continue with reddish orange realgar, and finish with orangey yellow orpiment. Keep in mind that these minerals are toxic. Please treat them with care. Store them safely and wash your hands after handling them.

References for Sulfides and Sulfosalts

• Anthony, J. W., Bideaux,R. A., Bladh, K. W., & Nichols, M C. (2001). Handbook of Mineralogy (Vol.1), Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/.

• Berry, L. G., Mason, B., & Dietrich, R. V. (1983). Mineralogy, 2nd ed. San Francisco: W.H. Freeman & Co., 561 pp.

• Bowles, J. F. W., Howie, R. A., Vaughan, D. J., & Zussman, J. (2011). Rock-Forming Minerals Vol. 5A - Non-Silicates: Oxides, Hydroxides, and Sulphides. The Geological Society, London, ISBN 978-1-86239-315-8, 920 pp

• Cook, N. J., Ciobanu, C. L., Pring, A., Skinner, W., Shimizu, M., Danyushevsky, L., … & Melcher, F. (2009). Trace and minor elements in sphalerite: A LA-ICPMS study. Geochimica et Cosmochimica Acta, 73(16), 4761-4791.

• Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons

• Pearce, C. I., Pattrick, R. A., & Vaughan, D. J. (2006). Electrical and magnetic properties of sulfides. Reviews in Mineralogy and Geochemistry, 61(1), 127-180.

• Vaughan, D. J., & Corkhill, C. L. (2017). Mineralogy of sulfides. Elements, 13(2), 81-87.

Olena Rybnikova, PhD

Olena Rybnikova is a gemologist and mineralogist. She has a PhD in mineralogy and petrology specializing in beryllium minerals and is a certified Applied Jewelry Professional accredited by the Gemological Institute of America. Her passion is actively promoting knowledge and appreciation of nature, geology, and gemstones.

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