DJURLEITE
Djurleite – Cu31S16 – is a fairly common sulfide found in copper ores.
The planetary abundance of copper and sulfur, their wide distribution, and the high reactivity of the two elements have generated nearly 300 different mineral species that contain them. There are nine different minerals formed only by Cu + S. Copper sulfide formed at low temperatures (T<~105ºC) is monoclinic chalcocite. At high temperatures (T>~105ºC) (including in blast furnaces!) digenite (trigonal Cu9S5) is formed. Another copper sulfide, djurleite (monoclinic Cu31S16), is very similar to chalcocite. To complicate matters, another copper sulfide, anilite (orthorhombic Cu7S4), transforms into digenite during the preparation of the polished section, by polishing. Other copper sulfides include roxbyite (monoclinic Cu9S5), spionkopite (trigonal Cu39S28), yarrowite (trigonal Cu9S8), geerite (trigonal Cu8S5), and covellite (hexagonal CuS).
The situation becomes even more difficult because in several of these minerals there are substitutions of Cu by varying amounts of Ag, Fe, and Mn, in addition to substitutions of S by Se and Te. Furthermore, there are crystals with digenite x chalcocite and digenite x covellite mixtures.
And there is a complete series of solid solutions between high-temperature digenite and berzelianite. There are serious sample preparation problems, which are discussed further below. This set of complications has historically caused much confusion and identification errors (More: Pósfai & Buseck 1994. American Mineralogist, 79, 308-315).
Djurleite is classified in the Chalcocite-Digenite Group and may contain Fe and Ag.
Macroscopically and microscopically, it is almost indistinguishable from chalcocite. Its identification requires the use of other analytical techniques. Even the X-ray diffractogram of djurleite is similar to that of chalcocite.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Black.
Habit: Compact, massive, more rarely thick or short tabular or prismatic, up to 1.5 cm.
Cleavage: No.
Tenacity: No information avaliable.
Twinning: Common, by {110}.
Fracture: No information avaliable.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: No information avaliable.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 5.75 (calculated).
2. Geology and Deposits
Djurleite is a mineral characteristic of secondary enrichment zones of Cu deposits such as Porphyry Copper, SEDEX (“Sedimentary Exhalative”), MVT (“Mississippi Valley Type”) and others.
3. Mineral Associations
It is evidently associated with a series of other Cu sulfides: anilite, bornite, chalcocite, chalcopyrite, digenite, spionkopite, yarrowite, and covellite.
It also occurs with secondary Cu minerals (malachite, azurite, tenorite, connellite, langite, shattuckite, atacamite, paratacamite, delafossite) and other sulfides (pyrite, sphalerite).
It occurs with carbonates (calcite, aragonite, dolomite), quartz, barite, gypsum, neptunite, benitoite, joaquinite-(Ce), natrolite, prehnite, willemite, franklinite, and carminite.
4. Transmitted Light Microscopy
This does not apply, as djurleita is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Djurleite easily acquires a great polish. Its hardness upon polishing is similar to the hardness of chalcocite and galena. Sphalerite has a much higher hardness.
Due to its similarity to chalcocite, the text below discusses the precautions to take when preparing a polished section of chalcocite. For djurleita, the same precautions probably apply:
“Chalcocite is relatively easy to polish, producing a high-gloss surface. The surface usually has many polishing grooves, especially when there are small (hard) pyrite grains included in the chalcocite. Care is needed during polishing, and long polishing sessions should be avoided because they produce excessively high relief in the associated minerals and because the chalcocite surface becomes blurred due to the exaggerated surface film. To avoid this surface blurring of the chalcocite, it should never be polished dry or under high pressure. Excessive pressure produces surface films on the chalcocite that darken to blue colors later. Furthermore, polishing grooves produced with coarse abrasives (which should be avoided in chalcocite ores) and not eliminated during polishing become filled with mineral paste during the polishing process, appear bluer than the correct reflection color, and can generate pseudo-structures.
Low-quality polishing makes the reflection color of the chalcocite appear bluer than normal. After polishing, the section It must be dried immediately and very carefully. Poor drying produces surfaces with abnormal blue colors. The section cannot be impregnated at high temperatures (T>60º) nor should it be observed under a microscope with oil immersion for long periods (except for microscopes with cold LED light), because the chalcocite structure begins to change and associated covellites are reabsorbed. The resin of the polished section cannot contain sulfur, because it reacts with the chalcocite producing a mold-like film.
The surfaces of the polished sections (and macro samples) in the collections change slightly due to the formation of a brown CuS crust. Leaving the sections exposed to a humid atmosphere causes air corrosion (clouding) to develop on their surfaces, revealing any existing structures in the chalcocite.”
PLANE POLARIZED LIGHT – PPL
Reflection color: Grayish white with a bluish tint.
Compared to the color of chalcocite, the color of djurleite is very similar.
Pleochroism: No.
Reflectivity: 27.44 – 28.38%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very weak anisotropy, difficult to observe, not always visible.
Internal reflections: No.
May be confused with: Chalcocite, but during polishing the individual djurleite grains show several different shades of gray and blue; only with continued (exaggerated) polishing do all the grains show the same color.
