ARSENIC
Arsenic – As – is a relatively rare native element. It is not an ore, as arsenic is recovered from arsenopyrite present in ores of other metallic elements.
It is classified in the Arsenic Group, forms a series with Sb, and is dimorphic with arsenolamprite (orthorhombic As). It always contains some Sb; it may contain Fe, Ni, Bi, Ag, S, and Se. When exposed to air, it oxidizes slowly, developing a thin crust formed by small crystals of arsenolite (As2O3).
Well-formed isolated crystals (rhombohedral, pseudocubic) are very rare and reach sizes of up to 1 mm. A typical habit of As is rhythmic crusts (with varying Sb contents) forming mammillated to botryoidal surfaces, called “Scherbenkobalt” (= cobalt in shards), which are very diagnostic. In these crusts, the crystals form tuffs oriented perpendicularly to the base.
Macroscopically, arsenic is very similar to several other minerals. It is indistinguishable from native antimony and stibarsen (SbAs). It reacts with H2O2 (hydrogen peroxide), which is a diagnostic aspect, but it is necessary to consider that oxides and hydroxides of Mn (also gray/black) also react with H2O2.
Caution: Arsenic is toxic. Always wash your hands after handling. Avoid inhaling dust during handling or breaking. Never lick or ingest. If beaten or heated, it releases toxic arsenic gases with a strong garlic smell. Chronic arsenic poisoning from contaminated drinking water is a health problem in several countries. In mining, the presence of native arsenic and/or minerals with arsenic generates highly toxic mine effluents.
1. Characteristics
Crystal system: Trigonal scalenohedral.
Color: White tin in fresh fracture, fades to dark lead gray to black.
Habit: Granular, massive, columnar, reticular, reniform, stalactitic, acicular, crusts of concentric layers.
Cleavage: {0001} perfect, {01-14} imperfect.
Tenacity: Brittle.
Twinning: It exhibits contact and pressure twinning.
Fracture: Unequal to granular.
Mohs Hardness: 3.5
Parting: No.
Streak: Gray to black.
Lustre: Almost metallic to dull.
Diaphaneity: Opaque.
Density (g/cm³): 5.63 – 5.78
2. Geology and Deposits
It occurs in mesothermal and epithermal Co-Ni-Ag-U veins and in deposits with other As minerals. Also in meta-dolomites.
3. Mineral Associations
It is associated with gangue minerals typical of hydrothermal veins: quartz, calcite, and barite.
Common sulfides are found in these veins, such as pyrite, marcasite, chalcopyrite, sphalerite, galena, and others.
Specific associated minerals are Ag minerals (silver, proustite, dyscrasite), Hg minerals (cinnabar), As minerals (loellingite, niquelite, safflorite, eskutterudite, rammelsbergite, arsenolite, ouropigment, realgar), and Sb minerals (antimony, stibnite, tetrahedrite-tennantite, breithauptite).
4. Transmitted Light Microscopy
This does not apply, as arsenic is completely opaque.
5. Reflected Light Microscopy
Sample preparation: It has low polishing hardness, but much greater than the hardness of bismuth, greater than the hardness of silver, dyscrasite and antimony; less than the hardness of loellingite.
It acquires a good polish without problems, including without polishing scratches.
As arsenic releases toxic fumes during the process of making the polished section, special care is needed when sawing, grinding and polishing, always working in a fume hood with the exhaust fan on.
PLANE POLARIZED LIGHT – PPL
Reflection color: White to gray, slightly brownish.
It tarnishes in a day or a few days, depending on weather conditions. This tarnishing can reveal textures and zoning, so it is advisable to observe the polished section before repolishing it.
Compared to the color of antimony, the color of arsenic is light gray, darker.
Compared to the color of galena, the color of arsenic is whiter and with a more pronounced cream tone.
Compared to the color of silver, the color of arsenic has a creamy hue and is much darker.
Compared to the colors of eskutterudite and safflorite, the color of arsenic is somewhat grayer and darker.
Compared to the color of bismuth, the color of arsenic is less creamy pink and much darker.
Compared to the color of proustite, the color of arsenic is less gray-blue.
Pleochroism: Very weak, clear only in intergranular contacts.
Reflectivity: 47 – 53%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: A very distinct anisotropy, ranging from yellow-brown and light gray to yellowish-gray.
In XPL, the polishing scratches located below the polished surface become visible again.
Internal reflections: No.
May be confused with: Some other native elements. The attackability of native arsenic with hydrogen peroxide is very diagnostic.
Native antimony is clearer and less anisotropic, it doesn’t tarnish;
Native bismuth is much clearer and softer.
General Characteristics:
Grain shape: generally presents with concentric structures (“Scherbenkobalt”). In compact masses it is finely granular, but may appear coarsely granular. Large grains may have a feathery appearance. Generally the texture is coloform with concentric layers or spherulites with radial texture (like chalcedony). In certain occurrences, native arsenic is present as inclusions in very fine grains in many other ore minerals. It can occur intergrown in various forms (fine veins, droplets, etc.) with stibarsene (SbAs; old name: allemontite).
Tarnishing of polished surfaces occurs in two to three days to dull, grayish-brown colors. This tarnishing, treated in the literature as air corrosion, is quite diagnostic and can reveal textures and zoning.
Basal cleavage 0001 (like that of micas) is often clearly visible.
Inclusions of dyscrasite (Ag3Sb) can occur in arsenic, around which “Scherbenkobalt” crusts are deposited. Furthermore, antimony can exhibit inclusions of silver, antimony, and rammelsbergite, the latter in the form of small spheres.
Twinning is very often visible in large quantities, forming thin pressure lamellae parallel to (01-12). Twinning rarely occurs along {01-14}.
Zoning can occur very well developed, but is rare. If present, it may become visible through tarnishing after a few days.
Rhythmic structures are frequent, partially in alternating layers with As minerals.
Deformations occur, showing crumpling, twisting, and the development of lamellar pressure twinning.
Graphic intergrowths with stibarsen may form, exhibiting various textures such as droplets, vermiform textures, and intersertal textures.
Alteration of geochronite (Pb14(Sb,As)6S23) can form idiomorphic arsenic crystals exhibiting twinning like that of tridymite. These large crystals tarnish very rapidly.
Substitutions 1: Arsenic replaces rammelsbergite, bismuth, and eskutterudite.
Substitutions 2: Arsenic is replaced by proustite surrounded by löllingite.
