Occurrence of Arsenic
|Arsenic is very widely distributed in Nature, but seldom in abundance. The element occurs in the free state, but in too small a quantity to be of economic importance. It is found more frequently in combination with sulphur and as metallic arsenides, sulpharsenides, arsenites and arsenates. The presence of arsenic in the sun has not been observed, but it is frequently detected in meteorites, and has been separated from siderites occurring in Mexico and Ontario. |
Native arsenic generally occurs in granular or lamellated masses, sometimes reniform, reticulated or stalactitic; more rarely it is found as rhombohedral crystals, usually acicular. It occurs in veins in crystalline rocks and the older schists, and when freshly exposed it is tin-white and possesses a lustre almost metallic, Tbut superficial oxidation gradually changes the colour to dark grey. The arsenic content varies from 90 to 98 per cent. The element is generally associated with ores of antimony, gold and silver, and with metallic arsenides and sulphides. The native element is brittle and gives an uneven granular fracture; it gives a white streak, has density 5.63 to 5.78, and hardness (Mohs' scale) 3.5. It is abundant in many parts of Germany - Bohemia (Joachimsthal), Harz, Saxony (Marienberg), Vosges; also in Alsace, Italy, Hungary and Norway. It occurs in reniform masses associated with calcite in the Caucasus (Vladikavkaz); in large masses near Kolyban Lake, south of Barnaul in Siberia, and in various localities in Japan, large crystals being found in the red vanadium clay of the Akadani district. In New Zealand it is plentiful in the Kapanga gold-mining district. In America it occurs in thin layers in blue mica slate in New Hampshire; as nodules in gold and silver mines near Leadville, Colorado; in Watson Creek, Fraser River, British Columbia, and in Mexico and Chile. The following are typical percentage analyses of native arsenic:
|Montreal||98.14||1.65||. . .||. . .||0.16||0.15|
|Cornwall||94.80||5.15||. . .||0.15||0.11||0.10|
A massive form of native arsenic having a fibrous foliated structure occurs in Saxony and Chile and is known as arsenolamprite (Gk. λαμπρος = lustrous). It differs from the ordinary form in possessing a brilliant lustre; it is lead grey in colour, has density 5.3 to 5-5 and hardness 2. The following are analyses of samples from the above two sources:
|Marienberg||95.86||1.61||1.01||0.99||. . .|
|Copiapo||98.43||. . .||1.00||. . .||0.05|
Werner found 3 per cent. Bi in a similar mineral which he called arsenik-wismuth.
Antimonial arsenic from California contains, according to Genth, 90.82 per cent. As and 9.18 per cent. Sb. Arsenical antimony, allemontite, is found in reniform masses in various parts of Germany. In colour it is white to reddish-grey, sometimes lustrous, of density 6.2 and hardness 3.5. In composition it approximates to SbAs3 (i.e. As 64.85, Sb 35.15). Thus a specimen from Allemont gave As 62.15 per cent., Sb 37.85 per cent.
Compounds of arsenic are extremely widespread. The black crust often found on native arsenic is a mixture of arsenic and arsenious oxide, As2O3. This oxide frequently accompanies other arsenical minerals and occurs in two crystalline varieties, arsenolite (cubic), usually as minute octahedra in capillary crystallisations or in stellar aggregates, and claudetite (monoclinic), in thin plates resembling selenite. Both forms are lustrous and may be transparent to translucent, colourless to white.
The most abundant mineral of arsenic is arsenopyrite, arsenical pyrites or mispickel, FeS2.FeAs2, which with other metallic arsenides and sulphides is found usually in igneous rocks. It occurs in veins near Freiburg (Germany), and in beds at Joachimsthal (Bohemia), Tunaberg (Sweden) and Skutterud (Norway). It is associated with tin and copper ores in Cornwall and Devon; with serpentine in Silesia; in the Island of Giglio it is associated with galena, sphalerite and traces of gold. For arsenopyrite from Trentino, Andreatta derived the formula Fe22As20S23. In many localities in New South Wales it is highly auriferous. It is widely distributed in North and South America, frequently associated with cobalt and nickel ores, especially in Quebec, Ontario, the Keewatin district, Utah, California and Bolivia. An auriferous deposit occurs in the Bella Coola coast district of British Columbia. It is also found in the Akadani district of Japan. The mineral occurs in orthorhombic crystals, sometimes prismatic or in columns, granular or compact. It is lustrous, white to grey in colour, giving a streak almost black, brittle, of hardness 5.5 to 6.0 and density 5.9 to 6.3. The arsenic content varies from 42 to 46 per cent. Cobalt is frequently present in place of iron, and danaite is a variety containing 5 to 10 per cent, of this metal; the name was first applied to New Hampshire specimens, but the mineral is also obtained in Norway and in Chile. Nickeliferous arsenopyrite containing about 5 per cent, nickel is found in Bolivia.
In Sweden the Skellefte district, which includes the important Boliden gold mine, contains the most extensive deposit of arsenic ore in the world. This is chiefly in the form of arsenopyrite, and is associated with ores containing iron, copper, silver and gold.
The sulphides realgar or ruby sulphur, As2S2, and orpiment, As2S3, are fairly abundant and are mined for their commercial importance both in Europe and Asia. Both are of historical interest and they are frequently found in association. Realgar, red arsenic sulphide, is found in short monoclinic prisms, orange-red in colour and generally translucent. It' gradually changes to orpiment and arsenolite on exposure to light. It occurs associated with ores of silver and lead in Hungary, Bohemia, Saxony and the Harz; in dolomite in Switzerland; on quartz in mica slate in Bosnia; in minute crystals in the Vesuvian lavas and the solfataras near Naples; in sandy clay below lava in Utah; in calcite in California; and as a deposition from the hot springs in Yellowstone National Park. It has been mined in small quantities in Washington. The yellow sulphide, orpiment, the auri pigmentum or golden pigment of the Romans, occurs in small orthorhombic crystals, usually in foliated or columnar masses, sometimes with reniform surface. Its yellow colour varies in depth, but it is generally lemon-yellow and translucent. It is found with realgar in Hungary, Bosnia, Albania, Switzerland, Italy, Utah and the Yellowstone Park. In Nevada it occurs in monoclinic crystals. It is mined at Acobambillo in Peru. It is found in brown coal in Styria. There is a large Turkish mine near Julamerk in Kurdestan. The chief Indian source of arsenic is Chitral in the extreme North, where realgar and orpiment occur associated with fluorspar.
Metallic arsenides are very widespread, the more common being those of cobalt, nickel, iron and copper. The arsenides of cobalt and nickel usually occur in intimate association. Smaltite is essentially cobalt arsenide, CoAs2, while chloanthite is nickel arsenide, NiAs2. Both occur in cubes or pyritohedra, often in distorted forms. Safflorite and rammelsbergite are orthorhombic forms of cobalt and nickel arsenides, respectively. Skutterudite, found in Norway and Alsace, is similar to smaltite but richer in arsenic, corresponding in composition to CoAs3. Niccolite, copper nickel or arsenical nickel approximates to NiAs. Iron arsenide occurs as lollingite, FeAs2, in orthorhombic prisms, but various other forms of this arsenide are found containing 55 to 70 per cent. As. All the above minerals, together with gersdorffite, NiAsS, associated with arsenopyrite and breithauptite (NiSb), occur in the Cross Lake ores of Keewatin, and in Ontario. Arsenoferrite is similar in composition to lollingite, but differs from it in being apparently isotropic in character (lollingite is anisotropic). It is similar in appearance and physical properties to smaltite. The following analysis is of arseno- ferrite from Jachymov, Czechoslovakia: Fe, 24.88; Cu, 1.34; Pb, 0.05; As, 66.84; S, 1.08; CaCO3, 4.00; MgCO3, 1.57. Total, 99.76 per cent. The crystals have been described as cubic, but X-ray examination shows that this is not the case and that the mineral is identical with lollingite. A similar mineral occurs as dark brown crystals on gneiss at the Binnenthal, Switzerland. Other minerals, mainly iron arsenide, are leucopyrite, geyerite (containing 5 to 8 per cent, of sulphur) and glaucopyrite, a cobaltiferous Spanish mineral. An arsenide of copper known as domeykite or arsenical copper, Cu3As, frequently accompanies arsenical ores. It occurs in Chile, mixed with niccolite in Michipicoten Island in Lake Superior, and in the deposits near Langban, Sweden. Arseniferous copper is also found as algodonite, Cu6As, whitneyite or darwinite, Cu9As.
Arsenic occurs also in a great variety of other minerals, generally as mixed arsenides and sulphides of the heavy metals or as metallic arsenites and arsenates, anhydrous and hydrated. The more common of these are listed below, with their approximate composition. They are to be found in small quantities widely scattered over Europe, Asia, America and Australia. Thus arsenical minerals in great variety are found in the blendes and lead glances of the Eastern Alps, the former containing more than the latter. The apomagmatic deposits contain less than the corresponding perimagmatic zones and, according to Tornquist, the arsenic content of these minerals indicates the possibility of three distinct mineralisation periods. Other arsenic-rich districts in Europe and America have been mentioned; in Asia they are to be found in Siberia, Turkestan, India, China, especially Yunnan Province, and Japan. These arsenical minerals are not, in themselves, of commercial value, but since they occur largely in ore veins and are always associated with other minerals, they frequently become of importance in metallurgical practice.
For the sake of easy reference the foregoing minerals are given in the accompanying tables, together with their approximate chemical composition and more important physical characteristics.
Minerals containing Arsenic
|Name||Approximate Composition.||Hardness (Mohs' Scale).||Density||Usual Form||Arsenic Content, per cent. Approx.|
|Native Arsenic.||As||3.5||5.63 to 5.78||Rombohedral||90 to 98|
|Arsenolamprite.||As||2||5.3 to 5.5||Rombohedral, fibrous||90 to 98|
|Arsenolite||As2O3||1.5||3.70 to 3.72||Octahedral||75.8|
|Claudetite||As2O3||2.5||3.85 to 4.15||Monoclinic||75.8|
|Orpiment||As2S3||1.5 to 2.0||3.4 to 3.5||Orthorombic||61|
|AsS||1.5 to 2.0||3.56||Monoclinic||70|
|Algodonite||Cu6As||4||7.62||Massive granules||15 to 17|
|Arite||Ni(SbAs)||. . .||7.1 to 7.5||Massive||11 to 33|
|Arsenoferrite||FeAs2||5.5||6.42||Granular, orthorhombic||66 to 67|
|Chloanthite||NiAs2||5.5 to 6.0||6.4 to 6.6||Pyritohedral||64 to 76|
|Domeykite||Cu3As||3.0 to 3.5||6.7 to 7.7||Reniform and botryoidal||26 to 29|
|Geyerite||FeAs2||. . .||6.25 to 6.8||Massive||55 to 62|
|Huntilite||Ag3As||. . .||6.27 to 7.47||Granular||18.8|
|Leucopyrite||Fe3As4 to Fe2As3||5.0 to 5.5||7.0 to 7.2||Orthorombic||64|
|Lollingite||FeAs2||5.0 to 5.5||7.0 to 7.4||Orthorombic||62 to 72|
|Niccolite||NiAs||5.0 to 5.5||7.33 to 7.67||Massive (crystals rare)||47 to 53.5|
|Rammelsbergite||(Ni, Co, Fe)As2||5.5 to 6.0||6.9 to 7.2||Orthorombic||68 to 71.5|
|Safflorite||(Co, Fe)As2||4.5 to 5.0||6.9 to 7.3||Orthorombic||66 to 71|
|Skutterudite||CoAs3||6||6.72 to 7.86||Pyritohedral||77.8 to 79.2|
|Smaltite||CoAs2||5.5 to 6.0||6.4 to 6.6||Pyritohedral||64 to 76|
|Sperrylite||PtAs2||6 to 7||10.6||Cubic||41|
|Whitneyite (Darwinite)||Cu9As||3.5||8.4 to 8.6||Massive, granular||11.5 to 12.3|
|Alloclasite||Co(As, Bi)S||4.5||6.6||Columnar||28 to 33|
|Arsenopyrite (Mispiekel)||FeAsS||5.5 to 6.0||5.9 to 6.3||Orthorhombic||42 to 46|
|Binnite||3Cu3S. As2S3||2.5 to 3.0||4.5||Massive||30|
|Clarite||3Cu3S. As2S5||3.5||4.46||Monoclinic||17.5 to 18.0|
|Cobaltite (Cobalt glance).||CoS2.CoAs2||5.5||6.0 to 6.3||Cubic, pyritohedral||42.5 to 45.5|
|Corynite||Ni(As, Sb)S||4.5 to 5 0||6||Octahedral||37 to 38|
|Dufrenoysite||2PbS.As2S3||3||5.56||Orthorhombic||20 to 22|
|Enargite||3Cu3S.As2S5||3||4.45||Orthorhombic||13.5 to 19.5|
|Epigenite||4Cu3S.3FeS.As2S5||3.5||4.45||Orthorhombic||12 to 13|
|Gersdorffite||NiS2.NiAs2||5.5||5.6 to 6.2||Cubic, pyritohedral||34.5 to 46|
|Glaucodote (Danaite)||(Fe, Co)AsS||5||5.9 to 6||Orthorhombic||43 to 44|
|Guitermanite||3PbS.As2S3||3||5.94||Massive||14 to 15|
|Jordanite||4PbS. As2S3||3||6.4||Orthorhombic||10 to 13|
|Pacite||FeS2.4FeAs2||. . .||6.0 to 6.3||Orthorhombic||. . .|
|Pearceite||9Ag2S. As2S3||3||6.1 to 6.2||Monoclinic||. . .|
|Proustite||3Ag2S.As2S3||2 to 2.5||5.57 to 5.64||Rhombohedral||14.8 to 15.6|
|Rathite||3PbS.2As2S3||3||5.3 to 5.4||Orthorhombic||. . .|
|Reniformite||5PbS.As2S6||. . .||6.45||Reniform aggregates||10|
|Rittingerite||Ag3As(S, Se)4||2 to 2.5||5.63||Monoclinic||. . .|
|Sartorite||PbS.As2S3||3||5.4||Orthorhombic||28 to 29|
|Seligmannite||CuPbAsS3||3||. . .||Orthorhombic||. . .|
|Tennantite||4Cu3S.As2S3||3 to 4.5||4.4 to 4.5||Tetrahedral||25 to 28|
|Tetrahedrite (Fahlerz)||(Sb, As)2S3||3 to 4.5||4.7 to 4.9||Tetrahedral||0 to 20|
|Wolfachite||Ni(As, Sb)S||4.5 to 5 0||6.4||Orthorhombic||38.8|
|Xanthoeonite||3Ag2S. As2S5||2||5.0 to 5.2||Rhombohedral||13.5 to 14.5|
|Arsenites and Arsenates|