THE MINERAL BEUDANTITE

Chemistry: PbFe₃AsO₄SO₄(OH)₆, Lead Iron Arsenate Sulfate Hydroxide.
Class: Sulfates; although sometimes classified as a Phosphate.
Group: Beudantite
Uses: As a very minor source of lead and as mineral specimens.
Specimens

Beudantite is a difficult mineral to classify in that it has both an arsenate anion group and a sulfate anion group. The arsenate anion would normally dictate that beudantite be classified in the Arsenate Subclass of the Phosphate Class of minerals. But beudantite's sulfate anion group is intricate and essential in its structure, while the arsenate anion group can be substituted for by a phosphate anion group to at least a limited degree (see the Beudantite Group below). Many classification schemes place beudantite in the Phosphate Class however.

Beudantite, which is named for the French mineralogist Francois S. Beudant, is at times a very attractive mineral. It can be beautifully colored although often too dark, appearing black. It has a nice high luster due to its lead content. It is often associated with other rare secondary minerals that are found in the oxidation zone of ore deposits. Often these assorted mineral specimens can be quite interesting and a bonus to the collector who likes lots of different minerals on one specimen (more for the money, so to speak).

Beudantite lends its name to a group of sulfate-arsenates and sulfate-phosphates called the Beudantite Group. These minerals are all trigonal, contain a sulfate ion group and have six hydroxides. The general formula of this group is AB₃{AsO₄ and/or PO₄}SO₄(OH)₆. The A cation can be either calcium, barium, cerium, lead, strontium and/or hydrogen. The B cation can be either iron, aluminum and/or gallium.
These are the members of the Beudantite Group:

Beudantite (Lead Iron Arsenate Sulfate Hydroxide)
Corkite (Lead Iron Phosphate Sulfate Hydroxide)
Gallobeudantite (Lead Gallium Arsenate Sulfate Hydroxide)
Hidalgoite (Lead Aluminum Arsenate Sulfate Hydroxide)
Hinsdalite (Lead Strontium Aluminum Phosphate Sulfate Hydroxide)
Kemmlitzite (Strontium Cerium Aluminum Arsenate Sulfate Hydroxide)
Orpheite (Lead Aluminum Phosphate Sulfate Hydroxide)
Schlossmacherite (Hydrated Hydrogen Calcium Aluminum Arsenate Sulfate Hydroxide)
Svanbergite (Strontium Aluminum Phosphate Sulfate Hydroxide)
Woodhouseite (Calcium Aluminum Arsenate Sulfate Hydroxide)

THE PHYSICAL CHARACTERISTICS OF BEUDANTITE:

Color is green, dark green, yellow-green, orange-brown, brown or black.
Luster is vitreous, adamantine to greasy.
Transparency: Specimens are transparent to translucent.
Crystal System is trigonal: bar 3
Crystal Habits include blocky rhombohedrons, sometimes pseudo-cubic, and platy to tabular crystals. also as druses, crusts and earthy masses.
Cleavage is perfect in one direction (basal), but not usually seen.
Fracture is conchoidal.
Hardness is 4.
Specific Gravity is approximately 4.3 - 4.5 (heavy for non-metallic minerals).
Streak is greenish yellow.
Associated Minerals include mimetite, jarosite, conichalcite, anglesite, galena, sphalerite, arsenopyrite, pyrite, pyrrhotite, descloizite, aegirine, hemimorphite, microcline, muscovite, arthurite, tetranatrolite and natrolite.
Notable Occurrences include Tsumeb, Namibia; Mont Saint-Hilaire, Quebec, Canada; several mines in Arizona, USA; laurion, Greece and Australia.
Best Field Indicators are crystal habit, density, color, streak, luster and hardness.