THE MINERAL MINIUM

Chemical Formula: Pb₃O₄, Lead Oxide
Class: Oxides and Hydroxides
Group: Spinel
Uses: As a minor ore of lead, a pigment and as mineral specimens.
Specimens

Minium, which has been known as "red lead", is an oxidation product of other lead minerals. It forms in extreme oxidizing conditions of lead ore bodies and its presence is very indicative of the degree of oxidation. Something of interest to petrologists (rock scientists). It is probably more common than is known as it exists as a slight reddish coating on many lead specimens, often to the detriment of the aesthetic appeal of the underlying mineral. However, some specimens are exceptional and boast a bright red color with a high adamantine luster. The best specimens in the world come from Broken Hill, New South Wales, Australia. Unfortunately, these specimens are the result of a mine fire and mineral purists do not like their human influenced origins.

Minium gets its name from the Minius River in Northwest Spain. The name, minium has also been applied to cinnabar, a red mercury sulfide, but now it is exclusively used world wide to refer to the red lead oxide. Minium is more well known to the makers of pigments than it is to mineral collectors. The artificially produced minium is used as a pigment in paints and dyes although the health concerns of lead have greatly diminished this role.

Minium is a member of the Spinel Group of oxide minerals. The general formula for the Spinel Group is AB₂O₄. Minium with its formula of Pb₃O₄, may not look like it belongs. But the lead in minium is actually composed of two different valence states (Pb +2) and (Pb +4). If its formula is rewritten with the different valence states indicated then the formula appears as in classic Spinel Group form as (Pb +4)(Pb +2)₂O₄. The two different valences of lead occupy different locations in the spinel type structure. Minium's structure is not exactly the same as spinel as spinel is an isometric mineral and minium is a tetragonal mineral. The small size of the lead ions, no doubt, cause a distortion in the spinel structure.

THE PHYSICAL CHARACTERISTICS OF MINIUM:

Color is red, scarlet to brick-red.
Luster is greasy to adamantine.
Transparency: Crystals are opaque.
Crystal System is tetragonal; bar 4 2 m
Crystal Habits include scaly aggregates and powdery coatings on other lead minerals. Also massive and granular. Crystals are generally well striated.
Cleavage is perfect in several directions.
Fracture is uneven.
Hardness is 2.5 - 3.
Specific Gravity is 8.9 - 9.2 (Well above average for any mineral).
Streak is orange yellow.
Other Characteristics: Some specimens fluoresce an orange color under ultraviolet light.
Associated Minerals include massicot, galena, wulfenite, pyromorphite, mimetite, bindheimite, cerussite, lead, duftite and sphalerite.
Notable Occurrences include its name originator, the Minius River in Northwest Spain as well as Inyo County, California; several localities in Arizona; Leadville, Colorado and at the Jay Gould Mine, Idaho, USA; Broken Hill, New South Wales, Australia; Altai Mountains, Russia; Eifel region of Germany and at Lavrion, Greece.
Best Field Indicators are color, crystal habit, streak, density and softness.