LEAD
Native lead is a fairly rare native element, recorded in just over 200 occurrences worldwide. It has also been recorded on the moon. It is not an ore due to its rarity.
It belongs to the Copper Group, along with copper, gold, maldonite (Au2Bi), and silver. When it forms crystals, they are octahedrons, cubes, and dodecahedrons, with sizes up to 6 cm. It may contain traces of Fe, Sb, Sn, Cu, and Ag.
Caution! Lead is toxic – wash your hands after handling any lead mineral or ore. Never lick or ingest it. Be careful not to produce dust that can be inhaled. Chronic and acute lead contamination is a serious public health problem, both historically and today. The dementia of Roman emperors is attributed by some researchers to the lead-based sweetener used in their wines, for example.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: White, immediately fades to gray, then develops an earthy white crust of weathering materials.
Habit: Rounded masses and plates weighing up to 60 kg. Wire-like, dendritic. Rare crystals.
Cleavage: No.
Tenacity: Ductile, very malleable.
Twinning: Twins are common, in the shape of a “herringbone,” with a 60º angle in {111}.
Fracture: Rough, uneven.
Mohs Hardness: 1.5
Parting: No.
Streak: Gray.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 11.37 (very high!)
2. Geology and Deposits
Native lead is very rare and only forms in the simultaneous absence of S and O. Because of this, in lead ores, lead occurs as lead sulfide (galena). Pb occurrences have generally been recorded in hydrothermal deposits and in correlatable placers. As an exception, the conditions for Pb formation can also occur in oxidation zones of lead ores.
It may occur as authigenic lead. It can replace tree roots.
3. Mineral Associations
Native lead is evidently associated with galena and secondary Pb minerals, such as Pb carbonates (cerussite, hydrocerussite) and Pb oxides (minium, massicot, and litharge).
It also occurs with secondary Mn minerals (oxides) such as pyrochroite, brandtite, allactite, caryopilite, sarkinite, and rhodonite.
With secondary Ba minerals such as celsian. It is also associated with garnet (andradite), willemite, axinite, dolomite, and native copper.
The assemblages of associated minerals vary greatly from deposit to deposit; there is no single typical paragenesis.
4. Transmitted Light Microscopy
This does not apply, as native lead is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Native lead has very low hardness and therefore will exhibit a large number of polishing scratches, like silver, copper, and galena. Its hardness upon polishing is much lower than that of galena. Furthermore, being very malleable, it can alter its optical characteristics if the grinding and polishing process of the sample is too vigorous. Therefore, it is necessary to use fine abrasives, low pressure, and plenty of lubricant. With the correct techniques, it is possible to produce a section without polishing scratches, which, on the other hand, are very useful as a diagnostic feature. Once exposed to air, the polished section immediately tarnishes, making observations very difficult or impossible.
PLANE POLARIZED LIGHT – PPL
Reflection color: Grayish-white with a brownish tint, fades very quickly to gray.
Compared to the color of silver, the color of lead is grayish-white on a fresh surface.
Pleochroism: No.
Reflectivity: 50.67% on a freshly polished surface, but decreases immediately due to tarnishing.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
As is generally the case with cubic minerals, it may exhibit anomalous anisotropy, both due to stresses suffered and stresses developed during grinding and polishing of the sample. The colors of this anomalous anisotropy are dark gray and blue-gray.
Lead grains can show isotropic portions mixed with portions that exhibit this anomalous anisotropy.
Internal reflections: No. Reddish-brown reflections rarely appear.
May be confused with: Native silver. However, silver has a much higher reflectivity. Nevertheless, its very low hardness and immediate tarnishing make it quite easy to identify.
General Characteristics:
Grain shape: occurs as granular aggregates, can be dendritic or skeletal. Isolated crystals may occur.
Polishing scratches will always be present, as in galena, native copper, and native silver.
Twinning may occur, in the form of translational lamellae.
Inclusions: very fine Pb inclusions are found in rhodonites from the famous Franklin mine (New Jersey, USA), a very special occurrence, without similar counterparts.
