Leadhillite

It's advisable to handle leadhillite carefully to avoid generating dust and wash hands thoroughly afterward. When cutting or polishing leadhillite, wear a dust mask to prevent inhaling heavy metal particles. Store leadhillite in a sealed container in a well-ventilated area, away from children and pets. For those involved in crystal healing, never put it in your mouth.

Leadhillite is a soft mineral, with hardness only ​2 ⁄2 to 3, a little less than that of calcite. It breaks with an irregular to conchoidal fracture and it is somewhat sectile. That is, thin shavings can be pared off it. It is heavy, due to the lead content, with specific gravity 6.55, similar to other lead minerals such as cerussite (6.5) and anglesite (6.3). Cleavage is perfect on a plane perpendicular to the c crystal axis. The mineral is usually twinned, according to a variety of twin laws, forming contact, penetration and lamellar twins. The typical habit is platy or tabular pseudohexagonal cyclic twinned crystals. Leadhillite is soluble with effervescence in nitric acid HNO3, leaving lead sulfate.

Crystals are usually small to microscopic, and nearly always pseudo-hexagonal, being tabular with a hexagonal outline. Prismatic forms also occur. The simplest form with faces parallel to the b axis and cutting the a and c axes (represented as {101}) may develop. When it does it may be striated or curved. The colour is white or pale shades of green, blue or yellow, but the commonest is clear to white. Leadhillite is transparent to translucent, with a white streak and a resinous to adamantine lustre, pearly on faces parallel to the plane containing the a and b axes. Tabular forms of susannite are very similar.

The type locality is the Susanna Mine at Leadhills, Strathclyde, Scotland, UK. Leadhillite is a secondary mineral found in the oxidised zone of lead deposits associated with cerussite, anglesite, lanarkite, caledonite, linarite and pyromorphite. It may form pseudomorphs after galena or calcite, and conversely calcite and cerussite may form pseudomorphs after leadhillite. Heating leadhillite causes it to reversibly transform into susannite.

Leadhillite has a layered structure. The mineral contains both carbonate and sulfate groups, and these are arranged in separate sheets. Pairs of carbonate sheets 8(PbCO3) alternate with pairs of sulfate sheets 8[Pb(SO4)0.5OH]. The carbonate sheets virtually have trigonal symmetry, but the sulfate sheets do not. All the lead (Pb) atoms in the carbonate sheets are surrounded by 9 oxygens from carbonate groups and by one hydroxyl from an adjacent sulfate sheet. The Pb atoms in the sulfate sheets are bonded to 9 or 10 oxygens.