[6] Leadhillite crystallises in the monoclinic system, but develops pseudo-hexagonal forms due to crystal twinning.

It forms transparent to translucent variably coloured crystals with an adamantine lustre.

It has a two-fold axis of symmetry perpendicular to a mirror plane, and the general form is an open-ended prism.

The simplest form with faces parallel to the b axis and cutting the a and c axes (represented as {101}) may develop.

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.

[2][5] The refractive indices are large, giving the mineral its high lustre, nα = 1.87, nβ = 2.00 and nγ = 2.01.

The typical habit is platy or tabular pseudohexagonal cyclic twinned crystals.

[8] Leadhillite is soluble with effervescence in nitric acid HNO3, leaving lead sulfate.

[2][3][5] The type locality is the Susanna Mine at Leadhills, Strathclyde, Scotland, UK.

[3] Leadhillite is a secondary mineral found in the oxidised zone of lead deposits associated with cerussite, anglesite, lanarkite, caledonite, linarite and pyromorphite.