Leadhillite

Key takeaways

Leadhillite belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Leadhillite to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Leadhillite from memory before moving on to harder problems.

Reference excerpt

Leadhillite is a lead sulfate carbonate hydroxide mineral, often associated with anglesite. It has the formula Pb4SO4(CO3)2(OH)2. 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 is quite soft with a Mohs hardness of 2.5 and a relatively high specific gravity of 6.26 to 6.55. It was discovered in 1832 in the Susannah Mine, Leadhills in the county of Lanarkshire, Scotland. It is trimorphous with susannite and macphersonite (these three minerals have the same formula, but different structures). Leadhillite is monoclinic, susannite is trigonal and macphersonite is orthorhombic. Leadhillite was named in 1832 after the locality.

Unit cell Leadhillite belongs to the monoclinic crystal class 2/m, which is the class with the highest symmetry in the monoclinic system. It has a two-fold axis of symmetry perpendicular to a mirror plane, and the general form is an open-ended prism. The space group is P21/a, meaning that the two-fold axis is a screw axis and the mirror plane is a glide plane. There are 8 formula units per unit cell (Z = 8) and the angle β is very nearly equal to 90°. The side-lengths of the unit cell are a = 9.11 Å, b = 20.82 Å and c = 11.59 Å.

Structure 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.

Appearance 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.

Optical properties Leadhillite is biaxial (-) with the optical Z axis parallel to the crystallographic b axis, and the optical X axis inclined to the crystallographic c axis at an angle of −5.5°. The refractive indices are large, giving the mineral its high lustre, nα = 1.87, nβ = 2.00 and nγ = 2.01. Compare these values with that for ordinary window glass, which is only 1.5. The refractive index depends upon the direction of travel of light through the crystal. The maximum birefringence δ is the difference between the highest and the lowest values. For leadhillite δ = 0.140. An important characteristic of a biaxial material is the angle between the two optic axes, called the optic angle and designated 2V. It is possible to calculate this value from the refractive indices, and also to measure it. For leadhillite both the measured and calculated values of 2V are 10°. When the colour of the incident light is changed the refractive indices change, and so does the value of 2V. This effect is known as dispersion of the optic axes. In leadhillite this effect is strong, with 2V smaller for red light than for violet light (r < v). The mineral may fluoresce yellowish longwave or shortwave ultraviolet light.

Physical properties Leadhillite is a soft mineral, with hardness only 2+1⁄2 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.

Occurrence 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.

References

Illustrations

Leadhillite: Leadhillite from the Beer Cellar Mine, Missouri, USA — Leadhillite from the Beer Cellar Mine, Missouri, USA

Worked examples

Example 1 — a first encounter with Leadhillite

Start with the simplest possible case. Write down what Leadhillite claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Leadhillite before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Leadhillite ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Leadhillite

In research: Leadhillite appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Leadhillite in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Leadhillite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Carbonate minerals, Lead minerals, Luminescent minerals, so understanding it makes those chapters shorter.
In everyday life: Look for Leadhillite outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Leadhillite in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Leadhillite means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Leadhillite out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Leadhillite in simple terms?

Leadhillite is a lead sulfate carbonate hydroxide mineral, often associated with anglesite. It has the formula Pb4SO4(CO3)2(OH)2.

Why does Leadhillite matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Leadhillite?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Leadhillite.