Murdochite

Key takeaways

Murdochite 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 Murdochite to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Murdochite from memory before moving on to harder problems.

Reference excerpt

Murdochite is a mineral combining lead and copper oxides with the chemical formula PbCu6O8−x(Cl,Br)2x (x ≤ 0.5). It was first discovered in 1953 in the Mammoth-Saint Anthony Mine in Pinal County, Arizona by Percy W. Porter, a mining engineer who handpicked a 401.5-mg sample. Porter would later submit for analysis and it was then that Fred A. Hildebrand suggested that the sample was a new mineral after taking a powder x-ray picture. It was named for Joseph Murdoch (1890–1973), American mineralogist. Murdochite was first suggested to be of a cubic structure. After this suggestion, the term "murdochite-type structure" began to be used when describing a structure that is similar to that of murdochite. Murdochite was later found to be octahedral.

Composition Various studies have examined the composition of murdochite. When murdochite was first discovered, its chemical formula was determined through stoichiometry to be Cu6·1Pb0·9O8·O. The ideal formula for murdochite was originally thought to be Cu6PbO8. This formula failed to take into consideration the significant amounts of chlorine and bromine present in murdochite. Using electron microprobe analysis, a new chemical composition for murdochite was determined that included chlorine and bromine. This new formula was Cu6+xPb+x(O,Cl,Br)8 and was created with regard to the Cu/Pb variance observed in samples of murdochite from both Hansonburg, New Mexico, and T. Khuni mine, Iran. This new composition was also proven incorrect and the formula that is widely accepted today, PbCu6O8−x(Cl,Br)2x (x ≤ 0.5), was proposed.

Structure The structure originally proposed for murdochite was a simple cubic structure. This structure was later proven incorrect and determined to be octahedral. The structure of murdochite is described as edges of [PbO8] cubes spanned by Cu2+ ions that give a square-planar CuO4 arrangement in a three-dimensional network. Halogen ions complete a (4 + 2_ elongated CuO4(Cl,Br)2 octahedron.

Physical properties Murdochite is an opaque mineral that exhibits a black color, adamantine luster, black streak, and isotropic optical class. Samples from T. Khuni mine indicate that polished sections of murdochite resemble the color and reflectivity of magnetite but vary because of zoning. The lighter zones in murdochite are caused because they are richer in lead than the darker zones. The mineral is known to be brittle, have a hardness of 4, and have {111} cleavage. Murdochite also has a pink tinge that is distinctive and its edges seem to be more reflective than the center because samples are commonly porous in the center and compact on the sides with clean-cut faces. The space group of murdochite has been confirmed to be Fm3m. Murdochite can be cubic, with a point group of 4/m 3 2/m, or octahedral and cubo-octahedral whose point group is 2 mm. Murdochite can occur in the forms (100) and (111) with crystal twinning being very common. The calculated density of murdochite was determined to be 6.1 g/cm3 and have a measured density of 6.47 g/cm3.

Geologic occurrence Naturally occurring murdochite, PbCu6O8−x(Cl,Br)2x (x ≤ 0.5), was discovered in Mammoth Mine, Arizona. The mine, about 46 miles northeast of Tucson, contains deposits of molybdenum, lead, gold, zinc, vanadium, sphalerite, and galena. In this mine, murdochite was found embedded in plates of wulfenite as well as on the surface of crystals of fluorite. Other crystals found in Mammoth Mine include hemimorphite, willemite, and quartz. Larger masses of murdochite have also been found at the T. Kguni Mine in Anarak, Iran. Here murdochite was found in conjunction with khuniite (iranite), chrominium (phoenicochroite), plumangite and an unknown mineral with the formula Pb9O16. All these minerals are lead oxide minerals, meaning that murdochite can be found in oxidized Pb-Cu deposits. More recently, small murdochite crystals were found encrusting aurichalcite crystals in the Granite Gap Mines in New Mexico.

Origin of the name The mineral murdochite is named after Joseph Murdoch, who was once a professor of geology at the University of California, Los Angeles and a past president of the Mineralogical Society of America. Professor Murdoch was born February 19, 1890, and died December 31, 1973, at 83 years of age. Murdoch obtained a baccalaureate degree, an M.S. and a Ph.D. in geologic science by 1915 from Harvard University and would first be involved in a non-geological business career before joining the faculty at the department of geology at the University of California, Los Angeles in 1928. It was here that Murdoch returned to work in the geologic field that would lead to the publishing of numerous papers which would include 23 publications ranging from the years 1913–1968, along with ten published abstracts and many book reviews.

See also List of minerals List of minerals named after people

References

Illustrations

Worked examples

Example 1 — a first encounter with Murdochite

Start with the simplest possible case. Write down what Murdochite 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 Murdochite 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 Murdochite 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 Murdochite

In research: Murdochite 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 Murdochite 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: Murdochite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Copper(II) minerals, Cubic minerals, Lead minerals, so understanding it makes those chapters shorter.
In everyday life: Look for Murdochite 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 Murdochite in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Murdochite 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 Murdochite out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Murdochite in simple terms?

Murdochite is a mineral combining lead and copper oxides with the chemical formula PbCu6O8−x(Cl,Br)2x (x ≤ 0.5). It was first discovered in 1953 in the Mammoth-Saint Anthony Mine in Pinal County, Arizona by Percy W.

Why does Murdochite 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 Murdochite?

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 Murdochite.