Segnitite

Key takeaways

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

Reference excerpt

Segnitite is a lead iron(III) arsenate mineral. Segnitite was first found in the Broken Hill ore deposit in Broken Hill, New South Wales, Australia. In 1991, segnitite was approved as a new mineral. Segnitite has since been found worldwide near similar locality types where rocks are rich in zinc and lead especially. It was named for Australian mineralogist, gemologist and petrologist Edgar Ralph Segnit. The mineral was named after E. R. Segnit due to his contributions to Australian mineralogy. Segnitite is not a primary ore mineral, but is found amongst other well known ore minerals such as galena, sphalerite, pyrite and more. Many minerals found with segnitite are important for industrial purposes. Minerals associated with segnitite make up metal alloys, batteries and even pigments. Carminite and beudanite are closely related to segnitite. In fact, forms of beudanite are commonly confused with segnitite, as the sulfate and arsenate anions are readily exchangeable with each other. Other mineral relations include, goethite, coronadite, agardite, bayldonite, and mimetite. Set parameters for the space group and chemical composition have yet to be confirmed. Segnitite has loose boundaries and conclusions about segnitite were derived from associated minerals. Chemically, there are many varieties of segnitite which affects space group measurements and chemical analysis interpretations. In other words, segnitite samples from around the world can be very different from one another making it difficult to provide accurate chemical measurements.

Chemical composition The mineral segnitite is a part of the alunite-jarosite family which is a part of the alunite supergroup, which has a general formula of AB3(TO4)2(OH)6. Minerals in the alunite-jarosite family have the general formula AB3(XO4)2(OH)6. More specifically, Segnitite is a part of the low-S lusungite group of alunite-jarosite minerals. The lusungite group falls on the lower third of the ternary diagram, with the jarosite group on top, having the highest content of sulfur. The beudantite-corkite group lies in the middle third on the ternary diagram, possessing intermediate amounts of sulfur. The lusungite group then has the lowest amount of sulfur, and has end members segnitite and lusungite, which then have varying amounts of arsenic to phosphorus; segnitite is the arsenic rich end member of the lusugnite group. Through electron microprobe analysis on the basis of 14 oxygen atoms, the empirical formula of segnitite was found to be (Pb1.05Ba0.03)1.08(Fe2.76Zn0.22Al0.17Cu0.01)3.16[(AsO4)1.85(PO4)0.05(SO4)0.04]1.94(OH,H2O)6.24. Four alternative and simplified versions include: (1) PbFe3(AsO4)2(OH)5·H2O, (2) PbFe3(AsO4)2(OH,H2O)6, (3) PbFe3(AsO4)(AsO3OH)(OH)5, (4) PbFe3H(AsO4)2(OH)6, although the preferred formula is PbFe3H(AsO4)2(OH)6, which is most consistent with the space group (R3m) for Segnitite.

Crystal structure and habit Segnitite belongs to the trigonal crystal system, which is also a subcategory of the hexagonal crystal system. Segnitite occurs most commonly as rhombohedral crystals that can grow to around 5mm in height and as pseudo-octahedral crystals that typically measure about 1mm across. The habit of these segnitite clusters is often found to be hemispherical in shape. Rhombohedron {112} and Pinacoid {001} are the two most common forms of segnitite.

Occurrence Segnitite was first discovered in Broken Hill ore deposit in Broken Hill, New South Wales, Australia. It was first found in the oxidized zone of lead and zinc sulfide ores bodies of the Broken Hill ore deposit and forms in similar settings around the world. Segnitite is a relatively uncommon mineral, and it not found in very large quantities, but is found worldwide in similar areas. Segnitite has since been found in many localities in Western Europe, specifically in sites near Switzerland, Germany and Austria. Segnitite is commonly found alongside many well known minerals including anglesite, galena, jamesonite, linarite, arsenopyrite, cerussite, covellite, cuprite, sphalerite, sulfur, beudantite, cassiterite, pyrite, smithsonite, carminite and plumbojarosite. Many of these minerals are important ore minerals. Segnitite poses some importance when considering the overall composition of surrounding rock.

Crystallography Through X-ray diffraction (XRD), which is either conducted through powder diffraction, or through single-crystal X-ray analysis, segnitite was found to belong to the hexagonal crystal system of minerals. Using angles 2θ< 66 degrees, cell parameters were estimated along with chemical composition. Before segnitite was identified as a mineral, it was commonly mistaken for beudantite. There are close similarities in diffraction data when comparing low sulfate beundantite from the beudantite-corkite group, and segnitite of the lusungnite group. Beundantite, segnitite and plumbojarosite form solid solutions with each other; conditions that form these minerals are similar. The space group R3m was determined for segnitite as the best fit based on the space group of beudantite, R3m as there is no well observed crystal structure. The difference between segnitite and beundantite is the replacement of the sulfate anion in beundantite with an arsenate anion to form segnitite.

Optical properties Segnitite exhibits weak pleochroism from a pale yellow to a more moderate yellow under different angles of plane polarized light, although the effects can be much more dramatic under polarized light. Dichroism and trichroism are both types of pleochroism. The term dichroism is used to describe optical properties of minerals that are uniaxial, including segnitite. Minerals that exhibit dichroism are generally trigonal, hexagonal and tetragonal. A response from UV light was not observed from segnitite.

References

External links MineralData.org

Illustrations

Worked examples

Example 1 — a first encounter with Segnitite

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

In research: Segnitite 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 Segnitite 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: Segnitite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Arsenate minerals, Crandallite group, Lead minerals, so understanding it makes those chapters shorter.
In everyday life: Look for Segnitite 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 Segnitite in 20 minutes

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

Frequently asked questions

What is Segnitite in simple terms?

Segnitite is a lead iron(III) arsenate mineral. Segnitite was first found in the Broken Hill ore deposit in Broken Hill, New South Wales, Australia.

Why does Segnitite 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 Segnitite?

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 Segnitite.