earth science

Nephelinite

Nephelinite is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Nephelinite rather than just read about it. In short: Nephelinite is a fine-grained or aphanitic igneous rock made up almost entirely of nepheline and clinopyroxene (variety augite). If olivine is present, the rock may be classified as an olivine nephelinite.

Key takeaways

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

Reference excerpt

Nephelinite is a fine-grained or aphanitic igneous rock made up almost entirely of nepheline and clinopyroxene (variety augite). If olivine is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may resemble basalt in hand specimen. However, basalt consists mostly of clinopyroxene (augite) and calcic plagioclase. Basalt, alkali basalt, basanite, tephritic nephelinite, and nephelinite differ partly in the relative proportions of plagioclase and nepheline. Alkali basalt may contain minor nepheline and does contain nepheline in its CIPW normative mineralogy. A critical ratio in the classification of these rocks is the ratio nepheline/(nepheline plus plagioclase). Basanite has a value of this ratio between 0.1 and 0.6 and also contains more than 10% olivine. Tephritic nephelinite has a value between 0.6 and 0.9. Nephelinite has a value greater than 0.9. Le Maitre (2002) defines and discusses these and other criteria in the classification of igneous rocks. Nephelinite is an example of a silica-undersaturated igneous rock. The degree of silica saturation can be evaluated with normative mineralogy calculated from chemical analyses, or with actual mineralogy for completely crystallized igneous rocks with equilibrated assemblages. Silica-oversaturated rocks contain quartz (or another silica polymorph). Silica-undersaturated mafic igneous rocks contain magnesian olivine but not magnesian orthopyroxene, and/or a feldspathoid. Silica-saturated igneous rocks fall in between these two classes. Silica-undersaturated, mafic igneous rocks are much less abundant than silica-saturated and oversaturated basalts. Genesis of the less common mafic rocks such as nephelinite is usually ascribed to more than one of the following three causes:

relatively high pressure of melting; relatively low degree of fractional melting in a mantle source; relatively high dissolved carbon dioxide in the melt. Nephelinites and similar rocks typically contain relatively high concentrations of elements such as the light rare earths, as consistent with a low degree of melting of mantle peridotite at depths sufficient to stabilize garnet. Nephelinites are also associated with carbonatite in some occurrences, consistent with source rocks relatively rich in carbon dioxide. Nephelinite is found on ocean islands such as Oʻahu, although the rock type is very rare in the Hawaiian Islands. It is found in a variety of continental settings. An example is the Hamada nephelinite lava flow in southwest Japan which occurred in the late Miocene age. Nephelinite is also associated with the highly alkalic volcanism of the Ol Doinyo Lengai volcanic field in Tanzania. Nyiragongo, another African volcano known for its semipermanent lava lake activity, erupts lava made of melilite nephelinite. The unusual chemical makeup of this igneous rock may be a factor in the unusual fluidity of its lavas.

Olivine nephelinite flows also occur in the Wells Gray-Clearwater volcanic field in east-central British Columbia and at Volcano Mountain in central Yukon Territory. Melilite olivine nephelinite intrusives of Cretaceous age are found in the area around Uvalde, Texas.

References Roger W. Le Maitre (Editor), Igneous Rocks: A Classification and Glossary of Terms. (Recommendations of the International Union of Geological Sciences Subcommission of the Systematics of Igneous Rocks). Cambridge University Press (2002). ISBN 0-521-66215-X Hamada nephelinite, SW Japan Oldoinyo Lengai volcano, Eastern Rift Valley, North Tanzania Demant, A.; Lestrade, P.; Lubala, R. T.; Kampunzu, A. B.; Durieux, J. (March 1994). "Volcanological and petrological evolution of Nyiragongo volcano, Virunga volcanic field, Zaire". Bulletin of Volcanology. 56 (1): 47–61. Bibcode:1994BVol...56...47D. doi:10.1007/BF00279728. S2CID 129031508. Daniel P. Miggins, Charles D. Blome, and David V. Smith. Preliminary Argon40/Argon39 geochronology of igneous intrusions from Uvalde County, Texas: Defining a more precise eruption history for the southern Balcones Volcanic Province. U.S. Geological Survey Open-File Report 2004-1031. In usgs.gov

Bibliography Flett, John Smith (1911). "Nephelinites" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 19 (11th ed.). Cambridge University Press. p. 384.

External links (in Spanish) Nefelinita in - Atlas de rocas ígneas

Illustrations

Nephelinite: Columnar jointing in melilite-olivine nephelinite from the Southern Balcones volcanic and intrusive rocks, Uvalde County, Texas. — Columnar jointing in melilite-olivine nephelinite from the Southern Balcones volcanic and intrusive rocks, Uvalde County, Texas.

Worked examples

Example 1 — a first encounter with Nephelinite

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

In research: Nephelinite 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 Nephelinite 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: Nephelinite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Igneous petrology, Ultramafic rocks, Volcanic rocks, so understanding it makes those chapters shorter.
In everyday life: Look for Nephelinite 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.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Nephelinite” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Nephelinite in 20 minutes

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

Frequently asked questions

What is Nephelinite in simple terms?

Why does Nephelinite 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 Nephelinite?

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 Nephelinite.