earth science

Oligoclase

Oligoclase 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 Oligoclase rather than just read about it. In short: Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).

Key takeaways

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

Reference excerpt

Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). The albite:anorthite molar ratio of oligoclase ranges from 90:10 to 70:30. Oligoclase is a high sodium feldspar crystallizing in the triclinic system. The Mohs hardness is 6 to 6.5 and the specific gravity is 2.64 to 2.66. The refractive indices are: nα = 1.533–1.543, nβ = 1.537–1.548, and nγ = 1.542–1.552. In color it is usually white, with shades of grey, green, or red. Oligoclase is a common mineral in the more silica-rich varieties of igneous rock and in many metamorphic rocks.

Name and discovery The name oligoclase was given by August Breithaupt in 1826 from the Ancient Greek: όλίγος, romanized: oligos, little, and Ancient Greek: κλᾶν, romanized: klân, to break, because the mineral was thought to have a less perfect cleavage than albite. It had previously been recognized as a distinct species by J. J. Berzelius in 1824, and was named by him soda-spodumene (Natron-spodumen), because of its resemblance in appearance to spodumene.

Occurrence Perfectly colorless and transparent glassy material found at Bakersville, North Carolina has occasionally been faceted as a gemstone. Another variety more frequently used as a gemstone is the aventurine-feldspar or sun-stone found as reddish cleavage masses in gneiss at Tvedestrand Municipality in southern Norway; this presents a brilliant red to golden metallic glitter, due to the presence of numerous small scales of hematite oriented within the feldspar structure. Oligoclase occurs, often accompanying orthoclase, as a constituent of plutonic igneous rocks such as granite, syenite, and diorite. It occurs in porphyry and diabase dikes and sills as well as in the volcanic rocks andesite and trachyte, and in mugearite where its presence is a defining feature. It also occurs in gneiss. The best developed and largest crystals are those found with orthoclase, quartz, epidote, and calcite in veins in granite at Arendal in Norway. The distinctive texture of rapakivi granite is due to oligoclase rims on orthoclase phenocrysts. Oligoclase is also found in metamorphic rocks formed under transitional greenschist to amphibolite facies conditions.

Schiller iridescence Some examples are called moonstone and show Schiller iridescence due to the presence of exsolution lamellae on cooling in the peristerite miscibility gap, ~An5-An18. One of the iridescent varieties of oligoclase, discovered in 1925 near the White Sea coast by academician Alexander Fersman, became widely known under the trade name belomorite.

References

Sources Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7

External links Webmineral data Mineral galleries Mindat

Illustrations

Worked examples

Example 1 — a first encounter with Oligoclase

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

In research: Oligoclase 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 Oligoclase 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: Oligoclase is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aluminium minerals, Aluminosilicates, Calcium minerals, so understanding it makes those chapters shorter.
In everyday life: Look for Oligoclase 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.

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How to study Oligoclase in 20 minutes

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

Frequently asked questions

What is Oligoclase in simple terms?

Why does Oligoclase 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 Oligoclase?

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 Oligoclase.