Identify the Mineral on the basis of the following characteristics relating to it: I. It is usually a greenish crystal. II. It is often found in basaltic rocks. III. Magnesium, iron and silica are its major elements. Select the answer using the code given below:

Correct — D, Olivine. Take the clues in order. A crystal that is 'usually greenish' points straight at olivine, which is named for its olive-green colour. The formula (Mg,Fe)2SiO4 accounts for clue III exactly — magnesium, iron and silicon with oxygen, and nothing else of consequence. And clue II is the standard occurrence: magnesium-rich olivine crystallises out of magma that is rich in magnesium and poor in silica, so it turns up in the mafic rocks, gabbro at depth and basalt at the surface. Put together, the three clues describe a magnesium-iron silicate, green, in basalt. Only one mineral in the list fits all three at once; olivine is also the single most abundant mineral of the Earth's upper mantle, which is why basaltic magmas carry it up so often.

• (a)Amphibolite — As printed, this is not a mineral at all — amphibolite is a metamorphic rock. The mineral group the paper is reaching for is amphibole, whose major elements are given as aluminium, calcium, silica, iron and magnesium, and whose colour runs green or black; the extra aluminium and calcium put it outside clue III.
• (b)Feldspar — The most abundant mineral group in the crust, but it is an aluminium silicate of potassium, sodium or calcium — magnesium and iron are not among its major elements. Feldspars are typically white, pink or grey, not green.
• (c)Pyroxene — The closest trap: pyroxenes are also magnesium-iron silicates common in basalt. But they usually carry calcium as well, and they are typically dark green to black rather than the clear olive-green of clue I. Where a stem says 'usually a greenish crystal' with no fourth element, olivine is the intended fit.

Rock-forming silicate minerals are sorted by how their silica tetrahedra are joined. Olivine is an isolated-tetrahedra silicate with the formula (Mg,Fe)2SiO4; pyroxene is a single-chain silicate; amphibole is a double-chain silicate that also carries hydroxyl; and feldspar is a framework silicate built on aluminium substituting into the silica network. That structural order also tracks the order in which minerals crystallise out of a cooling basaltic magma, olivine first.

This is a description-to-name item, so work the clues against each other rather than looking for a single give-away. Colour alone is weak evidence in mineralogy, and 'found in basalt' fits both olivine and pyroxene. The element list is what decides it: magnesium, iron and silica with nothing else named. Add calcium and you are in pyroxene territory; add aluminium and calcium and you are in amphibole; drop magnesium and iron altogether and you are in feldspar. One printed slip is worth noticing without letting it derail the answer — option (a) prints 'Amphibolite', a metamorphic rock, where a list of minerals wanted 'amphibole'. The slip does not change the answer.

• Olivine is a magnesium-iron silicate, (Mg,Fe)2SiO4, named for its typical olive-green colour, and is used as a gemstone (peridot).
• Magnesium-rich olivine crystallises from magma poor in silica, which is why it is common in basalt and gabbro and abundant in ultramafic rocks.
• Olivine and its high-pressure forms make up more than half the Earth's upper mantle by volume.
• Amphibole's major elements are aluminium, calcium, silica, iron and magnesium; it forms about 7 per cent of the crust and is green or black; hornblende is one of its forms.
• Feldspar is an aluminium silicate of potassium, sodium or calcium and is the most abundant mineral group in the Earth's crust.

• Choosing pyroxene because it too is a magnesium-iron silicate found in basalt; the missing calcium and the olive-green colour separate them.
• Treating amphibolite as a mineral — it is a metamorphic rock; the mineral group is amphibole.
• Assuming any green mineral must be olivine — chlorite, serpentine and some pyroxenes are green too, so use the element list as the decider.

As a three-clue identification of a mineral or rock, or as a match-list pairing minerals with their major elements, colour or characteristic occurrence.

• practice — not a real PYQ

  Which one among the following minerals is the most abundant constituent of the Earth's upper mantle?

  • (a)Quartz
  • (b)Olivine
  • (c)Calcite
  • (d)Mica

  Answer(b) Olivine — olivine and its high-pressure structural variants make up over half the upper mantle by volume.

• practice — not a real PYQ

  Aluminium, calcium, silica, iron and magnesium are the major elements of which one of the following mineral groups, of which hornblende is a form?

  • (a)Feldspar
  • (b)Amphibole
  • (c)Olivine
  • (d)Quartz

  Answer(b) Amphibole — hornblende is one of its forms, and it is green or black in colour.