Earth Materials Lecture 7: Ferromagnesian minerals

Question 1

Define Ferromagnesian minerals/Mafic

Answer 1

Mafic minerals are Silicate minerals that contain large amounts of Mg (Magnesium) and Fe (Iron).

Usually refers to dark, Fe-Mg minerals in igneous rocks:
− Olivine, Pyroxenes, Amphiboles

Question 2

Describe the chemical composition of Olivine and the Olivine group

Answer 2

(Mg,Fe)²[SiO⁴]

Independent SiO4
tetrahedra
linked laterally by divalent
cations: Mg, Fe2+,

Solid solution between Forsterite
(Mg) and Fayalite (Fe2+).

Question 3

Describe Olivine Paragenesis

Answer 3

Mostly igneous rocks:
- Lherzolites, dunite (>90% olivine) = mantle rocks.
- Basalts, dolerite, gabbro = mafic rocks.
- Lavas, dykes, sills, layered cumulates.
- Tends to crystallise early (Mg-rich).
- Occurs with pyroxenes, Ca-plagioclase, magnetite,
chromite.
- Readily alters to chlorite, serpentine, talc, Fe-oxides.
- Some marbles at high metamorphic grade.

Question 4

Describe the pyoxene group and chemical structure

Answer 4

Single chain silicates of
green / brown stumpy and
slender prisms elongate II to c-axis.

Structure of the pyroxenes is inosilicate tetrahedra
linked by various cations,
e.g. Ca, Mg, Fe etc.

• Various solid solution series.
• Orthorhombic (OPX) and
  monoclinic (CPX) members.
• Monoclinic predominate. ⚫ (Ca,Mg,Fe,Ti,Al)2
  [(Si,Al)2O6] – augite (titanaugite).

(Mg,Fe)[SiO3] – enstatite

Question 5

Describe Augite paragenesis

Answer 5

Wide range of igneous and metamorphic rocks,
occasionally as detrital grains in sedimentary rocks.

• Igneous rocks from ‘black rocks’ to ‘pale rocks’:
  ➢ Mantle rocks, intrusions and lavas.
• Olivine, labradorite, andesine, sanidine, biotite,
  magnetite.
• Diopside occurs in some marbles.
• Pyroxene granulites – high grade metamorphic rocks.

Question 6

Describe hornblende and the amphibole group

Answer 6

inosilicate minerals, forming prism or needlelike crystals, composed of double chain SiO 4 tetrahedra

➢ Ca, Mg, Fe: Actinolite
➢ Ca, Mg, Fe, Al: Hornblende
➢ Na, Mg, Al: Glaucophane
(mostly metamorphic).

Several solid
solution series.

Question 7

Describe Hornblende paragenesis

Answer 7

Common in intermediate to acid igneous rocks:
➢Hornblende as primary phase in:
– Andesites, diorites, granodiorites, ~granites.
– With pyroxenes, feldspars, micas, quartz.
- In some metamorphic rocks, e.g. amphibolites where it
is the main constituent along with plagioclase, quartz,
± epidote,± garnet.
- As detrital grains in some sedimentary rocks (lithic).