Rocks are aggregates of minerals. They are classified into three types—igneous, sedimentary, and metamorphic rocks—and each has characteristic mineral compositions and textures.
Basic Knowledge
The main rock-forming minerals are divided into colorless minerals and colored minerals.
Colorless minerals: quartz(SiO₂, colorless and transparent, breaks irregularly) and feldspar(white, breaks in definite directions).
Colored minerals: biotite(peels into thin plates), hornblende(greenish-black and elongated), pyroxene(dark green to black and short-prismatic), and olivine(yellow-green and granular).
Light-colored igneous rocks(rhyolite and granite) contain many colorless minerals, while dark-colored igneous rocks(basalt and gabbro) contain many colored minerals.
Mineral hardness is expressed using the Mohs hardness scale(1〜10), with diamond having the maximum value of 10.
Mineral(A natural inorganic solid with a definite chemical composition and crystal structure)
Rock-forming minerals(The main minerals that make up rocks. Six types are central)
Quartz(SiO₂. One of the hardest rock-forming minerals)
Feldspar(Potassium feldspar and plagioclase. The most abundant mineral in igneous rocks)
Mohs hardness(1 = talc, 10 = diamond. Determined by scratching)
Cleavage(The property of a mineral to break in definite directions. Prominent in feldspar and biotite)
In Depth (Background and Significance)
By examining the mineral composition of a rock, we can determine the environment in which it formed(temperature, pressure, and type of magma). For example, rocks containing much olivine(peridotite) originated deep in the mantle, while granite containing much quartz formed by slow cooling deep in the crust.
Minerals are also important as resources. Iron(Fe), copper(Cu), gold(Au), lithium(Li), and rare earth elements become concentrated in specific geological environments. More than 30 types of elements are used in a single smartphone, and urban mining(reusing materials from discarded electrical appliances) is attracting attention.
Learning about crystal structures is directly connected to chemistry(inorganic chemistry), and the tetrahedral [SiO₄] structure of silicate minerals is the basic unit of all rocks.
- Six basic rock-forming minerals: two colorless and four colored
- Quartz and feldspar = colorless(light-colored rocks)
- Biotite, hornblende, pyroxene, and olivine = colored
- White igneous rocks = rhyolite and granite(much SiO₂)
- Black igneous rocks = basalt and gabbro(little SiO₂)
- Mohs hardness 1(talc)〜10(diamond)
- Urban mining = reuse of metals from discarded electrical appliances
Points to Note (Common Confusions)
① Do not confuse minerals and rocks. A rock is an aggregate of minerals. ② Quartz breaks irregularly, while feldspar has cleavage(and breaks flatly). ③ Biotite is plate-shaped and peels into thin layers, while pyroxene and hornblende are prismatic and differ from it. ④ Remember that white rocks contain many colorless minerals, while black rocks contain many colored minerals.
Practice
- Name two colorless minerals.
- What mineral has a Mohs hardness of 10?
- Name a light-colored plutonic rock.