Minerals and Rocks
To appreciate the geology of California we must understand minerals and rocks. Harden does not go too much into minerals.
All minerals are made of chemical elements. So let’s look into this.
Chemical Element: the most fundamental substance into which matter can be separated by chemical means.
You should know the name and chemical symbols of the most abundant elements in the Earth’s crust.
Other elements to know are fluorine (F), Chlorine (Cl), sulfer (S), and carbon (C).
Let’s review the structure elements.
Atom: the smallest individual particle that retains all of the properties of a given chemical element.
In an electrostatically balanced atom, the number of electrons equals the number of protons. For example, oxygen (O) has eight protons, eight electrons, and usually eight neutrons (we will discuss isotopes when we talk about absolute dating techniques and Geologic Time).
However, the atom may gain or lose electrons. When this happens, the atom is no longer balanced and becomes an ION.
Chemical Compound: a substance composed of more than one element.
For example, common table salt is the chemical compound sodium chloride, NaCl.
Minerals: the basic building blocks of rocks, i.e. all rocks are composed of minerals. Some rocks may be formed of just one mineral, but most are composed of several, average of 5 to 10.
3000 recognized minerals, we will learn about 10 to 15 (the common rock forming minerals).
Definition of a Mineral:
A mineral can be composed of a single element.
Graphite, C. Pencil "lead" is graphite, very soft.
Can anyone think of another mineral composed entirely of carbon?
Diamond—the hardest of all minerals. These two minerals have the same chemical composition, but very different physical properties.
Polymorphs—same chemical composition, but different internal arrangement of atoms and different physical properties.
Mineral Composition Chemical Compound
Halite NaCl sodium chloride
Quartz SiO2 Silicon dioxide
Calcite CaCO3 calcium carbonate
These minerals are examples of minerals with fixed chemical compositions.
They basically have the same composition where ever you find them.
Feldspar: is just about everywhere and is an example of a mineral whose composition can vary.
Two types of feldspar K-feldspar and Plagioclase
Plagioclase feldspar varies in composition by its content of Na, Ca, Al, and Si.
Caà Na substitution
Alà Si substitution
Plagioclase can vary in composition from rock to rock, but its composition is constant in any single rock.
Crystalline Structure: the atoms of ions in a mineral are arranged in a specific 3-dimensional pattern. Each mineral has its own unique pattern or internal arrangement of atoms or ions.
This is how glass differs from crystalline material.
Glass is AMORPHOUS, no crystalline structure.
Now let’s discuss the physical properties of minerals.
Pearly (like talk)
Waxy (like jasper)
Resinous (like sphalerite)
Special Properties are used to identify minerals
Effervescence—Calcite reacts with HCL to form CaO and CO3 gas.
Tenacity – toughness, describes the minerals resistance to being broken. [Adjectives – flexible (mineral is easily bent), elastic (mineral rebounds), brittle (mineral fractures into many pieces), malleable (mineral is bend like soft metal), sectile (mineral can be cut by knife)]
Magnetism – attraction to a magnet (Magnetite is most common)
Taste, Odor, Feel – Some minerals have a distinctive taste (halite – rock salt), some a distinctive odor (powder of sulfide minerals like sphalerite smells like rotten eggs), and some a distinctive feel (talc feels slippery, greasy).
Striations and Exsolution Lamellae
Striations are very thin, parallel grooves on some mineral faces – most commonly Plagioclase feldspar, some pyroxenes, tourmaline, and pyrite. Key thing is that these are physical grooves in mineral.
Exsolution Lamellae are thin veinlets seen mainly in Potassium Feldspar. These have variable widths and are not physical grooves. They are also not parallel.
Introduction to Igneous Rocks
Rocks we will see in the class exercise are Basalt (14), Andesite (14), Dacite (4), Rhyolite (9), Obsidian (18), Volcanic Tuff (8), Pumice (12), Pegmatitic, large grain granite (25), course grained granite-Diorite (15), Diroite-Gabbro (20), and Gabbro (40).
Igneous rock: forms by the cooling and solidification of magma.
Magma: molten rock, which may include suspended minerals and dissolved gasses.
Two types of Igneous rock. Extrusive and Intrusive
Extrusive: lava, pryoclastic material, magma coming out of a volcano.
Intrusive: magma solidifies within the Earth’s crust or mantle.
Classification of Igneous Rocks.
Igneous rocks: classified based on Rock Texture and Mineral Assemblage.
Size of mineral grains—Intrusive rocks tend to be coarse grained because minerals grow slowly in the Earth’s crust or mantle. Extrusive rocks tend to be fine grained to glassy because they cool rapidly on the surface of the Earth.
Phaneritic—the minerals are visible to the eye.
Aphanitic—the minerals are not visible to the eye.
Porphyritic—When on mineral is considerably courser, larger than the other minerals. Often the groundmass or matrix is fine grained.
Pegmatitic—Pegmatite, when all minerals are enormous, i.e. big.
Glassy—basically aphanitic, but typical of volcanic glass, obsidian.
Phenocryst—Minerals in rocks are termed phenocrysts.
These consist of the Major Rock Forming Minerals.
Feldspar (K-spar and plagioclase)
Mica (Biotite and Muscovite)
Depending on what minerals you have in the rock, classifies or names the rock.
See Handout on igneous rocks.
Basically, Quartz is typical of rocks like Granite and Rhyolite, where as olivine is typical of rocks like basalt and Gabbro.
Note that rocks become darker in color as more of the darker minerals are present in the rock. We call this the COLOR INDEX. Granites have a CI of 25 or less. Basalts have a CI of 90 to 100.