What is a rock, a mineral, and what’s the difference between the two?
Tier II: High frequency words used across content areas. Key to understanding directions, understanding relationships, and for making inferences.
Tier III: Low frequency, domain specific terms
Building on what we already know
What vocabulary & concepts were learned about this earlier?
Rocks stay together in one piece because their atoms and molecules hold each other with strong bonds. But if a force becomes stronger than those bonds then it breaks apart: It will cleave or fracture.
What is a mineral?
Science terms have precise, specific definitions. Think of words like – atoms, compounds, ionic compounds, molecules.
But many common words don’t have a precise meaning, like “rock.” Even the word “mineral“ can have a couple of meanings, depending on context. In science class something can’t be called a mineral unless it meets these requirements.
formed through geological processes
definite chemical composition
they are crystalline – they have an ordered atomic structure
Now let’s explain what these mean!
Naturally occurring – People did not make it. Steel is not a mineral because it is an alloy produced by people.
Inorganic – Not made by an organism. Wood and pearls are made by organisms and thus are not minerals.
Solid – Not a liquid or a gas at standard temperature and pressure. (Many minerals can melt at very high temperatures, of course.)
Definite chemical composition – We find the same ratio of atoms in all parts of the sample.
For example, every part of a salt crystal has 1 Na and 1 Cl atom. Every part of a calcium carbonate sample has 1 calcium, 1 carbon, and 3 oxygen atoms
Ordered internal structure – The atoms exist in a 3D pattern. We can draw this pattern. As we look through a sample of a mineral, this pattern repeats over and over. The structure of the mineral halite is shown here.
Other uses of the word “mineral”
In the mining industry, a “mineral” is anything obtained from the ground. This includes:
crushed stone, which is a manufactured product made from crushed rocks
lime, which is a manufactured product made from limestone or marble (both composed of the mineral calcite)
coal which is organic
oil and gas which are organic fluids
rocks such as granite that are mixtures of minerals
rocks such as obsidian which do not have a definite composition and ordered internal structure.
What is the difference between a rock and a mineral?
A rock is usually an aggregate of one or more minerals, and/or one or more mineraloids.
Common rocks include granite, basalt, limestone, and sandstone.
Here we see silicon and oxygen (elements) combining to make a quartz crystal (which is a type of mineral.)
Then we see this crystal join with other minerals.
Finally, many pieces of this combination join together to make a rock.
However, a large chunk of just one mineral can be called a rock as well. This would be a mono-mineralic rock.
Is glacier ice a type of rock?
Glacier ice is actually a mono-mineralic rock (a rock made of only one mineral, like limestone which is composed of the mineral calcite).
The mineral ice is the crystalline form of water (H2O).
Glacier ice forms through the metamorphism of tens of thousands of individual snowflakes into crystals of glacier ice. Each snow flake is a single, six-sided (hexagonal) crystal with a central core and six projecting arms.
The metamorphism process is driven by the weight of overlying snow.
During metamorphism, thousands of individual snowflakes recrystallize into much larger and denser individual ice crystals. Some of the largest ice crystals observed at Alaska’s Mendenhall Glacier are nearly one foot in length.
What Is a Rock?
adapted from Andrew Alden
Everyone knows what a rock is, until you ask what it is exactly…. most people say: Rocks are hard solids, of natural origin, made of minerals. But all of those criteria have exceptions.
Rocks Are Hard
Not necessarily. Some can be scratched with your fingernail: shale, soapstone, gypsum rock, peat. Others may be soft in the ground, but they harden once they spend time in the air (and vice versa).
Rocks Are Solid
Well, some are far from completely solid. Many rocks include water in their pore spaces.
Some geodes—hollow objects found in limestone country—hold water inside them like coconuts.
Then there’s the matter of temperature. Mercury is a liquid metal at room temperature (and down to 40 below zero), and petroleum is a fluid unless it’s asphalt erupted into cold ocean water.
And ice meets all the criteria of rockhood too, in permafrost and in glaciers.
Rocks Are Natural
Not entirely. Concrete is a mixture of sand and pebbles (aggregate) and a mineral glue (cement) of calcium silicate compounds.
Concrete is a synthetic conglomerate, and it acts just like the natural rock, turning up in riverbeds and on beaches. Some of it has entered the rock cycle – to be discovered by future geologists.
Brick, too, is an artificial rock—in this case, an artificial form of massive slate.
Slag: the byproduct of metal smelting. Slag is a complex mixture of oxides that has many uses, such as in road building and concrete aggregate. It too has surely found its way into sedimentary rocks already.
Rocks are made of minerals
Most are, sure, but many are not. Minerals are inorganic compounds with chemical formulas and mineral names, like quartz or pyrite.
But what about coal? Coal is made of organic material, not minerals.
And what about coquina, a rock made entirely of seashells? Shells are made of mineral matter, but they aren’t minerals any more than teeth are. Rocks like these are called biogenic rocks.
And what about obsidian, a rock glass? None of its material has gathered into crystals. It is an undifferentiated mass of geological matter. While it has no minerals in it, it is called a rock.
Lustre: Minerals and rocks reflect light in special ways, called lustre. Let’s explore – open this document: Lustre
Mineral Identification properties
8.MS-ESS2-1. Use a model to illustrate that energy from Earth’s interior drives convection that cycles Earth’s crust, leading to melting, crystallization, weathering, and deformation of large rock formations, including generation of ocean sea floor at ridges, submergence of ocean sea floor at trenches, mountain building, and active volcanic chains.
MS-ESS2-2. Construct an explanation based on evidence for how geoscience processes have changed Earth’s surface at varying time and spatial scales.
Surface weathering, erosion, movement, and the deposition of sediment ranging from large to microscopic scales (e.g., sediment consisting of boulders and microscopic grains of sand, raindrops dissolving microscopic amounts of minerals).
S-ESS3-1. Construct a scientific explanation based on evidence for how the uneven distributions of Earth’s mineral, energy, and groundwater resources are the result of past and current geoscience processes.
MS-ESS1-4. Construct a scientific explanation based on evidence from rock strata for how the geologic time scale is used to organize Earth’s 4.6-billion-year-old history.
Students identify and describe* the evidence necessary for constructing the explanation, including:
i. Types and order of rock strata.
HS-ESS1-5. Evaluate evidence of the past and current movements of continental and oceanic crust and the theory of plate tectonics to explain the ages of crustal rocks.
Disciplinary Core Ideas ESS1.C: The History of Planet Earth. Continental rocks, which can be older than 4 billion years, are generally much older than the rocks of the ocean floor, which are less than 200 million years old.
HS-ESS2-3. Develop a model based on evidence of Earth’s interior to describe the cycling of matter by thermal convection.
… Matter is cycled between the crust and the mantle at plate boundaries. Where plates are pushed together, cold crustal material sinks back into the mantle, and where plates are pulled apart, mantle material can be integrated into the crust, forming new rock.