What are Minerals?
Minerals are naturally occurring, inorganic, solids with definite chemical compositions and ordered internal structures. Naturally occurring means that they form in nature. People do not create them. For example, steel is not a mineral, but gold is. Inorganic means they are not made by an organism. For example, pearls are not minerals because oysters make them. Solid means that they are not liquids or gases at standard temperature and pressure. A definite chemical composition means that the same number of atoms always combine to form specific minerals. For example, a molecule of quartz always has one silicon atom and two oxygen atoms. Finally, an ordered internal structure means that the atoms or molecules form regular and repeating patterns. For example, halite has a cubic crystal structure, and quartz has a hexagonal structure.
Minerals can either be made up of one type of element or multiple elements in molecules. When they are made up of one element, they are called native elements. Gold, silver, and carbon are all native elements. Multiple minerals can come from the same native elements. For example, diamond and graphite are both made up of pure carbon. Calcite, made up of calcium, carbon, and oxygen, is an example of a compound mineral. There are over 4,000 different minerals on Earth, and most of them are compounds.
How do minerals form?
Minerals form in two ways. First, they can form when magma or lava cools. The chemical composition of the magma or lava determines the type of mineral that develops. The slower the liquid rock cools, the larger the crystals grow. When lava cools quickly, only minuscule crystals appear. Sometimes, the lava cools so fast that no crystals have time to form. For example, obsidian has no crystals.
Crystals also form when cations and anions dissolved in water fall out of solution when the water evaporates. Most minerals are compounds made up of a positively charged ion called a cation and a negatively charged ion called an anion. Cations and anions separate when they dissolve in water and join together through ionic bonds when the water evaporates. Minerals can form from solutions anywhere water is found. For example, massive halite deposits formed in the Midwest and Southwest as ancient oceans evaporated. Gypsum and calcite also form from evaporating water.
Physical Properties of Minerals
Today we classify minerals based on their chemical composition, but for hundreds of years, mineralogists grouped them by their physical characteristics. Some physical characteristics include color, crystal form, hardness, luster, density, magnetism, and cleavage and fracture.
While easy to determine, color is unhelpful for classifying minerals because many of them are the same color, and one mineral can appear as several different colors. For example, quartz can be clear, smoky, pink, purple, or yellow. A mineral’s color is determined by the atoms within. Ruby and sapphire are varieties of the corundum mineral, but ruby has traces of chromium within it, and sapphire has traces of either iron or titanium.
A streak test gives the color of the mineral in powder form. It can be helpful for differentiating between two different minerals that look the same. For example, gold has a yellow streak and pyrite has a black streak. On the other hand, calcite crystals can be many different colors, but they all have a white streak. To perform a streak test, scrape a mineral over a piece of unglazed porcelain. Streaks of powder will be left on the porcelain for you to study.
The crystal form is the shape of a mineral’s crystals, and it is determined by the structure of the atomic bonds that make it up. Unfortunately, we only see crystals when they have space and time to grow. For example, quartz within a geode will form large hexagonal crystals, but will only form small amorphous crystals within granite.
The most common way to test the hardness of a mineral is a scratch test. Scratching a mineral with a nail will determine if it is harder or softer than the nail. In the 1800s, Friedrich Mohs, an Austrian mineralogist, created the Mohs scale to rank minerals based on their hardness between 1 and 10. Talc has a score of 1 on the Mohs scale, and diamond has a score of 10. Today, we have more accurate methods for determining a mineral’s hardness, but many mineralogists continue to use the Mohs scale because it is easy to remember and use in the field with few tools.
Luster describes how a mineral reflects light. The two main types of luster are metallic and nonmetallic. There are even more specific groups, including vitreous, pearly, waxy, resinous, adamantine, greasy, silky, and dull.
Density describes how tightly packed the atoms or compounds within a mineral are. Water has a density of 1 g/cm3, and all minerals are denser than water. Gold has a density of 17.65 g/cm3, and iron has a density of 7.6 g/cm3. In the field, experienced mineralogists can estimate a mineral’s density by holding it.
Magnetism describes how much a mineral is attracted to magnets. The movement of the electrons with the mineral’s crystaline structure determines the power of the magnetism. Iron and magnetite are examples of minerals that will stick to magnets.
Cleavage and Fracture
Cleavage and fracture describe how minerals break. Some of them have weaknesses within their atomic structures. These are places where the bonds between atoms or molecules are not as strong as the bonds around them. For example, biotite has layers held together by weak hydrogen bonds, so the layers break off in flat planes. When a layer of the face breaks away, a new face is left in its place.
While only some minerals have cleavage, they all fracture. A fracture is any break that doesn’t remove a complete layer. Mineralogists classify fractures based on how the edges of the minerals look after the break. A conchoidal fracture creates concentric ripples similar to those seen on a mussel shell. Minerals with conchoidal fractures usually cooled quickly and have tiny crystals. Earthy fractures look like freshly broken soil. They happen in soft minerals, such as limonite and kaolinite. Hackly fractures are jagged and sharp. They happen when metals like copper and silver tear.
From the Middle Ages until the 1800s, mineralogists carefully studied physical properties to organize minerals into groups with similar characteristics. Then, in the late 1800s, Yale mineralogist James Dwight Dana developed a classification system based on the chemical composition of the anions. He named seven major groups based on their anions, including silicates, carbonates, halides, oxides, phosphates, sulfates, and sulfides. Native elements are the eighth major mineral group. There are also minor mineral groups that are uncommon on Earth. Around 90% of the minerals in Earth’s crust are silicates. Silicates are also abundant in the mantle.
Rocks and Minerals
Minerals make up rocks. For example, granite, a common igneous rock, is mostly made up of the minerals quartz and feldspar. Limestone, a sedimentary rock, is mostly made up of calcite. Rocks are also different from minerals because they can contain organic material. For example, coal mainly comes from the remains of plants, so it is a rock, not a mineral.
How to Remember What You Learn
Now that you have learned about minerals, it is time to focus on remembering what you have learned. The first part of remembering is paying attention. Next, make sure you take the time to process what you learn. You can do this by answering comprehension questions, creating a mind map, drawing a picture, or talking about what you learned.
Next, you need to practice remembering what you have learned. My favorite tool for practicing remembering is flashcards. Flashcards are easy to make and force our brains to try to remember the answers to questions because when our brain sees a question, it attempts to answer it. So just trying to answer the question and then seeing the right answer will help you strengthen your neural networks on the topic.
Finally, if you are taking a test on a topic, make sure you study for about ten minutes a day for several days before the test. Then, the night before the test, get a good night’s sleep. Sleeping will help your brain clean itself and strengthen neural networks. Pulling an all-nighter may seem like a good idea, but it is terrible for your brain. Finally, while taking the test, take deep breaths to stay calm, and always go with your first instinct unless you can prove why your later guess is right. You are more likely to change a right answer to a wrong answer than the other way around.
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