Cleavage vs. Fracture

When we say a mineral exhibits "fracture," not "cleavage," we are talking about how the mineral breaks. "Fracture" in this instance doesn´t mean a hairline crack, the way you might fracture a bone - it means that the mineral breaks, but not along preferred planes. It´s a more random way of breaking - since the bonds between all the atoms are roughly equal, there aren´t layers of weakness, so the mineral when broken will crack and separate more or less randomly. If a mineral has cleavage, on the other hand, we´re saying that there are planes, or flat surfaces, that are more weakly bound - so if the mineral breaks, it will tend to break along that weak plane. That will produce a nice flat surface rather than a rough jagged edge.

Effervescence

Most minerals don´t react with acid, but carbonate minerals like calcite or dolomite, do. Calcite and dolomite can look alike, but you can differentiate between them using acid. If you drop some weak acid (even vinegar will do) on a piece of calcite, it will foam, fizz, or otherwise effervese. Dolomite, on the other hand, will only fizz on fresh surfaces, so you have to scratch the dolomite first, and drop the acid on the scratch.

Hardness I

The hardness is important because it is considered diagnostic - that is, you can use it to help identify the mineral. Moh´s Scale of Hardness runs from 1 (talc) to 10 (diamond), but here´s a tip: use a glass plate to separate "soft" minerals from "hard" ones. If the mineral scratches the glass, call it hard. If it can´t scratch the glass, consider it a soft mineral.

Mineral or Rock?

Rocks are defined as aggregates of minerals - in other words, it´s the little bits of minerals, stuck together, that make a rock. The tricky part is that sometimes you can´t see the individual minerals, and you might think you´re looking at one big mineral instead of lots of little ones stuck together.

Although this won´t always be true, as a good rule of thumb, if you find a specimen larger than an inch or so, it´s probably a rock rather than a mineral.

Hardness II

Moh´s scale of hardness is a way to decide what the hardness is of a particular mineral specimen you want to identify. The scale rates the hardness of ten minerals from talc (1) to diamond (10). We don´t usually have all the minerals on hand, so we substitute things like pennies and knives instead.

If you can scratch the mineral with your fingernail, it´s a 1 or a 2. If you can scratch it with a penny, that means the mineral is about 3.5 or less. If you can scratch it with a knife blade or a nail, the hardness of the mineral should be about 5.5 or less. If the mineral can scratch glass (you can get them in hardness kits) the hardness is about 8 or higher.

Crystal Habit

The crystal habit of a mineral is the characteristic shape it takes if it has room to "grow" freely. Table salt (halite), for example, will always grow in cubes, as will fool´s gold (pyrite). Garnets will grow in wonderfully complex dodecahedrons, and asbestos minerals are fibrous. These are all characteristic crystal habits, or crystal shapes, and all minerals will grow in a predicatable shape if they can.

Elements

There are 108 elements, and they can be combined in so many different ways you might wonder how many minerals can occur naturally. However, although 108 elements exist, they aren´t all common - the earth´s crust is composed *mostly* of eight important elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. Together, they account for 98.59% of the earth´s crust by weight.

Streak

The streak of a mineral is its color when it´s in powder form. We usually identify the streak of a particular specimen by rubbing it across a piece of unglazed porcelain (a ´streak plate´). Some minerals leave no streak, or leave a streak that is the same color as the mineral. However, many minerals do have a diagnostic color that is more reliable for identification purposes than the color of the specimen itself.

Crystals

A crystal is a substance whose atoms are arranged in a regular, repeating pattern. You can see an example of crystals by looking through a hand lens or magnifying glass at regular table salt (which is actually the mineral halite).

Window glass, on the other hand, is an example of a solid substance which isn´t made of crystals, even under a microscope.

Mineral or not?

Try this: why is it that a natural diamond is a mineral, but a synthetic one technically is not? Answer: because to a geologist, a mineral has meet certain specific criteria. It has to be solid, for starters. Also, it has to be inorganic (not a plant!), natural (man-made stones don´t count!), with a specific chemical composition (quartz is always SiO2) and a crystalline structure.

Specific Gravity

The specific gravity of a mineral is the its weight compared to water. For example, a one-inch-square cube of copper would weight 8.9 times as much as a one-inch-square cube of water.

Most common minerals have a specific gravity of about 2.7. If your specimen seems to be heavy compared to other minerals of about the same size, it probably has a significant amount of gold, lead, silver, or copper in it.

Identification

You can differentiate minerals by using certain characteristics or physical properties of the minerals. Generally those properties are crystal habit, cleavage, fracture, hardness, specific gravity, color, streak, and luster.

Luster

"Luster" refers to the way a mineral reflects light. For example, a mineral like quartz reflects light in a way similar to glass - it has a "vitreous" luster. "Vitreous" is a word related to glass, so a mineral with a vitreous luster will reflect light kind of the way a piece of glass will. Gold or pyrite (fool´s gold) has a metallic luster, it reflects light the way normal, everyday metallic objects do. "Greasy" or "earthy" are also lusters. Minerals have typical lusters associated with them - a minerals that looks like a lump of dirt (an "earthy" luster) won´t be quartz, since we already know quartz has a vitreous luster.

Hint: even if a mineral looks like a piece of glass, don´t say the luster is "glassy." In geology, glassy has a whole different meaning, related to rock texture, not mineral luster!

Color

Color isn´t the best diagnostic tool for some common minerals, but it´s great for others. Color can vary due to impurities in the mineral - for example, quartz can be clear, milky, pink (rose quartz), grey (smoky quartz), yellow (citrine), or purple (amethyst). But while you can´t rely solely on color to identify a mineral, it does help. A clear mineral won´t be hornblende. A pink mineral won´t be pyrite.

Cleavage vs. Crystal Shape

It can be hard to decide what´s a cleavage surface, and what´s a crystal face. Crystal shape has to do with the way a mineral will "grow" if it has enough room. (If it is crowded, the crystal shape might be clear only under a microscope, not in a hand specimen). Cleavage, on the other hand, refers to the way a mineral breaks - if you whack it with a hammer, does it break into cubes? Does it always break so that it´s like a sheet of paper? That´s cleavage. Some minerals don´t really have cleavage, they don´t always break a certain way. But others, for example, pyrite, break alone certain preferred planes. A cube of pyrite, if you whack it with a hammer, with break into - little cubes of pyrite. That´s cleavage, in the case of a cube, it´s cleavage in three directions.