Rocks and Minerals

I will start by reminding everyone that I am just a girl that loves crystals and I don’t have any ology titles behind my name. When you start to invest serious time and money in crystals, the details become a little more important and you may want to pursue a deeper understanding of your crystal investments.   

Before anyone gets too upset by words, I will clarify some terms. A rock is two or more minerals bonded together. A mineral is a naturally occurring inorganic element. A crystal refers to the structure of a mineral in crystalline form. A gemstone is a rare mineral of the purist quality and are classified as precious or semi-precious. Many people interchange these four terms, but to do so is technically inaccurate.

That’s out of the way. Let’s dig in. This blog explores a simplified version of the ology of rocks and minerals. Crystals will be discussed in a later blog.

A brief review of the layers of the Earth is required. The Earth is composed of the inner core, outer core, mantle, and crust. The inner core is mostly made of iron and nickel and it is extremely hot. Immense pressure exists in the inner core. The outer core is also iron and nickel but in liquid form. This liquid churns and generates the magnetic field of the Earth. The mantle is the thickest layer and is composed of iron, magnesium and silicon. It is dense, hot and semi-solid. It also churns but it does so much more slowly than the layer below. Near the mantle’s edge, the temperature reaches the melting point of rock but the most outer edge of the mantle is cooler and rigid. The Earth’s crust is thin, cold and brittle. The crust is made of oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. Now that we have reviewed the layers we can go back to rocks.

Rocks are divided into three main groups; igneous, sedimentary, and metamorphic. These group designations reflect the formation process that creates the rock. Considering how the rock is formed is important for correct identification and value.  

Igneous rocks form when magma crystallizes and solidifies. The word igneous is derived from the Latin word meaning fire. Igneous rocks are further divided into intrusive or extrusive. Intrusive rocks result when magma cools and crystallizes slowly and within the Earth’s crust. Extrusive rocks are created when magma exits the crust, cools quickly, and is unable to properly crystallize. As magma moves upwards through the crust, it picks up silica. Silica content is used to classify a rock as igneous.  

Sedimentary rocks are formed on or near the Earth’s surface. The proximity of the surface creates various influences on the rock. Physical (erosion and weathering) and chemical (dissolution) weather elements create alterations to the rock. Mineral deposits from surface water (precipitation) may introduce additional minerals to the rock. Ocean floor sediments (lithification) may be turned into rock. There are two categories of sedimentary rock. Detrital rock is formed organically when plants or animals decay in the ground and become compressed into rock. Inorganic rock is composed of various pieces of other rocks.

Metamorphic rocks are formed by exposing a rock to pressure and temperature conditions that are different from what originally created it. These conditions change the form of the original rock by causing recrystallization. The formation process of metamorphic rock determines its class. Contact metamorphism arises when magma heats the surrounding rock. Pressure metamorphism occurs deep beneath the surface when pressure influences formation more than temperature. Regional metamorphism happens when both temperature and pressure play a role. Metamorphic rock has two categories. Foliated rocks have banded or layered appearances caused by repetitive layering. Non-foliated rocks are not banded or layered.

Whew, that was a little intense. We’re going to keep going and have a look at minerals.

Rocks are composed of minerals. Minerals are composed of elements like oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, and potassium. Minerals are naturally occurring substances that are solid, crystalline, stable at room temperature, and inorganic. There are thousands of minerals but most rocks contain rock-forming minerals like feldspars, quartz, amphiboles, micas, olivine, garnet, calcite, and pyroxenes. Accessory minerals by the way, are minerals that occur in small quantities. They help to provide the age and history of a rock. Some accessory minerals are zircon, monazite, apatite, titanite, tourmaline, pyrite, and other opaques.

Minerals are identified and valued by physical properties like color, crystal habit, hardness, streak, lustre, and cleavage. Multiple physical properties may be required to identify a mineral. Here’s a brief break down of those properties.

Most minerals have a characteristic color that identifies it but impurities may cause variations making identification tricky. Color therefore, isn’t always an accurate identification method.

Crystal habit refers to the shape of the mineral unit. The shapes are; acicular, bladed, botryoidal, columnar, cubic, dendritic, fibrous, foliated/lamellar, hexagonal, massive, octahedral, platy, prismatic, and radial/stellate.

Hardness references how resistant the mineral is to scratching and this is measured by the Mohs scale. The Mohs scale uses ten minerals organized progressively by hardness. Talc, gypsum calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, and diamond are assigned numbers 1-10.

Streak is the color of the mark left behind when a mineral is rubbed against unglazed porcelain. Streak is only used with minerals that have a hardness of 7 or less on the Mohs scale.

Lustre describes how the mineral reflects light. The terms are adamantine, dull/earthy, greasy, pearly, vitreous, silky, resinous, and metallic.

Cleavage refers to the presence of weakness between the chemical bonds of the atoms that make up the structure of a mineral. The ease which the weakness breaks helps with identification of the mineral. Cleavage is described as perfect, good, distinct, or poor.

Still with me? Well done! That’s it for our intro into rocks and minerals. We will explore some ology about crystals next time.