Geology 101 - Introduction to Physical Geology
Photos of Pacific Coast, Cascades, Columbia Plateau
Wenatchee Valley College logo
Lab - Minerals

Introduction

A mineral is not the same thing as a rock. A mineral is an inorganic solid, made of of a specific combination of chemical elements, the atoms bonded together in a repeated, symmetric pattern. The symmetric, three-dimensional pattern in which a mineral's atoms are arranged is called the crystal lattice or atomic structure.

The physical properties of a mineral are determined by its chemical composition and its atomic structure.

To learn minerals involves learning new ways of testing and thinking about reality, based on more than looking and guessing. In this lab, you will work with real minerals to experience for yourself how minerals express their chemistry and symmetry in their physical properties. By being able to determine the physical properties of the minerals, you will be able to identify them, which is a key step on the way to the more important goal of using minerals to extract scientific information. The information we can scientifically determine from examining minerals tells us about the earth, how the earth works, and what has happened in earth history to form the mineral. (Or, going beyond geology to other basic sciences, we can extract information from minerals regarding physics, chemistry, or biology.

Advance Preparation and Materials Needed

Geology Learning Outcomes

By performing and completing this lab, you will progress toward the following learning objectives for this course:

Methods

Lab Procedures

In this lab you will determine the physical properties of minerals, identify minerals and take photographs of them.

Refer to the Lab Assignments Grading Rubric for a reminder of what constitutes a well-performed lab.

Minerals and their Identification

What is a mineral? While Wikipedia does not provide a primary source of information, it happens to have a good introductory paragraph to its article about minerals, as follows: "A mineral is a naturally occurring solid chemical substance that is formed through geological processes and that has a characteristic chemical composition, a highly ordered atomic structure and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids, and does not have a specific chemical composition. Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms. The study of minerals is called mineralogy." (article on "Minerals" http://en.wikipedia.org/wiki/Mineral, accessed 4/3/11.)

Physical Properties of Minerals

Learning to distinguish the physical properties of minerals provides you the diagnostic tools you need to distinguish similar looking minerals from each other and to identify unknown mineral crystals. With a little practice and some very simple tools, most minerals the size of samples you can hold in your hand can be quickly identified based on their physical properties. Determining the physical properties of minerals requires learning to routinely examine, describe, and test pieces of solids in unfamiliar ways. The physical properties of mineral hand samples include color, streak, luster, hardness, cleavage, fracture, and crystal form.

Most people, before they practice identifying minerals, only know about color, and tend to only look at the color of a mineral to try to identify it. Just looking at a mineral and trying to identify it on the basis of color leads to many mistaken identifications. This is because, on the one hand, many minerals have the same color, and on the other hand, many minerals come in a variety of colors. Therefore, to verify the identity of a mineral, you cannot just look at it.

You need to pick up the mineral, scrape it against a streak plate to see what color the mineral is when turned into a powder, examine the mineral's cleavage properties (which requires looking closely at how it reflects light as you rotate it around to different angles), discern the shape of fractured surfaces on the mineral, distinguish its luster (the manner in which it reflects light), and conduct scratch tests with materials of known hardness to determine the hardness of the mineral. To be sure of the shapes, colors, cleavages, and fracture properties, you will usually need to view the mineral under a magnifying glass or a low-power microscope. Often mineral hand samples do not show the crystal form of the minerals. You need to check each mineral and decide if crystal facets are visible and defining the form of a mineral crystal, or if the sample you have is a broken or fractured or irregular piece that does not show the crystal form.

As you analyze and identify minerals, you will need to keep in mind that a mineral is not a rock, and a rock is not a mineral. A rock is a solid piece of the earth composed of minerals or other solids. Most rocks consist of several minerals. Some rocks, such as volcanic glass, contain no minerals.

Take a close look at the photograph of a hand sample of granite. Click on the thumbnail below for a larger version of the image that opens in a new window.

photo of granite hand sample

Granite is a rock consisting of several visible minerals, including quartz, potassium feldspar (K-feldspar), sodium plagioclase (Na-plagioclase), and typically one or two other minerals such as biotite, hornblende, or muscovite. In the picture below, if you know the physical properties of the minerals, you can quickly make out which parts of the granite consist of which minerals, as follows:

Quartz: key physical properties include colorless and transparent in most granites, no cleavage, and glassy luster. The lack of color means the quartz is relatively nondescript and nearly lost in the shadows among the colored minerals. The lack of cleavage means the quartz will not have been broken into any flat surfaces or any square shapes or and 90? corners.

K-feldspar: key physical properties include pink color in most granites, two cleavages at right angles to each other, and pearly luster. The pink color is distinctive in this case. The rectangular shapes and 90? corners result from K-feldspar's cleavage. The pearly luster means the surface of the mineral shines in a different way from the simple glassy luster of quartz. Knowing the physical properties of K-feldspar as it typically occurs in granite allow one to quickly distinguish it from quartz and verify that is K-feldspar.

Na-plagioclase: key physical properties include white or colorless, two cleavages at right angles to each other, and pearly luster. The white color helps distinguish Na-plagioclase from the colorless quartz. However, in some granites the Na-plagioclase is also colorless. In that case, the rectangular shapes and 90? corners resulting from the cleavage of Na-plagioclase's, and the nearly iridescent pearly luster, seen best under a magnifying lens, help distinguish the Na-plagioclase from quartz.

In addition, plagioclase commonly exhibits the special property of striations (fine, straight, parallel lines) on some of its cleavage surfaces. Checking under a magnifying lens and spotting the striations on some of the cleavage surface allows you to verify that much of the granite does indeed consist of Na-plagioclase.

In a similar way, you can use a few key physical properties to determine if the other minerals in the granite include biotite, hornblende, muscovite, or other minerals. It takes a little bit of work, some close examination, some use of magnifying lenses and, if available, a low-power microscope, to be sure which minerals are in the granite and make an estimate of their relative abundances.

The point of studying minerals is to be able to use them as geological sources of information. Think of the study of minerals as a three-step process:

  1. Identify the physical properties
  2. Name the mineral
  3. Use the what you know about this type of mineral to answer questions such as:
    • What was the chemical environment in which the mineral formed?
    • What the pressure or temperature in which the mineral formed?
    • What other earth system processes occurred to create the mineral?

photo of mineral crystal Your grandmother knows you are studying geology and shows you a mineral she collected as a child when she was camping in Idaho.
She asks you, "What is this?" How would you go about identifying it?

« Back to TOP


Geology 101 - Introduction to Physical Geology
Lab--Minerals
Created by Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, including figures unless otherwise noted
updated: 9/11/13

Unless otherwise specified, this work by Washington State Colleges is licensed under a Creative Commons Attribution 3.0 United States License.

Creative Commons License