Testing Minerals

Image credit: Scott Brande

The Bottom Line on Testing Minerals - Here on Top!

  • Each of the 5,000+ named minerals identified on planet earth is a unique chemical compound that exhibits a specified set of chemical and physical properties that have been determined by analytical experiments.
  • In an introductory geology course, in the laboratory, you would handle unknown mineral samples, perform tests, make observations, and compare your results to an identification chart in your textbook or other resource.
  • In this online environment, you "perform" a more limited (but not that much more limited) suite of tests by observing images and short videos of the actual experiments through your web browser.
  • The online media provide the visual information you need to observe and interpret a property (for example, reaction to acid), and record results needed for identification.
  • Tests included here are designed to be quick - for example, in a 10 second video, you will observe that a mineral sample either reacts strongly, or not, when exposed to a drop of cold, dilute, hydrochloric acid.
  • Multiple examples enable you to form concepts needed for the recognition of properties - for example, a mineral softer than a streak plate will leave a crushed powder needed for assessment of its color.
  • Links and references are provided for further details.

Bottom Line: Learning to identify minerals requires instruction in observing, recording data, interpreting data, consulting reference tables, and the occasional experiment which can also be a fun activity (see electrical conduction as a replacement for luster to separate metallic from non-metallic minerals).

Errors/Suggestions: Contact Scott Brande (see footer).

Properties Chart for Observing Minerals in Online Media

Mineral Properties. Click image to enlarge in new browser tab. Close tab to return.

Instructions for Observing Mineral Properties

The following is not a comprehensive list of detailed mineral properties that may be found in your textbook or other resources. Properties listed below are those used in mineral identification that may be recognized when well-developed in mineral samples, and in the online media included in this web site.

Some properties if well-developed may be observed in images or video snips. These include

  • Luster: traditional test to distinguish non-metallic from metallic minerals, from observing the appearance of light reflecting off the surface of a mineral
    • Electrical conductivity: a new replacement test for luster - identify a metallic mineral (except hematite) with a flashlight
  • Hardness: from observing results of scratching a mineral against a glass plate
  • Streak color: from observing results of scratching a mineral against a streak plate
  • Magnetism: from observing results of moving a sample close to a paper clip
  • Chemical reaction to 10% HCl: from observing results of dropping cold dilute HCl onto a sample
  • Cleavage: from observing number and angles between planar surfaces of a broken fragment
  • Fracture: from observing the surface of a broken fragment
  • Distinctive crystal shape: from observing and recognizing the geometry of crystal shape

Sample Quality Determines Observable Properties

"Museum" Quality vs "Student" Samples

  • You will probably NOT be able to determine some (or many) properties on student samples of minerals because of various factors (e.g., the budget available for purchasing quality mineral samples).
  • Here are some examples that show how student samples may limit the properties you can observe and determine.

Single crystal, well-developed flat sides that represent crystal growth form. (Didier Descouens, Soufresicile2, CC BY-SA 4.0 )

What can, and cannot, be observed

  • note light transmission through crystal proves this sample is non-metallic
  • attached form and flat faces suggests a single crystal of growth
  • flat faces of single crystal are not cleavage planes (as cleavage planes result from breakage)
  • angle between flat faces may be estimated and used to evaluate crystal form (geometry)

Crystalline aggregate of multiple crystals, intergrown in a mass. Can't determine form of single crystal. (Daniel Schwen (https://commons.wikimedia.org/wiki/File:Min_sulfur.jpg ), „Min sulfur“, https://creativecommons.org/licenses/by-sa/2.5/legalcode )

What can, and cannot, be observed

  • note straight lines seen inside sample - these are aligned on a cleavage plane (would break along the line)
  • because lines are seen inside this sample, light is transmitted into the crystal and proves this sample is non-metallic
  • mass is irregular in shape, suggests multiple, intergrown crystals
  • surfaces are not planar, cannot be used to evaluate crystal form or cleavage angles/directions

Crystalline aggregate. Rock composed of microscopic crystals. Paperclip for scale. "Student" grade sample. Can't determine form of single crystal or cleavage.

Image credit: Scott Brande

What can, and cannot, be observed

  • mass is irregular in shape, suggests multiple, intergrown crystals
  • surfaces are not planar, cannot be used to evaluate crystal form or cleavage angles/directions

Sample Size

Mineral Property and Size

The identification of a mineral by name requires observation and determination of distinctive properties, such as luster, hardness and streak. These properties are inherent in all the material throughout a sample, so that a rice-sized crystal of calcite, for example, has the sample physical and chemical characteristics as a 100 kg boulder of calcite (as long as the boulder is in fact composed of 100% calcite).

"Bulk" vs "Magnified" Samples

Numerous properties of minerals may be observed in online media just as well, or better, than when handling a physical sample. In online media, features that may require magnification might be better viewed when magnified, such as cleavage and fracture. The size of a sample shown in an image or video may be assessed by the inclusion of one or more of three different objects along with the mineral sample.

  • a metric scale (in centimeters and millimeters)
  • a paperclip, the width of which is about 5 mm
  • a black background grid of numbered squares, the width of each is about 4 mm (this printed slide is called a "micropaleo" slide, developed for mounting and observation of microscopic fossils, such as diatoms and foraminifera).

Here are media examples that illustrate samples with a paperclip for the judgment of size.

Bulk sample. For scale, note paperclip, width about 5 mm.

Image credit: Scott Brande

Fragments, with paperclip for scale. Width about 5 mm.

Image credit: Scott Brande

Magnified fragments on micropaleo slide. Each numbered square about 4 mm wide.

Image credit: Scott Brande