Minerals Lab

Last Modified: 08/20/09

ROCK CYCLE: Where any rock type can become any other rock type

• There are 3 rock types: Igneous Rocks, Sedimentary Rocks, and Metamorphic Rocks
• DIAGENESIS: Cementation & Compaction

DEFINITION of a MINERAL: A substance that is...

• Crystalline in Nature - AND SO HAS AN ORDERED ARRAY OF ATOMS
• Definite Composition or range of compositions
  • quartz (SiO₂) or pyrite (FeS₂)
  • Can have some variance over a restricted range
  • Ex) An amphibole such as hornblende
  •  
• Naturally Occurring

• Inorganic (Never lived; though graphite, diamond, calcite may be bioproducts of organisms)

These 4 criteria cause each type of mineral to have its own unique properties.

Properties of Minerals: Tools for Identification

Click on one of these mineral properties to learn about the property and utility in mineral identification:

• Hardness
• Luster
• Color
• Streak
• Cleavage
• Fracture
• Cleavage vs. Crystal Form
• Miscellaneous Properties

HARDNESS: Resistance to Breakage??? NO!

You've heard that "diamond is the hardest mineral." You've also heard of the "test" for a diamond--scratching glass. Why does diamond scratch glass? If it's so hard, can you break a diamond?

Answers:

• Diamond scratches glass because a diamond is harder (geologically speaking) than glass. We'll explore that more in a moment.
• You CAN break a diamond! Don't be like the recently engaged woman who decided to "test" the diamond in her engagement ring by slamming it into the table (unfortunately it shattered)! Why? Let's find out what hardness really means...

Hardness refers to the ability of a mineral to SCRATCH something else. A mineral can only scratch something else if it has an equal or higher HARDNESS RATING than the object to be scratched.

An arbitrary scale was developed that ranks the relative hardness of minerals. This scale is called Moh's hardness scale (below). Remember, there are several thousands of minerals, so Moh's Hardness Scale only lists several common "index" minerals and their associated hardness values. Please also note that impurities can cause the hardness of a mineral to vary.

Moh's Hardness Scale (Softest = 1, Hardest = 10)

NOTE: If it's really hard to scratch a mineral (but it can be scratched), the 2 minerals are close in hardness value. If something is really easy to scratch, the object doing the scratching must have a much higher hardness value. So quartz (hardness = 7) will scratch anything with a hardness equal to 7 or less (e.g., quartz, calcite, etc.). It will also scratch calcite (a really low (relative) hardness value) MUCH EASIER than it can another sample of quartz. Similarly a mineral cannot scratch something with a higher hardness value (e.g., quartz won't scratch a diamond).

Remember, Moh's hardness scale just places various minerals into an arbitrary scale of relative hardness values. Materials listed in italics in the above scale are INDEX materials. They are listed because they are useful, common objects that can be used to pigeonhole the possible hardness of the mineral in question (i.e., it's harder than my fingernail, but softer than a penny and therefore must have a hardness greater than 2.25 and less than 3.5).

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LUSTER: The appearance of a mineral in light.

METALLIC: looks like a metal (Samples 9, 15); like gold, silver, iron, etc.

• Don't confuse w/ VITREOUS (see below)

NON-METALLIC: (need to be more descriptive)

• VITREOUS or Glassy (Samples 3, 12) - strong glint (shiny like glass)
• PEARLY (Talc, Some Gypsum) - looks like mother-of-pearl
• SILKY - made up of many silk-like "strands"
• RESINOUS - reflects light in a manner similar to tree sap or syrup ("glazed")
• EARTHY - dull, little (or no) reflection (Sample 10)

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COLOR: The actual color of the mineral.

NOTE: This is rarely diagnostic--usually a very poor identifier!! Some examples...

• Sulfur - Is almost always Yellow
• Pyrite - Is almost always Brassy
• Quartz - Can have almost any color known to man!

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STREAK: The color of the powdery residue of a mineral left behind when you drag a mineral across an unglazed porcelain plate (Hardness of plate = 6).

NOTE: Streak is not always the same color as the mineral! However, each mineral has only one streak color associated with it. For example, the metallic mineral Hematite may be red or silver in color, but its streak is always reddish-brown.

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CLEAVAGE: Planar breakage of a mineral.

Why does a mineral break along parallel planes of weakness?

Since minerals are crystalline in nature, there is an ordered atomic arrangement within each mineral. Due to this atomic symmetry (and the variation of bond strengths between different atoms), parallel planes of weak bonds may exist in a mineral. When a mineral is struck (by a hammer for example), the weakest bonds will preferentially break. If a plane of weak bonds exist in a mineral, the mineral will display planar breakage, which we term CLEAVAGE.

An analogy:

If you imagine that 2 sheets of paper are 2 adjacent layers of atoms in a mineral, the weakest bonds would be between the 2 sheets of paper. It is easier to slide the two sheets of paper across one another (break the weakest bonds) than it is to tear across them (breaking stronger bonds).

How do you know you're looking at a cleavage surface and not just an otherwise flat surface?

Since cleavage is due to breakage along parallel planes of weakness, all of those planes are in exactly the same orientation. Therefore, when you hold up the surface towards the light, ALL of those parallel planes will reflect the light at the same time. So if you can orient a side of the mineral so that the entire surface reflects light (a sharp "glint") at the same time, you're looking at a cleavage face. To double check, go to the exact opposite (parallel) side of the mineral sample and see if you can get that side to reflect light at the same orientation--if it does you know the mineral has at least 'one direction of cleavage'.

Okay, but how do I determine how many directions of cleavage there are?

For the purposes of the Geology 303 lab, at most you will only see 3 directions of cleavage. In other words, at most 3 differently oriented sets of parallel planes of weakness. In the example above, you may find a cleavage surface, turn the mineral over and see 'another' cleavage surface parallel to the first one--since they are parallel to each other it still only counts as one cleavage 'direction'. When I look at a mineral sample, I envision it as a box. A box has 6 sides, but in reality 2 of the sides are always parallel to each other (giving you only 3 possible orientations of parallel planes). The top and bottom of the box are parallel (one possible cleavage direction), the left and right sides are parallel (another possible cleavage direction), and the front and back of the box are parallel (the third possible cleavage direction). I then look at the "top" (or bottom) to see if it is a cleavage surface, the left (or right) side to see if it is a cleavage surface, and finally the front (or back) to see if it's a cleavage surface. (You can double check if a surface is a cleavage surface by looking at the parallel (opposite) side). Now you note if you have 1, 2, 3 (or no) directions of cleavage.

How To Describe Cleavage:

If you see no cleavage surfaces...

• Then the mineral only breaks in a non-planar manner which is called FRACTURE (see below)

If you only see 1 set of parallel planes...

• Denoted '1 Direction of Cleavage' (mica was used as an example in class)

If you only see 2 sets of parallel planes...

• You must give the intersection angle of the two planes
• Denoted '2 Directions of Cleavage at 90°' (if they intersect at a right angle)

If you only see 3 sets of parallel planes...

• Denoted '3 Directions of Cleavage at 90°' (termed CUBIC - salt has cubic cleavage) or
• Denoted '3 Directions of Cleavage not at 90°' (termed RHOMBOHEDRAL)

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FRACTURE: Breakage in an uneven manner (non-planar breakage)

• CONCHOIDAL: See Obsidian Sample or Quartz; breaks in a surface marked by concentric circles
• SPLINTERY: Like splinters in wood (Sample 16)
• UNEVEN: A "catch-all" term for non-diagnostic, non-planar breakage

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CRYSTAL FORM and FACES: The symmetry of a mineral - expressed by crystal faces (planar growth surfaces) and by the angles between faces and their relative arrangements.

Simply, as minerals "grow" (add the appropriate atoms into their crystal lattices), they may grow with faces parallel to planes of atoms, and of course, these planes are perfectly regular. The internal atomic symmetry may be expressed as an external crystal symmetry. This symmetry may cause smooth faces to form on the crystal which can be mistaken for cleavage...

How do you distinguish cleavage from crystal faces?

Since cleavage is only evident when the mineral is broken, that is easiest method available to tell the difference. If you break the mineral and it breaks along planes parallel to the suspected cleavage faces, it is indeed cleavage. If it doesn't, the flat, smooth faces were due to crystal form. However, since a lot of nice mineral samples would quickly be reduced to dust in the Geology 303 labs using this technique, we suggest that you refrain from this brute force methodology.

The importance of both crystal faces and form and of cleavage is that they are clues to the internal arrangement of atoms in the mineral. ANGLES between intersecting cleavage planes and between intersecting crystal faces can be powerful tools for identification of a mineral.

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MISCELLANEOUS: Some minerals have very helpful (unique) diagnostic properties.

• Magnetite exhibits magnetism
• Sulfur smells like rotten eggs (after scratching the sulfur)
• Calcite fizzes when HCl acid is applied to it
• Halite tastes salty
• Some minerals are unusually dense, due to heavy elements in them or to close-packed atoms.

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