Learning Geology: How Can You Tell One Mineral From Another?

How Can You Tell One Mineral From Another?

Amateur and professional mineralogists get a kick out of recognizing minerals. They might hover around a display case in a museum and name specimens without bothering to look at the labels. How do they do it? The trick lies in learning to recognize the basic physical properties (visual and material characteristics) that distinguish one mineral from another. Some physical properties, such as shape and colour, can be seen from a distance. Others, such as hardness and magnetization, can be determined only by handling the specimen or by performing an identiﬁcation test on it. Identiﬁcation tests include scratching the mineral by another object, placing it near a magnet, weighing it, tasting it, or placing a drop of acid on it. Let’s examine some of the physical properties most commonly used in basic mineral identiﬁcation.

Physical characteristics of minerals.

Colour: Colour results from the way a mineral interacts with light. Sunlight contains the whole spectrum of colours; each colour has a different wavelength. A mineral absorbs certain wavelengths, so the colour you see when looking at a specimen represents the wavelengths the mineral does not absorb. Certain minerals always have the same colour, but many display a range of colours (figure above a). Colour variations in a mineral are due to the presence of impurities. For example, trace amounts of iron may give quartz a reddish colour.
Streak: The streak of a mineral refers to the colour of a powder produced by pulverizing the mineral. You can obtain a streak by scraping the mineral against an unglazed ceramic plate (figure above b). The colour of a mineral powder tends to be less variable than the colour of a whole crystal, and thus provides a fairly reliable clue to a mineral’s identity. Calcite, for example, always yields a white streak even though pieces of calcite may be white, pink, or clear.
Luster: Luster refers to the way a mineral surface scatters light. Geoscientists describe luster by comparing the appearance of the mineral with the appearance of a familiar substance. For example, minerals that look like metal have a metallic luster, whereas those that do not have a nonmetallic luster the adjectives are self-explanatory (figure above c, d). Terms used for types of nonmetallic luster include silky, glassy, satiny, resinous, pearly, or earthy.
Hardness: Hardness is a measure of the relative ability of a mineral to resist scratching, and it therefore represents the resistance of bonds in the crystal structure to being broken. The atoms or ions in crystals of a hard mineral are more strongly bonded than those in a soft mineral. Hard minerals can scratch soft minerals, but soft minerals cannot scratch hard ones. Diamond, the hardest mineral known, can scratch most anything, which is why it is used to cut glass. In the early 1800s, a mineralogist named Friedrich Mohs listed some minerals in sequence of relative hardness; a mineral with a hardness of 5 can scratch all minerals with a hardness of 5 or less. This list, the Mohs hardness scale, helps in mineral identification. To make the scale easy to use, common items such as your fingernail, a penny, or a glass plate have been added (Table below).
Speciﬁc gravity: Speciﬁc gravity represents the density of a mineral, as represented by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water at 4°C. For example, one cubic centimetre of quartz has a weight of 2.65 grams, whereas one cubic centimetre of water has a weight of 1.00 gram. Thus, the specific gravity of quartz is 2.65. In practice, you can develop a “feel” for specific gravity by hefting minerals in your hands. A piece of galena (lead ore) feels heavier than a similar-sized piece of quartz.
Crystal habit: The crystal habit of a mineral refers to the shape of a single crystal with well-formed crystal faces, or to the character of an aggregate of many well-formed crystals that grew together as a group (figure above e). The habit depends on the internal arrangement of atoms in the crystal. A description of habit generally includes adjectives that highlight the shape of the crystal. For example, crystals that are roughly the same length in all directions are called equant or blocky, those that are much longer in one dimension than in others are columnar or needle-like, those shaped like sheets of paper are platy, and those shaped like knives are bladed.
Special properties: Some minerals have distinctive properties that readily distinguish them from other minerals. For example, calcite (CaCO3) reacts with dilute hydrochloric acid (HCl) to produce carbon dioxide (CO2) gas (figure above f). Dolomite (CaMg[CO3]2) also reacts with acid, but not as strongly. Graphite makes a gray mark on paper, magnetite attracts a magnet (figure above g), halite tastes salty, and plagioclase has striations (thin parallel corrugations or stripes) on its surface.
Fracture and cleavage: Different minerals fracture (break) in different ways, depending on the internal arrangement of atoms. If a mineral breaks to form distinct planar surfaces that have a specific orientation in relation to the crystal structure, then we say that the mineral has cleavage and we refer to each surface as a cleavage plane. Cleavage forms in directions where the bonds holding atoms together in the crystal are the weakest (figure below a–e). Some minerals have one direction of cleavage. For example, mica has very weak bonds in one direction but strong bonds in the other two directions. Thus, it easily splits into parallel sheets; the surface of each sheet is a cleavage plane. Other minerals have two or three directions of cleavage that intersect at a specific angle. For example, halite has three sets of cleavage planes that intersect at right angles, so halite crystals break into little cubes. Materials that have no cleavage at all (because bonding is equally strong in all directions) break either by forming irregular fractures or by forming conchoidal fractures (figure below f). Conchoidal fractures are smoothly curving, clamshell-shaped surfaces; they typically form in glass. Cleavage planes are sometimes hard to distinguish from crystal faces (figure below g).