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Learn about mineral cleavage, fractures, and luster properties with detailed examples of muscovite, halite, galena, calcite, potassium feldspar, and quartz. Explore the importance of cleavage in mineral identification.
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MINERAL PROPERTIES Prepared by Dr. F. Clark, Department of Earth and Atmospheric Sciences, University of Alberta Sept. 05
CLEAVAGE A mineral has cleavage if it has one or more directions or planes of weakness within the crystal structure along which it will break when stressed. Because of the regular, ordered arrangement of atoms that crystals have, this means that the same cleavage will repeat many times within the same crystal. Because a mineral is defined in part by its crystal structure, cleavage is a consistent and reliable property for identification. As well, when a mineral has more than one cleavage, those cleavages will always intersect at the same angles. Breakage of a crystal unrelated to a cleavage is called fracture.
MuscoviteThis sheet silicate has a single perfect cleavage, accounting for its flaky nature. The yellow stars are on parallel cleavage faces that all represent the same cleavage direction; the red arrows point to places where that same cleavage could be developed; the blue arrows point to cleavage flakes that have fallen off the specimen and reflect light.
Muscovite The cleavage flake balanced on the main specimen can itself be cleaved on progressively finer and finer scales (see red arrows). Nevertheless, it is a single cleavage because it represents a single direction in the crystal (see all five surfaces highlighted by yellow stars).
HaliteThis mineral exhibits three cleavages that are mutually perpendicular. This is called cubic cleavage, and produces rectangular specimens. Faces or portions of faces following the three cleavage directions corresponding to the stars on the right-hand specimen are highlighted by arrows; note that cleavages exhibit steps corresponding to one of the other cleavage directions.
GalenaThis metallic, lead-bearing sulphide exhibits cubic cleavage (three that meet at 90 degrees) just as was seen for halite. The discontinuous nature of cleavage is particularly well displayed by these specimens; note the large number of short steps and disruptions to the cleavage faces, such that they are recognized as representing the same cleavage only by virtue of reflecting light the same.
CalciteLike halite and galena, this exhibits three cleavages, but they do not meet at 90 degrees; this is called rhombic cleavage. The three cleavage directions are marked with their own distinctively coloured stars; notice that many faces have faint lines or traces on them, marked by appropriately coloured arrows, where the corresponding cleavage would develop if the specimens were struck.
Potassium FeldsparThis mineral has two cleavages that meet at 90 degrees, giving squared edges to the specimen. Highlighted by red arrows, one cleavage is expressed as several faces that are catching the light at the same time; the green arrows highlight the second cleavage (note that a small ridge is produced on the first cleavage); faces marked by blue stars show irregular fracture.
Potassium Feldspar There are two sets of planar cleavage faces developed on this crystal, one highlighted in green arrows, the other with red arrows; they meet at right angles. The face highlighted by a blue star exhibits irregular fracture; the steps or discontinuities in the cleavage planes (blue arrows) exhibit the same irregular fracture.
QuartzThis is a very common mineral that has no cleavages, but exhibits distinctive conchoidal fracture. Blue arrows point to the characteristically curved ridges that develop with this type of fracture; quartz typically grows as a hexagonal prism, with triangular faces at the end of the prism (see specimen on right); crystal faces should not be confused with cleavage or fracture.
QuartzThese three specimens, consisting of several crystals each, lack any trace of planar surfaces, either crystal faces or cleavages. Although quartz ideally and characteristically grows as long, six-sided prismatic crystals, it commonly occurs as irregular aggregates that fill fractures in rocks to produce bright white vein fillings that show up against the darker background of the host rock.
LUSTRE Lustre is a qualitative description of the appearance of reflected light from the specimen. It can be somewhat subjective, and may also depend on the quality of the specimen and the crystal size. The fundamental subdivision is into those minerals whose lustre is metallic, having the appearance of polished metal, and those that are non-metallic, which of course do not. There is a host of different lustres within the non-metallic category.
Metallic Lustre In the left photo, all three specimens are of the mineral pyrite, an iron sulphide commonly known as Fool’s Gold. In the right photo, the left specimen is pyrrhotite, also an iron sulphide, the center specimen includes bornite, a copper-plus-iron sulphide, and the right specimen is pyrite.
Galena + DolomiteThe contrast between metallic and non-metallic lustre is exhibited in this specimen from Pine Point. From this lead-zinc deposit near Great Slave Lake, metallic galena, highlighted by red arrows, has been deposited in cavities within the brown, non-metallic dolomite host rock. The two left arrows point to the same large mass of galena; the right arrows to smaller masses.
Halite + GalenaBoth minerals exhibit cubic cleavage, as highlighted by the three sets of arrows. Placing non-metallic halite (left) next to metallic galena (right) simply highlights the fact that minerals may share one or more properties with others. It is the combination of at least a few properties that allows us to uniquely identify minerals.
COLOUR Colour is an obvious property, but one which is often unreliable. Because of the variation in chemistry that is possible for many minerals, plus the inclusion of trace elements in the crystal structure which have a characteristic interaction with light and cause distinctive colours, for many minerals the colour is highly variable. When the mineral is reduced to powdered form, which is done by dragging the specimen across an unglazed porcelain streak plate, this variability disappears, and we get a consistent result which is reliable. We note that certain minerals do have a consistent and characteristic colour.
Quartz or SilicaAll specimens are composed of silica (SiO2), either as quartz, or as other materials with a different crystal form. For many minerals, colour is not a consistent and reliable feature, and other properties must be used to identify them. Observe the several different colours the same mineral may have, usually controlled by trace elements in the crystal structure.
Colour Variations in Silica On the left, we see various examples of Tiger’s Eye, an attractive stone produced by the formation of silica mimicking the structure of a fibrous, so-called asbestos mineral. On the right, the distinctive purple variety of quartz called amethyst is seen; the colour results from the presence of iron in a particular state.
HematiteThis iron oxide mineral exhibits a number of different colours, and even degrees of metallic lustre. The upper right specimen is distinctly red-brown, and rather dull; the upper left specimen a somewhat dull gun-metal grey colour, and the lower specimen is quite shiny and also a gun-metal colour. What property unites these specimens?
HematiteA variable mineral is consistent in one regard. Note that the streak of all three specimens is red-brown, as seen on the unglazed porcelain streak plates next to each one. Powdering the specimen on such a plate (the hardness must be less than 7 for this to work) eliminates the variation exhibited by the larger crystal sizes.
SPECIFIC GRAVITY Specific gravity is the ratio of the density of a material to the density of water at a specified temperature, and therefore has no units or dimension. It can be determined accurately using various techniques, and resolution between minerals is possible to very small differences. In practice, in hand specimen it is not possible to resolve most minerals, whose specific gravity lies in the range of 2.65 to 2.85 or so, but a few minerals are unusually “hefty” (have a very high specific gravity), and this will be readily detected in specimens which are unusually heavy for their size.
Specific Gravity, (or “heft”, if you will) On the left, its three cleavages highlighted by arrows of different colours, is galena, PbS, the ore mineral for lead. On the right is the mineral barite, BaSO4, which is ground up as an additive to drilling fluid or mud, to give the mud sufficient weight to control high pressure formation fluids when drilling exploration wells.
CRYSTAL HABIT Crystal habit is the shape that crystals of a particular mineral will exhibit when grown under ideal or favourable conditions. Normally, simultaneous growth of other crystals of the same or a different mineral will lead to mutual interference and the development of what are called compromise boundaries. As the term “habit” suggests, this is not an invariant property, and one notes that some minerals may exhibit different habits depending on the conditions and circumstances of growth.
Crystal Habit, Quartz.This broken crystal demonstrates part of the typical habit of this mineral. Under ideal conditions, a quartz crystal grows as a six-sided or hexagonal prism, and terminates with a set of faces, seen here, that form a hexagonal pyramid.
Crystal Habit, Calcite The sample on the left exhibits two habits for calcite, one as large, almost clear crystals, the other as very fine, virtually opaque crystals comprising the spaghetti-like masses. On the right, we see a specimen which exhibits another habit for calcite, as scalenohedral crystals (see reflective crystal in center of image in particular).
OTHER PROPERTIES There are other properties that help us identify minerals, a few of which are illustrated and/or discussed on the following slides. Opacity refers to the degree to which the specimen will transmit light. If a mineral is transparent, light readily passes through it; if it is translucent, one gets a vague impression or shadow of an object on the other side of the specimen. If the mineral is opaque, no light passes through it. A few factors control our perception of the opacity of a mineral as revealed by a particular specimen.
CalciteThese three calcite rhombs show distinct differences in opacity. The upper specimen is strongly clouded with all sorts of impurities and inclusions that have been incorporated in the crystal as it grew; the lower left specimen is more typical, and the lower right specimen is of an exceptional clarity and is a variety called Iceland spar.
Double Refraction in Calcite.The exceptional clarity of this specimen of Iceland spar demonstrates the fact that for many minerals, the passage of light through the specimen is influenced by the crystal structure. Specifically, the speed of light varies according to its direction within the structure. The net result in this case is that the single dot in the left side image is refracted to produce two apparent dots as the light passes through the crystal. If we rotated the crystal in the second case, the two dots would revolve around each other. The right side image simply shows the effect with a more complex source.
Opacity and Lustre: Influence of Crystal Size Both images show the mineral gypsum. On the left, the two specimens are large, single crystals that are transparent [opacity] and vitreous [lustre]. On the right, the specimens are composed of thousands of individual crystals each a small fraction of a millimetre; crystal boundaries take over to make the gypsum appear opaque and dull.
Carbonate Minerals: Effervescence The minerals calcite [CaCO3, on the left] and dolomite [CaMg(CO3)2, on the right] react with dilute hydrochloric acid. With calcite, the reaction is immediate and brisk, whereas dolomite needs to be powdered to get a sluggish reaction. Both exhibit rhombic cleavage (highlighted by arrows for calcite). The colour difference exhibited by these sets of specimens is not a consistent or reliable distinguishing feature; the hardness for dolomite is slightly higher.
