What are the properties of minerals that we use to identify them? (There are 5)

EENS 2110	Mineralogy
Tulane University	Prof. Stephen A. Nelson
Physical Backdrop of Minerals

Although we have discussed x-ray identification of minerals and later in the course will talk over techniques that tin exist used to identify minerals with the optical microscope, information technology is however necessary to develop techniques that can be used in the laboratory and field where instrumentation like 10-ray diffractometers or microscopes cannot be easily used.  Minerals have distinguishing physical properties that in most cases can exist used to make up one's mind the identity of the mineral.  In this course, you lot will develop a systematic approach to using the concrete properties of minerals as identifying tools.  If yous follow this approach you should be able to place most of the common minerals, or at the least exist able to narrow the possibilities to but a few.  Nosotros will first hash out each of the physical properties that tin be used, then develop a methodical arroyo to the identification of minerals using these physical properties.  Among the properties we will discuss are: crystal habit, cleavage, hardness, density, luster, streak, colour, tenacity, magnetism, and taste.

Crystal Habit

In nature perfect crystals are rare.  The faces that develop on a crystal depend on the infinite available for the crystals to grow.  If crystals abound into one some other or in a restricted environment, it is possible that no well-formed crystal faces volition be developed.  However, crystals sometimes develop certain forms more than commonly than others, although the symmetry may non be readily apparent from these common forms.  The term used to draw full general shape of a crystal is habit.

Some mutual crystal habits are as follows (discussed previously):

Individual Crystals

• Octahedral - shaped like octahedrons, every bit described above.

• Tabular -  rectangular shapes.

• Equant - a term used to describe minerals that have all of their boundaries of approximately equal length.

• Acicular -  long, slender crystals.

• Prismatic -  abundance of prism faces.

• Bladed -  like a wedge or knife bract.

Groups of Distinct Crystals

• Dendritic - tree-similar growths.

• Reticulated - lattice-similar groups of slender crystals.

• Radiated - radiating groups of crystals.
  
  

• Fibrous -  elongated clusters of fibers.

• Botryoidal - smoothen bulbous or globular shapes.

• Globular - radiating individual crystals that form spherical groups.

• Drusy - minor crystals that cover a surface.

• Stellated - radiating individuals that grade a star-like shape.

Some minerals characteristically bear witness one or more of these habits, so habit can sometimes be a powerful diagnostic tool.

Cleavage, Departing, and Fracture

Cleavage

Crystals often contain planes of atoms along which the bonding between the atoms is weaker than along other planes. In such a case, if the mineral is struck with a hard object, information technology will tend to break forth these planes. This property of breaking along specific planes is termed cleavage. Considering cleavage occurs along planes in the crystal lattice, it can be described in the aforementioned manner that crystal forms are described. For example if a mineral has cleavage along {100} it will break easily forth planes parallel to the (100) crystal confront, and any other planes that are related to it by symmetry. Thus, if the mineral belongs to the tetragonal crystal system it should also cleave forth faces parallel to (010), considering (100) and (010) are symmetrically related past the 4-fold rotation axis. The mineral volition be said to have ii directions of cleavage. [Note that in the tetragonal organisation, the form {100} has four faces: (100), (00), (010), and (00). Simply if we are referring to cleavage directions, the mineral only has two, because the cleavage planes (00) and (00) are parallel to, and thus in the same direction as (010) and (100).]

The cleavage can also be described in terms of its quality, i.e., if information technology cleaves along perfect planes it is said to be perfect, and if it cleaves forth poorly divers planes it is said to be poor.

Note: Delight exercise not attempt to cleave the minerals in the laboratory. Many of the specimens you examine cannot exist readily replaced. Cleavage is usually induced in the mineral when it is extracted from the stone when information technology is found, and can usually be seen equally planes running through the mineral. Therefore, you do not have to interruption the mineral in social club to run across its cleavage.

Cleavage tin can as well be described by general forms names, for example if the mineral breaks into rectangular shaped pieces it is said to have cubic cleavage (3 cleavage directions), if it breaks into prismatic shapes, it is said to have prismatic cleavage (2 cleavage dir ections) , or if it breaks along basal pinacoids( one cleavage management) it is said to accept pinacoidal cleavage.  For examples, run into figure two.12 on folio 29 of your text.

Parting

Parting is also a plane of weakness in the crystal structure, but information technology is forth planes that are weakened by some applied strength.  It therefore may not exist apparent in all specimens of the same mineral, merely may announced if the mineral has been subjected to the right stress weather.

Fracture

If the mineral contains no planes of weakness, information technology will break along random directions called fracture.  Several dissimilar kinds of fracture patterns are observed.

• Conchoidal fracture - breaks along smooth curved surfaces.
  
• Fibrous and splintery -  similar to the style forest breaks.
  
• Hackly - jagged fractures with abrupt edges.
  
• Uneven or Irregular - rough irregular surfaces.

Hardness

Hardness is determined by scratching the mineral with a mineral or substance of known hardness. Hardness is a relative scale, thus to determine a mineral's hardness, you must decide that a substance with a hardness greater than the mineral does indeed scratch the unknown mineral, and that the unknown mineral scratches a known mineral of bottom hardness.

Hardness is determined on the ground of Moh's relative scale of hardness exhibited by some common minerals. These minerals are  listed below, along with the hardness of some common objects.

Hardness	Mineral	Common Objects
i	Talc
2	Gypsum	Fingernail (2+)
3	Calcite	Copper Penny (3+)
iv	Fluorite
5	Apatite	Steel knife blade (5+), Window glass (5.5)
six	Orthoclase	Steel  file
seven	Quartz
8	Topaz
9	Corundum
10	Diamond

Several precautions are necessary for performing the hardness examination.

• If you attempt to scratch a soft mineral on the surface of a harder mineral some of the softer substance may leave a marker of fine powder on the harder mineral.  This should not exist mistaken for a scratch on the harder mineral.  A powder will easily rub off, but a scratch volition occur as a permanent indentation on the scratched mineral.

• Some minerals have surfaces that are contradistinct to a dissimilar substance that may exist softer than the original mineral.  A scratch in this softer alteration product will not reverberate the true hardness of the mineral.  Always use a fresh surface to perform the hardness test.

• Sometimes the habit of the mineral will make a difference.  For example aggregates of minerals may break apart leaving the impression that the mineral is soft. Or, minerals that show fibrous or splintery habit may break hands into fibers or splinters.  It is therefore wise to always perform the hardness exam in opposite.  If one mineral appears to scratch another mineral, make sure that the other mineral does not scratch the apparently harder mineral before you declare which of the minerals is harder.

• In some minerals hardness is very dependant on management, since hardness is a vectorial property.  When in that location is meaning difference in hardness in different directions, information technology tin be a very diagnostic property of the mineral.  It is thus wise to perform the hardness examination past attempting to scratch the mineral in different directions.  Two minerals of note have differences in hardness depending on direction:

  • Kyanite has a hardness of five parallel to the length of the crystal, and a hardness of 7 when scratched forth a direction perpendicular to the length.

  • Calcite has a hardness of iii for all surfaces except the {0001} plane.  On {0001} it has a hardness of ii.

Tenacity

Tenacity is the resistance of a mineral to breaking, burdensome, or bending.  Tenacity tin exist described past the following terms.

• Brittle - Breaks or powders easily.

• Malleable - can exist hammered into sparse sheets.

• Sectile - tin can be cut into thin shavings with a knife.

• Ductile - bends easily and does not return to its original shape.

• Flexible - bends somewhat and does not return to its original shape.

• Elastic - bends simply does return to its original shape.

Density (Specific Gravity)

Density refers to the mass per unit volume.  Specific Gravity is the relative density, (weight of substance divided past the weight of an equal volume of water).  In cgs units density is grams per cmⁱⁱⁱ, and since water has a density of 1 grand/cm³, specific gravity would take the same numerical value has density, but no units (units would cancel).   Specific gravity is oftentimes a very diagnostic property for those minerals that have high specific gravities.  In general, if a mineral has college atomic number cations information technology has a college specific gravity.  For case, in the carbonate minerals the following is observed:

Mineral	Composition	Diminutive # of Cation	Specific Gravity
Aragonite	CaCO3	40.08	2.94
Strontianite	SrCO3	87.82	three.78
Witherite	BaCO3	137.34	4.31
Cerussite	PbCO3	207.xix	6.58

Specific gravity tin usually be qualitatively measured by the heft of a mineral, in other words those with high specific gravities usually feel heavier.  Virtually common silicate minerals take a specific gravity between near ii.v and 3.0.  These would feel light compared to minerals with high specific gravities.

For comparison, examine the following tabular array:

Mineral	Composition	Specific Gravity
Graphite	C	ii.23
Quartz	SiO2	2.65
Feldspars	(K,Na)AlSi3Oviii	ii.6 - 2.75
Fluorite	CaF2	iii.18
Topaz	Al2SiOfour(F,OH)ii	3.53
Corundum	Al2Othree	four.02
Barite	BaSOfour	4.45
Pyrite	FeSii	5.02
Galena	PbS	vii.5
Cinnabar	HgS	8.1
Copper	Cu	8.ix
Silver	Ag	10.5

Color

Colour is sometimes an extremely diagnostic holding of a mineral, for example olivine and epidote are almost always green in color.  But, for some minerals information technology is non at all diagnostic because minerals tin accept on a variety of colors.  These minerals are s assist to be allochromatic. For case quartz can be articulate, white, blackness, pink, blue, or imperial.     Read in your textbook, pp. 234-241 , about what causes minerals to take colour.

Streak

Streak is the color produced by a fine powder of the mineral when scratched on a streak plate. Often it is different than the color of the mineral in not- powdered form

Luster

Luster refers to the general appearance of a mineral surface to reflected low-cal. Two full general types of luster are designated as follows:

1. Yard etallic - looks shiny like a metal.  Unremarkably opaque and gives black or dark colored streak.
   

2. Not-metal - Non metallic lusters are referred to as

   1. vitreous - looks glassy - examples: clear quartz, tourmaline

   2. resinous - looks resinous - examples: sphalerite, sulfur.

   3. pearly - iridescent pearl-like - example: apophyllite.

   4. greasy - appears to be covered with a sparse layer of oil - case:  nepheline.

   5. silky - looks gristly. - examples - some gypsum, serpentine, malachite.

   6. a damantine - brilliant luster similar diamond.

Play of Colors

Interference of calorie-free reflected from the surface or from within a mineral may crusade the color of the mineral to change as the bending of incident light changes.  This sometimes gives the mineral an iridescent quality.  Minerals that prove this include: bornite (Cu₅FeS₄) , hematite (Fe₂O_three), sphalerite (ZnS), and some specimens of labradorite (plagioclase).

Fluorescence and Phosphorescence

Minerals that lite up when exposed to ultraviolet calorie-free, x-rays, or cathode rays are called fluorescent.  If the emission of light continues after the light is cut off, they are said to be phosphorescent.

Some specimens of the aforementioned mineral evidence fluorescence while other don't. For example some crystals of  fluorite (CaF₂) show fluorescence and others practice non.  Other minerals bear witness fluorescence frequently, merely not e'er.  These include - scheelite (CaWO_four), willemite (Zn₂SiO_four), calcite (CaCO_iii), scapolite (3NaAlSi₃O8^.(NaCl - CaCO₃), and diamond (C).

Magnetism

Magnetic minerals result from properties that are specific to a number of elements. Minerals that practise non accept these elements, and thus accept no magnetism are called diamagnetic .  Examples of diamagnetic minerals are quartz, plagioclase, calcite, and apatite.  Elements like Ti, Cr, Five, Mn, Fe, Co, Ni, and Cu tin can sometimes consequence in magnetism.  Minerals that contain these elements may exist weakly magnetic and can be separated from each other by their various degrees of magnetic susceptibility. These are called paramagnetic minerals.  Paramagnetic minerals only evidence magnetic properties when subjected to an external magnetic field.  When the magnetic field is removed, the minerals have no magnetism.

Ferromagnetic minerals have permanent magnetism if the temperature is below the Curie Temperature .  These materials will become magnetized when placed in a magnetic field, and will remain magnetic after the external field is removed.  Examples of such minerals are magnetite, hematite-ilmenite solid solutions (Fe_iiO_iii - FeTiO_three), and pyrrhotite (Fe_1-xS).

Other Backdrop

Other backdrop that may be diagnostic include  chatoyancy, asterism, piezoelectricity,  and sense of taste. Familiarize yourself with the meanings of these terms. And spotter for these backdrop as you examine minerals.

Tables for Identification of Minerals Starting time on page 604 of the Text by Klein and Dutrow are determinative tables which should assistance you in using concrete backdrop of minerals to identify them.  Note that the tables are broken first into two dissimilar groups based on Luster.  Inside each group, the minerals are then further divided on the basis of streak, hardness, and cleavage.  In the remarks column are listed other useful diagnostic holding for each mineral.  Again, I encourage yous to develop a systematic arroyo to identifying minerals. Luster - Metallic or Submetallic Hardness < ii� Hardness > ii�. <five�. Hardness > 5�. Luster - Nonmetallic Streak Colored Streak Colorless Hardness < two� Hardness >two�, < 3 Cleavage prominent. Cleavage non prominent. Hardness >three, <v�. Cleavage prominent. Cleavage not prominent. Hardness >five�, <7 Cleavage prominent. Cleavage not prominent. Hardness >7 Cleavage prominent Cleavage not prominent.

Questions from this textile that could be asked on an exam. The use of physical properties to identify minerals will be necessary for the second lab exam, so you lot should become very familiar with using physical properties and the mineral identification charts to identify hand specimens of minerals.  Other example questions that could appear on the lecture midterm are as follows: Ascertain the following: (a) cleavage, (b) departing, (c) fracture, (d) hardness, (e) luster, (f) streak, (yard) density. How are density and specific gravity related? What factors control the density of a mineral? What is the departure betwixt paramagnetic, diamagnetic, and ferromagnitic minerals?

parsonsexpont.blogspot.com

Source: https://www.tulane.edu/~sanelson/eens211/physprop.htm

Share this post