Mineral diagnostic properties - Civil Engineering

notes on mineral identification (Charly Bank, UofT dept of geology) 1
Mineral diagnostic properties
hardness: determined by how easy you can scratch a mineral (Mohs scale, Fig 1).
Try the following three approaches:
• scratch one mineral with another using sharp corners and smooth surfaces, and
switch them (minerals with nealy same hardness scratch one another)
• hold a needle close to its tip and run across the mineral,
• hold a glass plate flat to the table and draw the mineral across it (never scratch
the glass plate over the mineral, the glass may splinter in your hand),
Check to see if you have made a scratch (rub off dust and use handlens).
Try to judge how hard you pressed to produce a scratch, or how deep a groove you have
produced using uniform pressure.
Be aware that specimen may contain a mixture of minerals, or that minerals may have
become altered and softer (altered minerals are less glossy and fresh looking).
crystal shape: may reflect crystal form (e.g., hexagonal quartz prisms, cubic galena) or
habit (e.g., platy, equant, columnar). Some minerals often form aggregates (e.g., massive
pyrite).
striations: These are grooves that developed during growth of the crystal. They may be
a result of twinning (e.g., plagioclase) or a fast spatial change in crystal faces (e.g.,
pyrite).
To distinguish between plagioclase and alkali feldspar look for striations or perthitic
structure on a cleavage plane (Fig 2).
Fig. 1: Mohs hardness scale
Fig. 2: striations and perthitic structure in feldspars

notes on mineral identification (Charly Bank, UofT dept of geology) 2
cleavage/fracture: some minerals split along smooth planes (cleavage), while others break
along irregular surfaces (fracture). Cleavage is described by a number (1, 2, 3, 4, 6),
an angle (larger - equal - smaller 90 ◦ ), and a quality (excellent, good, poor), see Fig 3.
Cleavage is a very useful property (e.g., cubic cleavage in halite, rhombic cleavage in
calcite). Cleavage is an easy way to distinguish between pyroxenes and amphiboles (2
cleavages at 90 ◦ or 120 ◦ ). Fracture ranges in type and quality; two distinctive types are
conchoidal (shell-shaped, like quartz) and hackly (like torn metal). To observe cleavage,
turn the mineral until you see a flash of reflected light from the specimen (Fig 3).
Fig 3: observation and description of cleavage
colour/streak: Note that colour is diagnostic only for very few minerals. Streak is the colour
of a mineral in powdered form, it tends to be more uniform (e.g., black streak of galena).
lustre: defines how light is reflected and refracted by a mineral. Some terms to describe
lustre (and examples) are:
• metallic: bright (galena), dull (magnetite)
• non-metallic: brilliant (diamond), glassy (quartz), greasy (massive serpentine),
pearly (talc), silky (asbestos), dull (kaolinite)
density: Opaque metallic minerals (like pyrite or galena) tend to be denser than non-metallic
ones (like quartz or feldspar). Departures from this general rule may be diagnostic (e.g.,
barite).
tenacity: or how minerals break. Many minerals are brittle, that is they will break into
powder (quartz, feldspar, halite). Elastic minerals will bend and spring back (e.g.,
mica), while malleable can be hammered flat (e.g., copper).
unusual properties: Halite (rock salt) tastes salty. Carbonate minerals will evernesc (fizz
when acid is dropped on). Always handle acid carefully, and blot off with paper towel!
A few minerals are magnetic (e.g., pyrrhotite vs. pyrite, magnetite vs. chromite).

notes on mineral identification (Charly Bank, UofT dept of geology) 3
mineral H cl(eavage)/fr(acture) other properties uses
talc Mg, (OH) sheet sil. 1 1 excellent cl pearly lustre; white, pale green, tan; soapy feel talcum powder
graphite C 1 1 excellent cl metallic lustre; silvery gray to black, dark gray streak (writes lubricant, pencil lead
on paper); very soft, greasy feel
gypsum Ca[SO4] 2 1 good, 2 weak cl glassy to pearly crystals, pale colours plaster, dryboard
galena P bS 2.5 3 perfect cl (cubes) bright silver to dull gray, black streak; very heavy! lead ore
muscovite mica
K, Al sheet silicate
2.5 1 excellent cl colourless, pearly to glassy lustre; sheets elastic and flexible for electrical insulation
biotite mica
K, Mg, F e, Al sheet sil.
2.5 1 excellent cl black to dark amber, pearly to glassy lustre; sheets elastic
and flexible
for electrical insulation
halite Na[Cl] 2.5 3 good cl (cubes) pale colours, glassy to greasy lustre, salty taste table salt, de-icing
calcite Ca[CO3] 3 3 exc. cl (rhombs) glassy, pearly lustre, variable colours; reacts with acid calcium supplement
barite Ba[SO4] 3-3.5 1 perfect cl, 2 good colorless to white; short tabular crystals; very heavy rubber, paint, glass
chalcopyrite CuF e[S]2 3.5-4 2 poor cl metallic lustre; golden or brassy yellow color, tarnishes
brown or iridiscent
copper ore
sphalerite ZnS 3.5-4 6 exc. cl (dodec.) sub- to non-metallic lustre; usually yellow-brown to black zinc ore
fluorite Ca[F ]2 4 4 exc. cl (octahed.) transparent; colourless, purple, green, yellow; often cubes source of fluorine
apatite Ca5F [P O4]3 5 conchoidal fr pale to dark green, blue, white; hexagonal prisms fertilizer
hematite F e2[O]3 5-6 uneven fr metallic lustre; silvery gray, black, or brick red, red-brown pigment, iron ore
streak; often massive
magnetite F eF e2[O]4 5.5 uneven fr bright-dull metallic; black streak; magnetic; octahedrons iron ore
pyrite F eS2 5.5-6 conchoidal to uneven
fr
brassy yellow, may tarnish iridescent; greenish-brownishblack
streak; commonly found as cubes or pyritohedrons
K-feldspar K[AlSi3O8] 6 2 exc. cl at 89 ◦ glassy to pearly (alters to dull and softer); variable colour
(often pink); may show perthitic structure
plagioclase feldspar
(Na, Ca)[AlSi3O8]
amphibole group
double-chain sil.
pyroxene group
single-chain sil.
6 2 exc. cl at 87 ◦ glassy to pearly (alters to dull and softer); variable colour,
pale (Na-rich) to dark (Ca-rich); (twinning) striations on
one cleavage face; may show iridescence
6 2 good cl at 124 ◦ glassy (alters to dull and softer); variable colour: white to
dark greenish-black; hornblende has gray/green streak
6 2 fair cl at 87 ◦ glassy (alters to dull and softer); variable colour: pale to
green-black; augite: gray/green streak
ore mineral
glass, ceramics; very
common in rocks
glass, ceramics; very
common in rocks
rock-forming min
rock-forming min
garnet group inosilicate 7 conchoidal fr glassy; many colours (often red); dodecahedral crystals sandpaper, gemstone
quartz SiO2 7 conchoidal fr glassy; clear, variable col.; sometimes 6-sided crystals glass