Download The Mohs Scale of hardness of minerals and important rock building

The Mohs Scale of hardness of minerals and important rock building minerals to know Use all the tools to identify a mineral: Hardness (steel needle, coin, finger nail, glass and other minerals), Lustre, Fracture, Colour, Habit, Transparency, Cleavage, all are important to identify the mineral! Identify all the minerals you can in a rock. Remember, the hardness of a mineral is something different to the hardness of a rock! The cleavage of the rocks is not a mineral cleavage! In a rock identify internal structures, crystals or grains, their size and / or roundness, cleavage, schistosity, flow structures. etc! From the above conclude the processes: crystallisation, intrusion, extrusion, precipitation or deposition. Using all the knowledge above try to classify the rock! Important Rocks:: Granite, andesite, sandstone, rhyolite, basalt, diorite, trachyte, shale, mudstone, conglomerate, quartzite, limestone, dolomite and their various metamorphic counterparts. Mohs hardness
Mineral
Chemical formula
1
Talc
Mg3Si4O10(OH)2
2
Gypsum
CaSO4·2H2O
3
Calcite
CaCO3
4
Fluorite
CaF2
5
Apatite
Ca5(PO4)3(OH–,Cl–,F–)
6
Orthoclase Feldspar
KAlSi3O8
7
Quartz
SiO2
8
Topaz
Al2SiO4(OH–,F–)2
9
Corundum
Al2O3
10
Diamond
C
Image
Mohs Scale Minerals: Talc
General chemistry: [Mg3Si4O10 (OH)2] (Phyllosilicate)
Hardness: 1
Streak: White
Lustre: Dull, pearly or greasy
Cleavage: One perfect to poorly visible plane
Colour: apple-green, grey, white or dark-grey
Comment: Has a soapy texture (‘soapstone’) normally
formed as secondary mineral when alteration of silicate
minerals occurs (i.e. olivine)
Gypsum
General chemistry: CaSO4·2H2O (sulphate)
Hardness: 2
Streak: White
Lustre: Vitreous; sub-vitreoous
Transparency: Translucent to milky
Cleavage: In 3 directions: Perfect, almost micaceous in
some samples; distinct, yielding a surface with a
conchoidal fracture; yielding a fibrous fracture
Colour: Colourless to white, light grey or brown
Comment: Found as massive variety, including the
alabaster and as clear crystals, the selenite variety; and,
parallel fibrous, the satin spar variety.
Calcite
General Chemistry: [CaCO3] carbonate
Hardness: 3
Streak: White to greyish
Lustre: vitreous
Cleavage: Perfect
Colour: White, colourless, grey, rarely red, brown, green
or black
Comment: Calcite is transparent to translucent. It can
form nice hydrothermal crystals or a carbonate rock and
be there usually organogenic.
Fluorite
General chemistry: [CaF2]
Hardness: 4
Streak: White
Lustre: Vitreous
Cleavage: Perfect on 3 sides; Fracture is uneven
Colour: Great variety, ranging from purple, green, colourless,
white, brown, and yellow
Comment: Is very common in hydrothermal veins, and
around hot springs. It is associated with common minerals
such as quartz, calcite, dolomite, and galena. It falls under
flouride group
Apatite
General chemistry: Ca5(PO4)3 (F,OH,Cl) (phosphate)
Hardness: 5
Streak: White
Lustre: Vitreous
Cleavage: Poor
Colour: green, brown, blue, yellow, violet, colourless
Comment: Common in organic-rich sedimentary rocks, and
hydrothermal veins. Used as source of phosphor in fertilizers
Orthoclase; Alkali Feldspar (K-feldspar)
General chemistry: ([KAlSi3O8]) Microcline or Orthoclase,
Tectosilicate
Hardness: 6
Streak: White
Lustre: Dull to vitreous
Cleavage: Normally 2 planes at ~90 but can also have max.
3 planes at ~90 (Perfect – good – poor visibility)
Colour: Green (microcline = Amazonite), white, clourless,
red, grey or yellow.
Comment: Commonly occurs in granites, granodiorites,
syenites, and sedimentary arkose and conglomerate rocks
Quartz
General chemistry: [SiO2]
Hardness: 7
Streak: none
Lustre: Vitreous on fresh surface
Cleavage: None – (Uneven fractures and Conchoidal
Fracture)
Colour: Smoky, milky, purple, pink, yellow, and rose
quartz are most common
Comment: Occurs commonly in SiO2 rich- igneous,
metamorphic and in almost all sedimentary rocks but not
on carbonates and in igneous rocks never with olivine!
Topas
General chemistry: Al2SiO4(F,OH)2 + Cr
Hardness: 8
Streak: None
Lustre: Vitreous
Cleavage: Perfect in one direction
Colour: Great variety, ranging from purple, green, colourless,
white, brown, and yellow
Comment: In igneous environments, in granite pegmatites
and in rhyolite. Also in sedimentary, alluvial deposits as
heavy mineral.
Corundum
General chemistry: [Al2O3] is an oxide mineral
Hardness: 9
Streak: none
Lustre: Vitreous or dull when weathered
Cleavage: One plane at 90° to C-axis is generally visible
as good to poor striations
Colour: varying as a result of traces of Cr, Fe and Ti,
hence blue (sapphire), grey, yellow, or brown.
Comment: Present as accessory mineral in metamorphic
rocks and sometimes in silica poor rocks.
Diamond
General chemistry: C
Hardness: 10
Streak: none
Lustre: Greasy
Cleavage: Perfect octahedral
Colour: Colourless, yellowish, brown, black, blue, green or
red, pink, champagne-tan, cognac-brown, lilac (very rare).
Comment: Diamond is the hardest natural substance known.
It is formed deep in the mantle, and is only brought to the
surface via kimberlite pipes, lamprophyres, eclogites
intruding thick continental crust. It is also found in alluvial
deposits.
Other important minerals, not part of the Mohs Hardness Scale: Olivine
General chemistry: [(Fe, Mg, Ca)2SiO4] Nesosilicate
Mineral
Hardness: 6.5 – 7
Streak: White
Lustre: Vitreous or greasy looking
Cleavage: None, hence conchoidal fracturing
Colour: Pale yellow-brownish-green or olive-green
(chemistry dependent)
Comment: Olivines are generally associated with
ultramafic rocks and there never come together with
quartz!
Pyroxene
General chemistry: [(Ca, Na) (Mg, Fe, Al, Ti)(Si,Al)2O6]
Inosilicates
Hardness: 5 -6.5
Streak: grey-green or white or grey
Lustre: vitreous or pearly or dull
Cleavage: Orthopyroxene: 2 planes at ~90o (good to poor
visibility); clinopyroxene: at c. 60° and 120°
Colour: dark green or black, rarely light green to
Comment: occurrence mostly in ultramafic to mafic rocks
and some metamorphic rocks.
Amphibole (Hornblende)
General chemistry (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22 (OH)2
Hardness: 5 - 6
Streak: White
Lustre: Pearly to greasy -glassy
Cleavage: 2 perfect planes crossing at c. 60° and 120°
Colour: mostly black to dark greenish or brown
Comment: It is an important constituent in acid and
intermediate igneous rocks such as granite, diorite,
syenite, andesite and rhyolite. Also in metamorphic rocks
such as gneiss and schist. Metamorphic rocks with
abundant hornblende are called amphibolites.
Biotite Mica
General chemistry: [K(Mg,Fe2+)3(Al, Fe3+)SiO10(OH, F)2]
Phyllosilicate
Hardness: 2.5 - 3
Streak: White
Lustre: Pearly or vitreous
Cleavage: One perfect plane
Colour: Dark green, brown- black, rarely light colour even
individual sheets have smoky colour, translucent
Comment: It occurs in a very wide array of environments
in igneous and metamorphic and sedimentary rocks.
Muscovite (White Mica)
General chemistry: [KAl2(Si3Al)O10(OH,F)2] Phyllosilicate
Hardness: 2 - 2.5
Streak: White
Lustre: Pearly or vitreous
Cleavage: One perfect plane
Colour: Transparent, translucent (individual sheets),
white, brown, shades of yellow or green
Comment: Is very common in silica-rich igneous rocks
and forms characteristic ‘foliation’ planes in regionally
metamorphosed rocks. As flakes in sedimentary rocks.
Plagioclase Feldspar
General chemistry: [Anorthite (CaAl2Si2O8) – Albite
(NaAlSi3O8)] Tectosilicate
Hardness: 6 – 6.5
Streak: White
Lustre: Vitreous to glassy
Cleavage: 2 planes with perfect to good visibility
Colour: Translucent, milky white, grey, yellowish or fleshred (very wide range of colours due to chemistry range)
Comment: Present in wide array of rocks therefore poor
tool for discriminating rocks at hand specimen.
Fe oxide minerals Magnetite
General chemistry: Fe3+2Fe2+O4 (Fe3O4; oxide)
Hardness: 5.5 - 6
Streak: black to dark brown (often together with haematite)
Lustre: metallic to dull when weathered
Cleavage: None
Colour: greyish black to brown, when with haematite
Comment: strongly magnetic, occurring in magmatic rocks
and in metamorphic rocks and some sedimentary rocks like
banded iron formations, or as heavy mineral in sandstones
Haematite
General chemistry: Fe2O3 (oxide)
Hardness: 5 - 6
Streak: red to reddish-brown
Lustre: Metallic to dull
Cleavage: One perfect plane
Colour: reddish brown, sometimes black to silver grey
Comment: Occurs in all different sedimentary, igneous,
and metamorphic environments and in oxidation zones of
Fe-rich rocks. Important Fe-ore mineral, paramegnetic
Fe‐sulpide: Pyrite
General chemistry: FeS2 (sulphide)
Hardness: 6 - 6.5
Streak: black
Lustre: Metallic (goldish)
Cleavage: None
Colour: golden- silvery
Comment: Pyrite forms in almost all types of
environments, including sedimentary, igneous, and
metamorphic environments and hydrothermal veins at
the absence of oxygen.