Transcript Color

Minerals II: Physical Properties and Crystal Forms
From: http://webmineral.com/data/Rhodochrosite.shtml
The Physical Properties of Minerals
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Color
Luster
Streak
Hardness
Cleavage
External Crystal Form
The Physical Properties of Minerals (cont.)
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Fracture
Specific Gravity
Special Properties
Other Properties
Chemical composion
Important Physical Properties II
• Color - with color or without (glassy) Although an
obvious feature, it is often unreliable to use to
determine the type of mineral.
– Not original: Color arises due to electronic transitions,
often of trace constituents . For example, quartz is found
in a variety of colors .
• Color of a mineral may be quite diagnostic for the
trace element and coordination number of its bonding
environment.Quartz (yallow ,pink or white)
• False color :which reflect different color labrodorite
Hope Diamond: 44.5 carats
http://www.nmnh.si.edu/minsci/hope.htm
Important Physical Properties I
• Luster - - This property describes the appearance of
reflected light from the mineral's surface. Metallic:
Gold(Au),Cupper(Cu),silver(Ag),Pyrite (FeS2).
• Nonmetallic minerals are described using the
following terms: vitreous(‫)زجاجي‬: Quartz(SiO2),
Calcite(CaCO3) ,Halite (NaCl) pearly)‫(لؤلؤي‬: Mica,
Muscovite KAl2(AlSi3)O10 (OH,F)2.
silky )‫ (حريري‬Fibrous Gypsum , resino )‫ (صمغي‬Sulfur S,
and earthy Kaolin)‫ (ترابي‬,Diamond C :Diamond
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Important Physical Properties III
• Streak - The color of a mineral in its powdered
form, obtained by rubbing the mineral against an
unglazed porcelain plate.
– Streak is usually less variable than color.
– Useful for distinguishing between minerals with
metallic luster.
– Talc is in different color but its streak is white
Important Physical Properties V
• Hardness - This is the resistance of the mineral
to abrasion or scratching. This property doesn't
vary greatly from sample to sample of the same
mineral, and thus is highly diagnostic. It also is a
direct reflection of the bonding type and internal
atomic arrangement. A value is obtained by
comparing the mineral to a standard scale devised
by Moh, which is comprised of 10 minerals
ranging in hardness from talc (softest) to
diamond (hardest).
Mohs’ Hardness Scale
Fingernail Hardness (2.5) Scratches Gypsum (2)
Density and Specific Gravity
• Density - Defined as the mass divided by the
volume and normally designated by the Greek
letter, rho, 
mass/volume; geologists often use g/cm3 as
the unit of choice.
• Specific Gravity - Ratio of the mass of a
substance to the mass of an equal volume of water.
Note that water = 1 g cm-3. S.G. is unitless.
• Examples - quartz (SiO2) has a S.G. of 2.65 while
galena (PbS) has a S.G. of 7.5 and gold (Au) has a
S.G. of 19.3.
Important Physical Properties VI
• Cleavage - Orientation and number of planes of
weakness within a mineral. Directly reflects the
orientation of weak bonds within the crystal structure.
This feature is also highly diagnostic, Mica, muscovite and
Biotite broken in one direction.
• Fracture - This describes how a mineral breaks if it is not
along well defined planes. In minerals with low symmetry
and highly interconnected atomic networks, irregular
fracture is common. Conchoidal Fracture in Glass, chert –
planey , Pyrite not planey , Gypsum fibrouse
Conchoidal Fracture in Glass
Planer Cleavage in Mica
Weak Bonding Yields Planer Cleavage
Amphibole Cleavage ~120/60°
Rhombohedral Cleavage in Calcite
Special and Other Properties
• Striations - Commonly found on plagioclase
feldspar. Straight, parallel lines on one or more
of the cleavage planes caused by mineral
twinning.
• Magnetism - Property of a substance such that it
will spontaneous orient itself within a magnetic
field. Magnetite (Fe3O4) has this property and it
can be used to distinguish it from other nonmagnetite iron oxides, such as hematite (Fe2O3).
• Double Refraction - Seen in calcite crystals.
Light is split or refracted into two components
giving rise to two distinct images.
Plagioclase
striations
Calcite Double Refraction
Color and Density
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Two broad categories are ferromagnesian and nonferromagnesian
silicates, which simply means iron and magnesian bearing or not. The
presence or absence of Fe and Mg strongly affects the external appearance
(color) and density of the minerals.
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Ferromagnesian silicates - dark color, density range from 3.2 - 3.6 g/cc
– Olivine - high Temp. , low silica rocks; comprises over 50% of upper
mantle
– Pyroxenes - high Temp. , low silica rocks
– Amphiboles - esp. hornblende; moderate Temp. , higher silica rocks
– Mica - esp. biotite; moderate Temp. , higher silica rocks
– Garnet - common metamorphic mineral
Nonferromagnesian silicates - light color, density close to 2.7 g/cc
– Mica - exp. muscovite; moderate Temp, higher silica rocks
– Feldspars - plagioclase and orthoclase; most common mineral in crust;
form over a wide range of temperatures and melt compositions
– Quartz - low Temp. , high silica rocks; extremely stable at surface,
hence it tends to be a major component in sedimentary rocks.
– Clay - esp. kaolinite; different types found in different soil
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Crustal Minerals
More Zoning
From http://www.geo.wvu.edu/~lang/Geol284/Min8IgFels
Zoning in Plagioclase Feldspar
Na-rich rim
Ca-rich core
Important Physical Properties IV
• Crystal form or habit - The external
morphology of crystals generally reflect the
internal arrangement of their constituent
atoms. This can be obscured, however, if the
mineral crystallized in an environment that did
not allow it to grow without significant
interaction with other crystals (even of the
same mineral).
Chrysotile Asbestos
Belongs to the Serpentine mineral family hydrated ferromagnesian silicate.
Crystal Forms: Quartz
Feldspar
Intergrown cubic crystals of fluorite
Quartz Interfacial Angles
Perfectly
Proportioned
Crystals
Misshapen
Crystals
Steno’s Law (1669): Crystal face internal angles remain constant!
Macroscopic Forms and Microscopic Blocks
Cubes
Macroscopic
Crystal Forms
Rhombs
Unit Cells and Crystal Structure
Cubic unit cell:
smallest repeatable unit
Polymorphism and polymorphs
• Substances having the same chemical composition
but different crystal structures.
– e.g. diamond and graphite
• Both minerals are composed of pure carbon, but
diamond is the high pressure polymorph of
graphite.
• This gives rise to extremely different physical
properties.
Polymorphism
3 mm
Natural Octahedral Diamond
From: http://www.phy.mtu.edu/~jaszczak/diamond.html
Graphite & Calcite
Diamond vs. Graphite Crystal Structures
Hardness: 10
Hardness: 1-2
From: http://www.molecules.org/elements.html#diamond
X-ray diffraction: Laue
photographic method
Bragg Relationship
From: http://www.geology.wisc.edu/~g203/xray.htm
Laue X-ray photograph of Vesuvianite
From: http://www.geology.fau.edu/course_info/fall02/gly4200/X-RAY.htm
Vesuvianite: Contact Metamorphic Mineral
Formula:
Ca10(Mg,Fe)2Al4Si9O34(OH)4
System: Tetragonal
Hardness: 61⁄2
http://www.mindat.org/min-4223.html