Macroscopic Characteristics of Minerals Part 1: Luster and Color
Summary
TLDRThis tutorial expands on previous lessons about minerals, focusing on identifying minerals through their luster and color. Luster describes how light reflects off a mineral's surface, classified as metallic or non-metallic. Non-metallic minerals can exhibit various lusters, including adamantine, vitreous, pearly, silky, resinous, greasy, and dull. The tutorial also explains the influence of transition metals on a mineral's color and the use of a streak test for identification. By understanding these key properties, geologists can accurately identify minerals in nature.
Takeaways
- 💡 Luster is a key property used by geologists to identify minerals, with two main categories: metallic and non-metallic.
- 🔎 Metallic luster minerals are opaque with a mirror-like shine, while non-metallic minerals can range from opaque to transparent and are less shiny.
- 🌟 There are seven types of non-metallic lusters: adamantine, vitreous, pearly, silky, resinous, greasy, and dull.
- 💎 Adamantine luster minerals are the shiniest non-metallics, like diamonds and Herkimer quartz.
- 🔮 Vitreous luster resembles clean glass, found in minerals like quartz and calcite.
- ✨ Pearly luster is iridescent, seen in minerals with layered structures like muscovite.
- 🧵 Silky luster is found in minerals with parallel fibers, such as ulexite and satin spar.
- 🌳 Resinous luster looks like hardened resin, as seen in sphalerite, which can have different lusters.
- 🖤 Greasy luster, like graphite, appears and feels oily due to its atomic structure.
- 🎨 Color is influenced by transition metals in a mineral, but streak tests provide more consistent identification than visible color.
Q & A
What is the primary property geologists use to identify minerals by eye?
-Geologists primarily use the property of luster to identify minerals by eye. Luster describes the way light reflects off a mineral’s surface.
What are the two main categories of luster?
-The two main categories of luster are metallic and non-metallic. Metallic minerals have a shiny, mirror-like appearance, while non-metallic minerals can range from opaque to transparent and tend to be less shiny.
How is luster related to a mineral’s refractive index?
-Luster is related to the speed that light travels through a mineral, which is determined by its refractive index. Minerals with a refractive index greater than 3 generally have a metallic luster.
What are the seven types of non-metallic lusters?
-The seven types of non-metallic lusters are adamantine, vitreous, pearly, silky, resinous, greasy, and dull.
Which mineral is commonly known for having an adamantine luster?
-Diamonds are commonly known for having an adamantine luster, which is the shiniest of the non-metallic lusters.
What causes the pearly luster in minerals?
-Pearly luster is caused by the iridescence that occurs when light reflects and refracts off different crystallographic planes in a mineral, creating a shifting rainbow effect based on the angle of observation.
What is the difference between a silky luster and a fibrous luster?
-A silky luster is caused by thin, parallel fibers in the mineral that resemble silk, while a fibrous luster occurs when these fibers are coarser, more like twine.
How do transition metals affect the color of minerals?
-Transition metals affect the color of minerals due to their ability to absorb specific wavelengths of visible light through their partially filled d-orbitals, which influences the color reflected off the mineral's surface.
What is the difference between allochromatic and idiochromatic minerals?
-Allochromatic minerals get their color from trace amounts of transition metals, while idiochromatic minerals are 'self-colored' due to the presence of a transition metal in their standard chemical formula.
What is the purpose of a streak test in mineral identification?
-A streak test helps identify minerals by observing their powdered form, as the streak color is more consistent than the mineral's surface color. It is especially useful for distinguishing between minerals with similar appearances.
Outlines
🔍 Understanding Mineral Luster
The first paragraph introduces the concept of luster, a key property used by geologists to identify minerals. Luster refers to how light reflects off a mineral's surface, which can be categorized into metallic and non-metallic types. Metallic minerals have a mirror-like, shiny appearance due to their high refractive index, while non-metallic minerals are less shiny and can be transparent or opaque. There are seven types of non-metallic lusters, including adamantine, vitreous, pearly, silky, resinous, greasy, and dull. Each type is briefly described, along with examples such as diamonds (adamantine), quartz (vitreous), and muscovite (pearly). The paragraph also touches on the science behind these appearances, like the way light interacts with a mineral's crystal structure.
🎨 Color and Streak Tests in Mineral Identification
The second paragraph focuses on color as another important property for identifying minerals, but it also warns that color can be misleading due to factors like the presence of transition metals, which give minerals their hues. Corundum is used as an example, with trace amounts of different metals turning it into sapphires (blue) or rubies (red). The paragraph also introduces idiochromatic minerals, which get their color from metals in their chemical structure, such as malachite’s green hue from copper. It explains structural defects, like those in fluorite, which create unique colors. Lastly, the streak test, where a mineral's powdered form reveals a more consistent color, is described as a useful tool for distinguishing between minerals with similar macroscopic colors, like pyrite and gold.
Mindmap
Keywords
💡Luster
💡Metallic Luster
💡Non-metallic Luster
💡Adamantine
💡Vitreous Luster
💡Iridescence
💡Refractive Index
💡Transition Metals
💡Streak Test
💡Idiochromatic Minerals
Highlights
Luster is a fundamental property geologists use to identify minerals by eye, describing how light reflects off a mineral's surface.
Luster is categorized as metallic or non-metallic, based on the refractive index of the mineral.
Metallic minerals have a refractive index greater than 3, making them opaque with a mirror-like, shiny appearance.
Non-metallic minerals can be opaque to transparent, with lusters ranging from adamantine to dull.
Adamantine luster is the shiniest of the non-metallic lusters, with examples like diamonds and cubic zirconium.
Vitreous luster resembles the appearance of freshly cleaned glass, found in minerals like quartz and calcite.
Pearly luster is associated with minerals having a subtle iridescence, such as muscovite, which reflects light in rainbow-like colors.
Silky luster is seen in minerals made up of fine parallel fibers, with ulexite and satin spar as examples.
Resinous luster appears like hardened resin or maple syrup, with minerals such as sphalerite exhibiting this property.
Greasy luster gives minerals an oily appearance and sometimes a greasy feel, as seen in graphite.
Color is a prominent property of minerals but can be misleading due to the presence of transition metals that absorb specific wavelengths of light.
Allochromatic minerals get their colors from trace amounts of transition metals, as seen in corundum turning into sapphire or ruby based on different metal traces.
Idiochromatic minerals have color due to a transition metal in their chemical formula, such as malachite’s green color from copper.
Structural defects can also influence mineral color, such as fluorite’s purple hue caused by Frenkel defects in its crystal lattice.
Streak testing is a reliable method to identify minerals by observing their powdered form's color, which is more consistent than their surface appearance.
Transcripts
In the previous tutorial, we talked about minerals and the different types of bonding
they can exhibit. Now we are going to take that knowledge and expand upon it
so that you can start identifying minerals in the wild. Geologists start with one simple property to
identify minerals by eye: luster. Luster describes the way light reflects off a mineral’s surface.
A mineral’s luster should first be grouped into one of two categories:
metallic or non-metallic. Luster is related to the speed that light travels through a mineral,
or its refractive index, and minerals that transmit light slowly, with a refractive index
greater than 3, generally have a metallic luster. Metallic minerals are opaque with a mirror-like,
shiny luster, while non-metallic minerals, which can range from opaque to transparent,
tend to be less shiny than metallic minerals. There are seven types of non-metallic lusters:
adamantine, vitreous, pearly, silky, resinous, greasy, and dull.
Minerals with adamantine luster are transparent to translucent and are the shiniest of the
non-metallics. Diamonds, with their high refractive index, are the most well-known example,
but other minerals, like Herkimer diamonds, a particularly shiny type of quartz from upstate New
York, as well as the popular diamond knock-off, cubic zirconium, also get a chance to shine.
Vitreous luster looks like a freshly cleaned pane of glass.
These minerals are transparent to translucent and have a low refractive index.
Some of the more common minerals such as quartz and calcite have a vitreous luster.
Pearly luster resembles the subtle iridescence, or play of colors, found in natural pearls.
Iridescence occurs when light reflects and refracts off different crystallographic planes
as it passes through a crystal. The merging of two or more light waves that have been reflected off
atomic planes creates a new light wave with a new and distinct waveform. Because the
distance that a light wave travels between two layers is a function of the angle of incidence,
changing it will result in different wave combinations, creating a shifting rainbow effect,
where the color reflected changes based on the angle of observation.
It makes sense then, that minerals with a pearly luster consist of transparent, coplanar sheets.
A common mineral with a pearly luster is muscovite, which is sometimes used as glitter.
Minerals with a silky luster are composed of thin, parallel fibers, making them reminiscent of silk.
If these “fibers” are particularly coarse, more like twine then silk, it is said to have
a fibrous luster. Ulexite, a rare hydrated borate, is an excellent example of silky
luster. Another example is a variety of gypsum, a mineral composed of hydrated calcium sulfate,
called satin spar. Resinous luster resembles hardened maple syrup or frozen pine resin.
Sphalerite, which is named after the Greek word for deceptive, due to its many different crystal
appearances, can sometimes be found in nature with a resinous luster, along with metallic,
or even adamantine lusters. Minerals with a greasy luster appear as though
they are covered in a thin layer of oil, and many minerals with a greasy luster also feel greasy.
An excellent example of this is graphite. Comprised of sp2 hybridized carbon atoms forming
an extended pi system, it both has a greasy luster and feels greasy. Minerals with a dull,
or earthy luster are the least shiny of them all, as light is primarily scattered at the surface
instead of reflected. The clay mineral kaolinite is an example of a mineral with a dull luster.
Color, one of the most obvious properties of a mineral, is also the easiest to misinterpret.
Minerals typically get their coloration from the presence of transition metals. Due to
their partially filled d-orbitals, transition metals are very efficient at absorbing specific
wavelengths of visible light, which affects the color that is reflected off a mineral’s surface
into your eye. Allochromatic minerals only contain trace amounts of the transition metals. Corundum,
Al2O3, is colorless until about one percent of the aluminum ions are replaced with a transition
metal, such as iron, titanium, or chromium. Corundum with trace amounts of iron or titanium
is blue in color and is called sapphire, whereas corundum with trace amounts of chromium is red,
and is called ruby. Transition metals do not color all minerals identically. For example,
chromium makes rubies red, but it also makes emeralds green.
Idiochromatic minerals are “self-colored” by the presence of a transition metal in
their standard chemical formula. The carbonate mineral malachite appears bright green due to
the absorption of red light by copper ions, which makes up over half of its chemical composition.
Other sources of color include structural defects in the crystal lattice. The mineral fluorite
gets its purple color from Frenkel defects, which occur when a small ion slips out of its position,
leaving an electron behind in its hole, which can absorb and reemit light.
One of the ways that color can be useful for identifying a mineral
is by observing its color in a powdered form, which is done by a streak test. The streak is
the powdered residue left behind after rubbing a mineral on a piece of unglazed porcelain.
The streak of a mineral is much more consistent than its macroscopic color.
This is because when a non-opaque material is finely powdered, most of the light is reflected
off its surface, rather than being transmitted and subsequently absorbed by its electrons.
Most non-metallic minerals have a white streak, while metallic minerals tend to have dark-colored
streaks. For the ferric oxide mineral hematite, the streak test is one of the quickest ways to
identify it. Although it can appear black, brown, or even silver in color, its streak will always be
a reddish brown. Similarly, calcite, which can appear in a range of colors from pink to blue,
always has a white streak. If you want a way to tell the difference between pyrite,
also known as fool’s gold, and real gold, pyrite always has a dark gray streak,
while gold has a pale-yellow streak. That covers two very important macroscopic
characteristics of minerals, luster and color. Let’s move forward and learn about a couple more.
関連動画をさらに表示
The Composition of Rocks: Mineral Crystallinity and Bonding Types
Identifying Mineral Samples
Earth and Life Science Rewind 3 Physical and Chemical Properties of Minerals
Mineral Identification
Minerals Earth Revealed
ROCK-FORMING MINERALS (Physical & Chemical Properties) | EARTH AND LIFE SCIENCE | Science 11 MELC 3
5.0 / 5 (0 votes)