Macroscopic Characteristics of Minerals Part 1: Luster and Color

Summary

This 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

00:00

🔍 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.

05:00

🎨 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.

Keywords

💡Luster

Luster refers to the way light reflects off a mineral's surface. It is a key property used by geologists to identify minerals. The script divides luster into two main categories: metallic and non-metallic, with metallic minerals reflecting light like a mirror and non-metallic ones having varying degrees of shine.

💡Metallic Luster

Metallic luster describes minerals that reflect light like a polished metal. These minerals are opaque and have a mirror-like shine. The video explains that minerals with a refractive index greater than 3 often exhibit metallic luster, making them easily distinguishable from non-metallic minerals.

💡Non-metallic Luster

Non-metallic luster is exhibited by minerals that range from opaque to transparent and are less shiny than metallic minerals. The script lists seven types of non-metallic lusters: adamantine, vitreous, pearly, silky, resinous, greasy, and dull, each with distinct light-reflecting properties.

💡Adamantine

Adamantine luster is the highest shine of the non-metallic lusters, found in minerals that are transparent to translucent. The script mentions diamonds and cubic zirconium as examples, emphasizing how these minerals' high refractive indices contribute to their brilliance.

💡Vitreous Luster

Vitreous luster resembles the appearance of freshly cleaned glass. Minerals with this type of luster are typically transparent to translucent, like quartz and calcite. These minerals reflect light like glass and have lower refractive indices than those with metallic or adamantine luster.

💡Iridescence

Iridescence refers to the phenomenon where light reflects and refracts off a mineral's crystallographic planes, creating a rainbow-like effect. This occurs due to the interference of light waves, with muscovite, a mineral with a pearly luster, being an example given in the script.

💡Refractive Index

Refractive index is the measure of how much light is slowed down as it passes through a mineral. The script notes that minerals with a refractive index greater than 3 tend to have a metallic luster, which helps distinguish them from non-metallic minerals.

💡Transition Metals

Transition metals contribute to the color of minerals due to their ability to absorb specific wavelengths of light. The script explains that small amounts of these metals, like iron, titanium, or chromium, can drastically change a mineral’s color, as seen in corundum, where the presence of these metals creates sapphires and rubies.

💡Streak Test

The streak test involves rubbing a mineral on unglazed porcelain to observe the color of its powdered residue. The script explains that the streak color is more consistent than a mineral's surface color, making it a reliable method for identification. For example, hematite has a reddish-brown streak, regardless of its surface appearance.

💡Idiochromatic Minerals

Idiochromatic minerals are those whose color comes from the presence of a transition metal within their chemical formula. Malachite is given as an example, with its bright green color resulting from copper ions that make up over half of its composition. This distinguishes them from allochromatic minerals, where color is due to trace amounts of metals.

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.