Methods Used in Identifying Minerals

Over 4,000 minerals are known to man, and these minerals are identified by their physical and chemical properties. The physical properties of minerals are determined by the atomic structure and crystal chemistry of the minerals. The most common physical properties are crystal form, color, hardness, cleavage, and specific gravity.

  • Crystals
  • Cleavage and Fracture
  • Color
  • Hardness
  • Streak
  • Luster
  • Specific Gravity
  • Tenacity
  • Acid Test
  • Magnetism
  • Fluorescence

 

Crystals

One of the best aids in the identification of a mineral is its crystal form (external shape). A crystal is defined as a homogenous solid possessing a three-dimensional internal order defined by the lattice structure.

Crystals developed under favorable conditions often exhibit characteristic geometric forms (which are outward expressions of the internal arrangements of atoms), crystal class, and cleavage. Large, well-developed crystals are not common because of unfavorable growth conditions, but small crystals recognizable with a hand lens or microscope are common. Minerals that show no external crystal form but possess an internal crystalline structure are said to be massive.

A few minerals, such as limonite and opal, have no orderly arrangement of atoms and are said to be amorphous.

Crystals are divided into six major classes based on their geometric form: isometric, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic. The hexagonal system also has a rhombohedral subdivision, which applies mainly to carbonates.

Cleavage and Fracture

After minerals are formed, they have a tendency to split or break along definite planes of weakness. This property is called cleavage. These planes of weakness are closely related to the internal structure of the mineral, and are usually, but not always, parallel to crystal faces or possible crystal faces. Minerals may have one, two, three, four, or six directions of cleavage. These cleavage forms are (1) cubic, (2) octahedral, (3) dodecahedral, (4) rhombohedral, (5) prismatic, and (6) pinacoidal. Minerals that break easily along these lines of weakness yield shiny surfaces. Many crystals do not cleave, but fracture or break instead. Quartz, for example, forms well-developed crystal faces but does not cleave at all; instead it fractures or breaks randomly with a conchoidal fracture.

Color

The color of a mineral is the most important identifying characteristic for the amateur mineralogist. Many minerals exhibit various colors; the varieties are mainly due to impurities or a slight change in chemical composition. For example, calcite can be white, blue, yellow, pink, or fluorescent. Surface tarnish may have changed the color of the specimen; therefore, a fresh surface should be examined.

Hardness

The hardness (scratchability) of a mineral can be measured by its resistance to scratching or abrasion. Mohs scale is a set of 10 common minerals chosen for comparative hardness. The minerals are arranged in order of increasing hardness; each mineral will scratch all that precede it, and be scratched by all that follow it. Mohs scale is as follows:

  1. talc
  2. gypsum
  3. calcite
  4. fluorite
  5. apatite
  6. orthoclase
  7. quartz
  8. topaz
  9. corundum
  10. diamond

Streak

The streak of a mineral is the color of the powder produced when the mineral is rubbed against an unglazed porcelain plate or other fine-grained, hard, abrasive surface. The color of a particular mineral may vary, but the streak is generally constant. The streak may be the same color as the mineral or an entirely different color, but the streak of all white minerals, including calcite, is white.

Luster

Luster refers to the brightness of light reflected from the mineral's surface. The main types of luster are metallic and nonmetallic. Some of the more important nonmetallic lusters are:

Adamantine: brilliant, like that of a diamond.

Earthy: dull, like kaolin.

Silky: having the sheen of silk, like satin spar, a variety of gypsum.

Greasy: oily appearance.

Resinous: waxy appearance, like sphalerite.

Vitreous: the appearance of broken glass, like quartz.

Nacreous (pearly): like mother of pearl; for example, pearly luster on fossil gastropods and cephalopods.

Specific Gravity

The specific gravity (relative density) of a mineral is its weight compared to the weight of an equal volume of water; thus, a mineral with a specific gravity of 4 is four times heavier than water. Special instruments are needed to measure the specific gravity.

Tenacity

Tenacity is the measure of a mineral's cohesiveness or toughness. Tenacity terms are:

Brittle: breaks or powders easily; for example, pyrite or marcasite.

Ductile: can be drawn into a wire; for example, copper.

Elastic: bends and resumes its original position or shape when pressure is released; for example, biotite or muscovite.

Malleable: can be hammered into thin plates or sheets; for example, gold or copper.

Sectile: can be cut or shaved with a knife; for example, gypsum or galena.

Acid Test

When carbonates (especially calcite) are treated with cold, dilute hydrochloric acid, they will effervesce, foam, and bubble, and give off carbon dioxide gas. When sulfides, such as galena, pyrite, and sphalerite, are treated with dilute hydrochloric acid, they will give off the rotten-egg odor of hydrogen sulfide.

Magnetism

A few minerals, such as magnetite and pyrrhotite, are attracted by a magnet and are said to be magnetic. Magnetic minerals are rare in Kentucky, but do occur in the kimberlite in Elliott County. If you find a large piece of highly magnetic material, it may be a meteorite or a furnace product.

Fluorescence

Some minerals, such as calcite, gypsum, halite, uranium minerals, and fluorite, will fluoresce in brilliant colors when viewed with an ultraviolet (UV) light. UV light is not normally visible to the human eye, and you should avoid looking directly at the UV source, as it can damage eyesight.

 

Did I find a meteorite?
Stephen Greb and Warren Anderson

Twenty-seven confirmed meteorites have been found in Kentucky. These are meteorites that people saw fall, or were found and verified in laboratories. Many more pieces (100’s to 1000’s) of naturally occurring iron ores or man-made materials are misinterpreted as meteorites every year. Be cautious when buying “meteorites” on the internet or in stores. Make sure they have been authenticated or verified by a professional research institution. Most specimens that are brought to labs and professional meteorite researchers for verification are actually something else. In Kentucky, most of the brown-red, iron-like specimens that are brought to the state’s Universities for identification are man-made iron slag, rather than meteorites. Shiny, silver-metallic specimens brought in for verification as meteorites tend to be man-made metallic silica, which is a slag or byproduct of glass production. Before and during the Civil War, iron furnaces were common in Kentucky. In the 1830’s, Kentucky ranked third among U.S. states in iron production. Slag is the left-over material from those furnaces. It looks like iron and can easily be mistaken for a meteorite; even by well-meaning teachers and scientists that are familiar with rocks and minerals. Slag can be found far away from known furnaces because the iron (and sometimes slag) were shipped in all directions from the furnaces to industries and people who needed the iron.

Some steps to take to determine if your specimen might be a meteorite.

Determining if a rock is a meteorite can be difficult even for professionals. The following is a list of questions concerning the physical description of your specimen and the location where your specimen was found that might help you to answer if the specimen you found could be a meteorite. You can also see examples of iron meteorites on display at the Kentucky Geological Survey (KGS), or see images and descriptions in Space Visitors in Kentucky…, an educational KGS publication that can be viewed online at http://kgsweb.uky.edu/olops/pub/kgs/sp01_12.pdf  for comparison to the specimen you found.

A) Physical description.

1). Is the specimen heavy for its size? Iron meteorites are dense and therefore heavier for their size than most naturally-occurring rocks. Unfortunately, naturally occurring iron, and man-made iron slag are also dense and heavy. Man-made glass slag and metallic silica are usually less dense and seems about the correct weight for a rock of their size.

2) Is the specimen rust-like, brown, or orange colored? Weathered meteorites (found buried or in soil) will tend to be dark and rusty looking. Man-made iron slag has similar colors. Naturally-occurring iron compounds in Kentucky can also be dark and rusty, but sometimes have a purplish color or hue.


Specimens with iron-colored or drab-colored exteriors and metallic interiors
(silver or gold) that have been found in Kentucky, but are not meteorites

3) Is the specimen metallic, shiny, and silver?  Meteorites are rarely bright and metallic on the outside, although they can be on the inside. Specimens that are bright and metallic on the outside, and have been found in the ground in Kentucky, may be man-made metallic silica slag. If the specimen is shiny on the outside it is not a meteorite.


Specimens with silver or metallic, highly reflective exteriors that have been found in Kentucky,
but are not meteorites.

4) If the specimen is iron-looking, rust-colored, or metallic, and you think it could be a meteorite, look for fragments or pieces of light gray or white, cement-looking pieces or coal fragments in the specimen. If it contains pieces of other types of rocks or minerals, its probably not a meteorite. Iron or metallic meteorites are usually relatively homogenous, which means that any specimen will look like it is composed of one material. Some meteorites may contain greenish minerals (olivine), but so do many igneous rocks. Metallic (iron) meteorites do not contain pieces of cement or limestone. Iron slag from Kentucky often contains fragments of other lighter colored rocks from the smelting process. There are stoney meteorites that look like they contain pieces and specks of a wide variety of rock types, but these are not iron-colored, rusty, or metallic specimens most people report as possible meteorites.  


Iron-appearing specimen that contains light-colored fragments that was found in Kentucky,
but is not a meteorite.

5) Is the specimen irregularly shaped or round? Meteorites are not round. Most are irregularly shaped. Round rocks that look like what amateur collectors think a meteorite should look like are usually man-made materials or natural concretions and geodes, which are common in Kentucky. Round, heavy white and gray specimens from 1 to 3 inches in diameter may be ceramic “mixing balls”, which used to be used for mixing paints and dyes and are sometimes found in Kentucky.

6) Does the specimen have circular holes, markings, or bubbles on its outer surface?  This is called vesicular texture and is common in volcanic rocks and some slags, but not iron meteorites. Meteorites are solid and lack a bubbly texture. If the specimen has circular holes or bubbles it is not a meteorite.

7) Does the specimen have a dark rind or outer covering, when compared to the inside surface (if the inside is exposed)? Many meteorites develop a fused crust or rind when they enter the earth’s atmosphere. Unfortunately, many natural and man-made iron-bearing rocks also can have an exterior rind. Liesegang banding is a type of iron precipitation common in rocks along the Cumberland escarpment (Greenup to McCreary counties) in eastern Kentucky. These bands are formed naturally from iron in the rock. They tend to have a dark brown, orange, red, or purplish color. Pieces can weather out of the rock or be eroded in streams.

8). Does the specimen have striations (scratches) or colored streaks in one direction on its side? When meteorites enter the atmosphere they may streak or partially melt resulting in streaks. Slag can also sometimes have streaks, especially yellowish streaks from limonite in the slag when weathered.

9) Does the specimen have small depressions (not holes) that look like someone pressed their thumb all over the outside of the rock?  If yes, these might be regmaglypts, which are characteristic of meteorites (see pictures in Space Visitors… at http://kgsweb.uky.edu/olops/pub/kgs/sp01_12.pdf).

10) Is the specimen magnetic?  You can use a magnet, metal shavings, or you can place the specimen next to a compass to see if the compass needle is deflected by the specimen to determine if it is magnetic. Iron meteorites are magnetic, while most naturally-occurring rocks in Kentucky, and silica slag are not. Some people think this test proves a specimen is a meteor but this is not true. Iron ore and iron slag can also both be magnetic.

B) Location description

11) Do you live in Bath, Bell, Boyd, Bullitt, Calloway, Campbell, Carter, Crittenden, Edmonson, Estill, Greenup, Hart, Lawrence, Lewis, Livingston, Lyon, McCracken, Menifee, Muhlenberg, Powell, Russell, Scott, or Trigg counties? If yes, these were all counties in which iron furnaces are known to have operated, and it is very likely that you have a piece of slag from an old furnace. More furnaces were operated than are known from historic records so not living in these counties does not mean your specimen is a meteorite. Also, living in these counties does not preclude the possibility that a meteorite could be found there, but the likelihood is that the iron-like material found was from one of the old furnaces.

12) Do you live along a historic road to a major Kentucky town, an old railroad, or the Ohio, Cumberland, or Kentucky rivers?  These were all transportation corridors along which iron (and slag) were transported from the furnaces to people and industries that needed iron. Certainly, living along a transportation corridor does not preclude the possibility that a meteorite could be found there, but the likelihood is that the iron-like material found was from an old furnace and fell off a cart, train, etc.

C) More detailed description and testing

13) Does the specimen leave a streak on ceramic tile?  One of the tests geologists use in mineral identification is called a streak test. Different minerals leave different colored streaks when scratched across an unglazed ceramic tile. According to the University of Arizona Center for Meteorite Studies, meteorites do not commonly leave a streak unless they are highly weathered. In contrast, naturally occurring iron will leave a brown streak, and natural magnetite will leave a gray-black streak. Iron slags may or may not leave a streak.

14) Does the specimen contain Widmanstatten patterns?  These are characteristic cross-hatched patterns on the inside of nickel meteorites (see pictures in Space Visitors), which might be visible in broken specimens. In some cases, Widmanstatten patterns might also be visible on weathered specimens without treatment. They do not occur in slag. Although diagnostic for meteorites, a specimen usually needs to be cut with a rock saw to see the patterns. Then, sawed surfaces must be etched with acid. This requires professional laboratory services. Warning: cutting steel-hard specimens is dangerous and should not be attempted by amateurs. Likewise, working with acid is dangerous and should not be attempted by individual’s not trained to work with acids or outside of laboratories. Some metallic compounds in specimens that look like meteorites can react with acids or produce hazardous dust that can cause harm if inhaled or put in contact with your skin or eyes. Do not saw or apply acids to your specimen!


Cut and etched iron meteorite showing characteristic Windmanstätten patterns.
Scale at the bottom is in centimeters.

If you go through the questions listed above and think there is still a good chance you may have a meteorite, there are meteorite research labs associated with major universities that offer cutting and testing services for small fees.

Gold and Silver

Gold and silver are commonly found in areas where igneous and metamorphic activity has occurred and are generally associated with silicic types of intrusives and Precambrian metamorphic rocks. When gold-bearing rocks weather, the liberated gold, because of its higher specific gravity, is mechanically separated from the accompanying lighter material and concentrated in the stream bottoms in the valleys. This type of mineral concentration is called placer.

In Kentucky, the geology is not favorable for the natural occurrence of gold or silver. Most of the surface and near-surface rocks are sedimentary, and there has not been any igneous, metamorphic, or tectonic event to allow the gold or precious metal to be concentrated in economic quantities.

The most common mineral mistaken for gold is pyrite, an iron sulfide, commonly referred to as "fool's gold." Another mineral that often confuses an amateur rockhound is muscovite, often called white mica. It is nonmetallic, but has a shiny luster, and when weathered it turns brownish gold. Small flakes of muscovite occur disseminated throughout some shales and sandstones in Kentucky. It is remotely possible that minute, noncommercial amounts of gold could be found in glacial outwash deposits along the Ohio River Valley. Some gold has been recovered from glacial debris in southern Ohio near the Kentucky border. If this gold exists in Kentucky, it was derived from sediments in the northeastern part of the United States.

Some silver was recovered from galena concentrates in the Western Kentucky Fluorspar District in the 1960's, but the amount was very small and uneconomic. The silver occurred in the crystal lattice structure of the galena.

Vein Mineral Deposits

In Kentucky, many economic minerals occur in tabular or lenticular bodies known as veins. The mineralization is commonly localized in faults, fractures, and joints. A fault is a fracture or break in the continuity of a body of rock along which observable displacement or movement has occurred parallel to the plane of fracture. In many instances, faults of large displacement are accompanied by many small faults spread over a wide area. This type of complex faulting is referred to as a fault zone. Because of the resistance to movement of one large rock mass against another, much of the material along a fault is crushed or ground into a fine-grained, clay-like mass called fault gouge, or broken into larger angular fragments called fault breccia.

In the Central Kentucky Mineral District and the Western Kentucky Fluorspar District, minerals occur in veins associated with the fault zones.These minerals are barite, sphalerite, fluorite, galena, celestite, dolomite, and calcite. Occasionally, some secondary minerals such as smithsonite, cerussite, and anglesite may be observed. Some witherite has also been observed in central Kentucky veins.

Other Mineral Deposits

Iron

Iron deposits near Owingsville in Bath, Powell, and Estill Counties were mined as late as the 1900's. These iron ores occur in an oolitic limestone in the Brassfield Formation of Silurian age. Specimens of limonite and hematite may also be found in this area. Iron ore was also mined in northeastern Kentucky in Greenup, Boyd, and Carter Counties, where iron ore occurs in the Upper Mississippian and Lower Pennsylvanian rocks. Siderite and limonite nodules also occur at this locality. Some iron ore occurs in the Tertiary gravels of western Kentucky, and they were mined at one time. Numerous ferruginous nodules, concretions, and irregular bodies occur in the sandstones of eastern Kentucky. These are mainly massive ironstone bodies with few or no crystals visible.

Most mining of these iron deposits was halted in the early 1900's with the discovery of the massive iron deposits in the Lake Superior region.

Phosphate

Phosphate deposits consisting dominantly of microcrystalline apatite were mined from the Lexington Limestone in central Kentucky in Woodford, Fayette, Franklin, and Jessamine Counties in the early 1900's. These phosphates are thin bedded and weather easily in streams to form soft, dark-colored phosphate sands. Any cave or outcrop in the phosphatic portion of the Tanglewood Member would yield specimens of collaphane. It is possible that some nodules or geodes might contain crystals of some phosphate minerals. Phosphate is commonly used as a fertilizer; thus, the exceptional fertility of some Blue Grass soils.

Geodes

Geodes are defined as spherical or oblong bodies filled or partially filled with layered mineral matter, some with crystals projecting inward. Their sizes range from less than an inch to several feet in diameter. Most geodes have an outer shell of chalcedony, and an interior lined with quartz crystals projecting inward. Less commonly, calcite or dolomite crystals are found on the inside, either alone or associated with bitumen, barite, galena, fluorite, quartz, limonite, sphalerite, pyrite, selenite, or celestite. Geodes differ from concretions in that they are hollow and the crystals grow inward from an outer shell; concretions are solid, grow from the center outward, and are generally noncrystalline, although some crystals have been observed. The Fort Payne and Warsaw-Salem Formations of Mississippian age, which crop out in a general semicircle around the Outer Blue Grass and southward into Tennessee, are noted for their abundant geodes. In many places, creeks that drain these formations are filled with geodes, and several mineral varieties can be collected, particularly during low-water stages. Other locations for geode collecting include the tributaries of the Green River in south-central Kentucky and along ancient terraces of the Kentucky River. The Green River has produced some very large geodes (2 feet in diameter) and countless smaller ones. The terraces along the Kentucky River are usually situated near the present course of the river and their locations can be found on geologic quadrangle maps.

Concretions and Nodules

Concretions are formed by the deposition of distinct minerals, different from the surrounding rock, very firmly cemented around a nucleus. They are generally lens shaped, although some have irregular, complex forms. The most common cementing materials are calcite, siderite, and silica. Parts of plants and animals may serve as nuclei, and well-preserved fossils may be found in concretions. In Kentucky, large concretions of siderite and calcite, which used to be called ironstones, are found in shales associated with coal beds. Nodules are another type of irregularly shaped minerals that occur in sedimentary rocks. The most common minerals that occur in nodules are siderite, gypsum, calcite, quartz, and barite/celestite. Siderite nodules, concretions, and liesegang (iron-stained) banding are a very common type of mineralization found in eastern and western Kentucky sandstones.

Meteorites

A meteorite is a fragment of stony or metallic interplanetary rock thought to have formed in the asteroid belts billions of years ago when the earth and solar system were first formed. The most common are the stony meteorites, which are composed of silicate minerals (mainly olivene) and the iron meteorites (siderites) composed of iron, nickel, and accessory minerals. Meteorites have been recovered in 27 locations in Kentucky, including Bath, Bullitt, Livingston, Franklin, Allen, Carroll, Grant, and McCreary Counties. Another unconfirmed report of a meteorite has come from Lewis County. Once a large meteorite strikes a planet, the energy and shock waves eject rock material and brecciate other strata, forming the circular structures associated with meteorite impact structures. The deformation caused by the meteor impact is a very rapid geological process. These impact structures are also called astroblemes. Kentucky has three and possibly four meteorite impact structures. Jeptha Knob in Shelby County, the Versailles structure in Woodford County, and the Middlesboro structure in Bell County are all circular, faulted structures that have some shatter breccia. The Muldraugh Dome or Knob structure in Hardin County may also be an impact structure. Because the earth has weathered and erosion is ongoing, traces of impact structures are uncommon. Approximately 200 impact structures are known on the earth.

Some minerals or unusual rocks may be found near these impact structures, namely part of the meteorite itself, brecciated rocks struck by the meteorite, and possibly some iron, nickel, or sphalerite minerals formed during or after the impact.