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photo: ConexionCristalina


Common Rock-Forming Minerals

Since minerals are the building blocks of rocks, it is important that you learn to identify the most common varieties. Minerals can be distinguished using various physical and/or chemical characteristics, but, since chemistry cannot be determined readily in the field, geologists use the physical properties of minerals to identify them.

These include features such as crystal form, hardness (relative to a steel blade or you finger nail), colour, lustre, and streak (the colour when a mineral is ground to a powder). Generally the characteristics listed above can only be determined if the mineral grains are visible in a rock. Thus the identification key distinguishes between rocks in which the grains are visible and those in which the individual mineral components are too small to identify.

The descriptions below use various terms or numbers to describe the mineral's shape, hardness, appearance after breaking, or other attributes. If you're not sure what these mean, refer to How to Identify Minerals in 10 Steps (Photos).

Crystalline minerals are most often quartz.

Quartz cluster

Quartz is an extremely common mineral, and its glittering or crystalline appearance catches the eye of many collectors. Quartz has a hardness of 7 on the Mohs scale, and demonstrates any type of fracture when broken, never the flat surface of cleavage. It does not leave a noticeable streak on white porcelain. It has a glassy luster, or shine.



  • Milky quartz is white quartz.  The white color comes from carbon dioxide gas trapped within the quartz structure. Milky quartz is usually massive, but well well-formed crystals are also common.  In the Huachuca mountains, milky quartz occurs a a filling material in fractures (mineral veins). All quartz has a hardness of 7 on Mohs scale of hardness and can easily scratch glass. Milky quartz is shiny and translucent.  Quartz has no cleavage and breaks with a fracture that ranges from conchoidal to irregular.
  • Rose quartz is a variety of massive, translucent quartz with a pink color. It has no cleavage, it breaks with a conchoidal fracture, and it has a shiny surface. Depending on quality, it can be used as a gemstone or a decorative garden stone.  Two major occurrences of rose quartz are Maine and the Black Hills of South Dakota.
  • Amethyst is purple quartz.  It can occur as well-formed quartz crystals in geodes or deformed crystals in a mineral vein.  If the quality is high enough, amethyst is used as a gemstone. The luster of amethyst is usually vitreous (shiny).
Read: Major varieties of quartz with photos

Hard, glassy minerals without crystals may be a different type of quartz, called chert.

Chert with flint

All types of quartz are crystalline, but some varieties, called "cryptocrystalline," are made of minuscule crystals not visible to the eye.If the mineral has a hardness of 7, fractures, and has a glassy luster, it may be a type of quartz called chert. This is most commonly brown or grey.
  • "Flint" is one variety of chert, but it is categorized in many different ways. For instance, some people may refer to any black chert as flint, while others may only call it flint if it has a certain luster or was found among certain types of rock.

Minerals with striped bands are usually a type of chalcedony.


Chalcedony is formed from a mixtures of quartz and another mineral, moganite. There are many beautiful varieties, typically forming striped bands of different colors. Here are two of the most common:
  • Onyx is a type of chalcedony that tends to have parallel bands. It is most often black or white, but can be many colors.
  • Agate has more curving or "wiggly" bands, and can show up in a wide variety of different colors. It can form from pure quartz, chalcedony, or similar minerals.
Read: Types of agate with photos
See also: Funny weird agate specimens you should see
See if your mineral's characteristics match feldspar's

Orthoclase,plagioclase and amazonite photo:  StudyBlue Inc

Besides the many varieties of quartz, feldspar is the most common type of mineral found. Feldspar is the other common, light-coloured rock-forming mineral. Instead of being glassy like quartz, it is generally dull to opaque with a porcelain-like appearance. Colour varies from red, pink, and white (orthoclase) to green, grey and white (plagioclase). Feldspar is also hard but can be scratched by quartz. Feldspar in igneous rocks forms well developed crystals which are roughly rectangular in shape, and they cleave or break along flat faces. The grains, in contrast to quartz, often have straight edges and flat rectangular faces, some of which meet at right angles.





If the mineral peels when rubbed, it is probably mica.
 
Muscovite and Biotite


Mica is easily distinguished by its characteristic of peeling into many thin flat smooth sheets or flakes. This is similar to the cleavage in feldspar except that in the case of mica the cleavage planes are in only one direction and no right angle face joins occur. Mica may be white and pearly (muscovite) or dark and shiny (biotite).
  • Muscovite mica is colorless to a very pale brown in color. It peels easily into very thin, flexible, elastic sheets that are nearly colorless. Muscovite is also known as white mica.
  • Biotite mica ranges from dark brown to black. It also peels in very thin, flexible, elastic sheets like muscovite mica. Biotite is also known as black mica.
Learn the different between gold and fool's gold: Pyrite

Copyright ©Mark Steinmetz

Pyrite is also known as "fools gold" because it has a yellow metallic color. Pyrite can be distinguished from native gold by several different properties.  Pyrite is much harder than gold; it cannot be scratched by a steel straight pin.  Pyrite is brittle; it can be crushed to a powder, whereas gold simply flattens out because it is a metal. A streak test can also distinguish pyrite from gold; pyrite produces a greenish black streak and gold produces a yellow streak.
  • Marcasite is another common mineral similar to pyrite. While pyrite crystals are shaped like cubes, marcasite forms needles.

Green and blue minerals are often Malachite or Azurite. 

Stunning piece of azurite and malachite from China!

Both of these minerals contain copper, among other minerals. The copper gives malachite its rich green color, while it causes azurite to appear bright blue. These often occur together, and both have a hardness between 3 and 4.



  • Azurite is a bright blue mineral associated with copper ore. It may occur with green malachite, also a copper ore.  It is relatively soft at 3.5 on Mohs scale of hardness.
  • Malachite is a rich green to dark green copper mineral.  It can occur on its own or with azurite, a mineral that it is closely related to in chemistry.  It is relatively soft at 3.5 on Mohs scale of hardness.

Soft, light-colored mineral is Gypsum

Gypsum from Annabel Lee mine, Hardin Co., Illinois, United States. credit: Dakota Matrix Minerals

Gypsum is a soft, light-colored mineral.  Its color can be colorless and transparent  (selenite) or white, pale pink or pale brown. If crystallized, it displays one direction of excellent cleavage, but the cleavage fragments are much thicker than those of  mica and the fragments are not elastic. Generally, it lacks the greasy feel of talc.
  • One form of gypsum tends to form with a fibrous structure (satinspar).

Transparent crystals exhibit strong double refractionand React with even weak acids: Calcite

  
variety of colors and forms of calcite

Calcite is a very common mineral. The difficulty in identifying it is that can occur in a very large variety of colors and forms.  One of the most common forms of calcite crystals are pointy pyramids that resemble a dog's canine tooth (dogtoothspar).  Large, pure pieces of crystalline calcite display three directions of cleavage that are inclined (not at 90 degrees). 
Calcite ranges from transparent to translucent.  Colors may be colorless, white, cream, pale yellow, yellow-brown, brown, and even red due to impurities.  The easiest way to distinguish calcite is with an acid test; concentrated hydrochloric acid with cause abundant bubbles to form as it reacts with the calcite.




Commonly fluorescent in a variety of colors: Fluorite


Fluorite-Octahedron. Photo: Hannes Grobe

It is sometimes easy to mistake fluorite for calcite on a quick examination.  However, if you pay careful attention, fluorite has four directions of cleavage compared to three directions of cleavage for calcite. Fluorite is also harder than calcite (4 on Mohs scale of hardness) and can scratch a piece of calcite.  Fluorite is often more colorful than calcite and can be purple, green, yellow, pink, brown, or colorless and may even show two or more colors on the same specimen.  Fluorite crystals are usually cubes or octahedrons.  Above all, fluorite does not fizz in contact with hydrochloric acid.
Read also: Why Fluorite Comes in Different Colors? With Examples

It is strongly attracted to magnetic fields: Magnetite

Perfect octahedron Magnetite from Formazza Valley, Verbano-Cusio-Ossola
Province, Piedmont, Italy. Collection & Photo M.Chinellato

Magnetite is common in igneous and metamorphic rocks, and some sediments, though usually in only small amounts (1 - 2 %). It is black in colour with a metallic lustre, occurring in small octahedra (like two pyramids stuck together). Easily recognized by its strongly magnetic character.

Use a mineral guide or website to identify other types. 

A mineral guide specific to your area will cover other common types of mineral found in that region. If you are having difficulty identifying a mineral, some online resources such as minerals.net will let you search for the results of your tests and match them to possible minerals.


Sources and Citations

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