Is limonite metallic or nonmetallic

MINERAL IDENTIFICATION TABLE 1 – MINERAL LUSTER Luster – The most useful subdivision of luster is whether a mineral has metallic or nonmetallic luster.

Metallic Luster Streak color can help in identification, especially for hematite. Some observers see more subtle variations in streak color.

Mineral Name Specific Luster Streak Color

Chalcopyrite brass or gold dark gray

Native Copper coppery copper

Galena silvery dark gray

Graphite silvery dark gray

Hematite steel, but see below brick red

Magnetite broken cast iron dark gray

Pyrite brass dark gray

Sphalerite steel, but see below pale yellow

Nonmetallic – All other minerals in the lab set including hematite and sphalerite have nonmetallic lusters. A more detailed analysis of nonmetallic luster can help. Some minerals may exhibit more than one type of nonmetallic luster depending on crystal habit. Following are some examples of specific lusters. Different observers often have different opinions on luster appearance. Semantics aside, experience is the best way to develop expertise.

Specific Luster Example Minerals

Glassy or Vitreous olivine, quartz

Waxy cryptocrystalline quartz, talc

Pearly muscovite, biotite

Earthy or Dull nonmetallic hematite, limonite

Greasy calcium-rich plagioclase feldspar

Resinous nonmetallic sphalerite

Porcelaneous hornblende, potassium feldspar

MINERAL IDENTIFICATION TABLE 2 – HARDNESS

The following table lists mineral hardness using the Mohs scale. TEE Table 4.2, page 88 lists the index minerals on this scale. Topaz and Diamond are not

included in the mineral identification exercise, but all other minerals in Table 4.2 are. The scale is not linear. In practice, the scratch tests are performed with

commonly available objects. These also are listed in Table 4.2, but not

accurately. Remember that harder materials with higher Mohs number scratch softer materials with lower Mohs number. Substances of equal hardness scratch

each other with great difficulty.

Other common objects that can be used for hardness testing include copper penny (Mohs 3.5), wire nail (Mohs 4.5), concrete nail (Mohs 5.5) steel file (Mohs 6.5), streak plate (Mohs 6.5), and quartz (Mohs 7). Nails and streak plates are

provided in the laboratory. Noting the ease or difficulty of scratching a material will provide a useful clue about the precise hardness of a mineral. Some

pressure is needed to scratch glass with potassium feldspar, but quartz and harder materials scratch the glass easily.

Minerals harder than a knife blade or glass plate; Mohs > 5.5

Mineral Mohs Hardness

Comment

Corundum 9 Index mineral on Mohs scale

Garnet 7

Olivine 7

Quartz 7 Index mineral on Mohs scale

Cryptocrystalline Quartz 7 Be sure to test fresh mineral

Pyrite 6-6.5

Magnetite 6

Plagioclase Feldspar 6

Potassium Feldspar 6 Index mineral on Mohs scale

Hematite, metallic 6.5

Augite 5.5-6

Hornblende 5.5 Very difficult to scratch glass

Minerals softer than a glass plate but harder than a fingernail; Mohs 2.5-5.5

Mineral Mohs Hardness

Comment

Hornblende 5.5 Very difficult to scratch glass

Hematite, nonmetallic 5

Goethite 5-5.5

Limonite 1.5-5.5 Varies because of structure

Apatite 5 Index mineral on Mohs scale

Fluorite 4 Index mineral on Mohs scale

Chalcopyrite 3.5-4

Azurite 3.5-4

Sphalerite 2.5-4

Dolomite 3.5-4

Barite 3-3.5

Calcite 3 Index mineral on Mohs scale

Biotite 2.5-3

Copper 2.5-3

Galena 2.5

Halite 2.5 Crumbles - false low hardness

Muscovite 2-2.5

Minerals softer than a fingernail; Mohs < 2.5

Muscovite 2-2.5 Difficult to scratch

Chlorite 2-2.5 Difficult to scratch

Gypsum 2 Index mineral on Mohs scale

Limonite 1.5-5.5 Fine grained crumbly form is soft

Graphite 1 Messy to handle

Talc 1 Index mineral on Mohs scale

MINERAL IDENTIFICATION TABLE 3 – CLEAVAGE

Minerals can be classified into two broad groups by whether or not cleavage is obvious. This works well with large mineral grains with obvious broken

surfaces. Determining if cleavage is present can be difficult in fine-grained specimens, though use of magnification can help. When uncertain as to the

presence of cleavage, ignore that property and try to identify the mineral with other diagnostic properties.

If cleavage is observed, then determining the number of directions of cleavages and the angle between cleavages will help to define the mineral. For example,

fluorite is the only mineral we study with 4 directions of cleavage.

TEE does not use the term direction of cleavage. Cleavage planes in a direction of cleavage will all be parallel to each other. Any other directions of cleavage in the mineral grain will have different orientations. When rotating a single mineral grain under a light, the parallel planes of a direction of cleavage will all flash at

the same orientation of the mineral grain.

Cleavage can be classified further as to its quality and ease of forming. This ranges from fair through good to perfect. In the latter case, cleavages will be

smooth and easily formed. This is less so with good cleavage, and poor cleavage is difficult to form.

Two other types of surfaces occur on mineral grains. Fractures are irregular

surfaces from breakage on other than cleavage planes. The nature of the fracture may be useful in determining the mineral name. When minerals grow

unrestricted, they develop crystals with well defined crystal faces. A crystal form is all crystal faces related by symmetry. Some information about crystal forms

will be acquired in the basic study of minerals, and can be very useful for identification. The study of crystal symmetry is beyond the scope of GO110,

though it is a fascinating subject.

Minerals that do not exhibit cleavage

Mineral Name Comments

Apatite Conchoidal fracture

Chalcopyrite Poor cleavage rarely seen

Copper Hackly fracture tends to grab skin when rubbed

Corundum

Garnet Widely spaced parting sometimes confused with cleavage

Goethite/Limonite

Hematite Brick red streak is diagnostic!

Magnetite Magnetic property is diagnostic!

Olivine Conchoidal fracture

Pyrite

Quartz Conchoidal fracture

Quartz, cryptoxln Conchoidal fracture

Talc has one direction of cleavage, but it often is to fine-grained to show cleavage

Minerals that do exhibit obvious cleavage

Mineral Name

Cleavage Directions Comments

Augite 2 at nearly 900

Barite 2 good; 1 excellent Cleavage results in platy appearance

Biotite 1 excellent Easy to cleave

Calcite 3 excellent not at 900 Rhombohedral cleavage

Chlorite 1 excellent Difficult to see when fine grained

Dolomite 3 excellent not at 900 Similar to calcite, usually small grains

Feldspars 2 good at 900

Fluorite 4 excellent Octahedral cleavage; diagnostic!

Galena 3 excellent at 900 Cubic cleavage

Graphite 1 excellent Excellent lubricant

Gypsum 2 poor; 1 excellent Difficult to see when fine grained

Halite 3 excellent at 900 Cubic cleavage

Hornblende 2 good at 600 and 1200

Muscovite 1 excellent Easy to cleave

Sphalerite 6 excellent Diagnostic, but finding all 6 is difficult