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3.4.4: Streak

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    3.48 Red hematite streak

    Although it would never occur to many people to check a mineral’s streak, streak is sometimes a key diagnostic property. It is not a useful property for identifying most silicates, but is especially useful for distinguishing oxide and sulfide minerals. The streak of a mineral is the color it has when finely powdered. For mineral identification, it is much more reliable than mineral color, and it is easy to determine. The usual method of determining streak is to rub the mineral against a ceramic streak plate or other piece of unglazed ceramic. Figure 3.48 shows a red streak from hematite, and the photo below (Figure 3.49) shows a yellow streak from sulfur. Mineralogists routinely use streak when identifying minerals, both in the laboratory and in the field, but it cannot be determined for minerals harder than the hardness of a streak plate.

    3.49 Yellow sulfur streak

    Steak color is a good diagnostic property because when a mineral is finely powdered, structural and other nonchemical effects are minimized. Calcite, for example, comes in many different colors, but calcite’s streak is always white. Pyrite (fool’s gold) is yellow but has a dark-colored streak, as does chalcopyrite. Gold, which has a color similar to pyrite’s (in hand specimen), has a yellow-gold streak.

    Streak can be extremely useful for telling dark-colored minerals apart, especially metallic ones. For example, hematite may be red, gray, or black in hand specimen and may or may not have a metallic luster. It always, however, has a diagnostic red streak that helps distinguish specular hematite from galena, or other kinds of hematite from similarly colored minerals.

    Unfortunately, most minerals have a white or colorless streak. Yet, streak is a key property for identifying a small number of minerals. The table below lists some minerals that often have diagnostic streak colors.

    Examples of Minerals that Do Not Have a White Streak
    mineral streak color
    magnetite, ilmenite
    lead gray
    pale or light brown
    yellow brown to ocher yellow
    white to light brown or yellow
    very pale blue to white
    light blue
    very pale blue to gray or tan
    pale green
    pale yellow to yellow
    light yellow to yellow
    orange or reddish yellow
    dark red to scarlet
    rust red to blood red

    This page titled 3.4.4: Streak is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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