The answer to whether all minerals can produce a streak on a porcelain plate is no. The streak is a diagnostic property in mineral identification, defined as the color of a mineral when it is reduced to a fine powder. This property is revealed by rubbing the mineral across a specialized piece of unglazed porcelain called a streak plate. The test’s success depends entirely on the relative hardness of the mineral compared to the plate.
Defining the Mineral Streak Test
The streak test is performed by firmly scraping the mineral sample across the rough, unglazed surface of the porcelain plate. This action grinds a small amount of the mineral into a powder, leaving a line of residue whose color is the mineral’s streak. This powdered color is often a more reliable property for identification than the mineral’s external or “body” color.
A mineral’s surface color can be highly variable due to impurities, weathering, or structural defects. However, the chemical composition that determines the powder’s color remains consistent. For instance, hematite can appear black, silver, or reddish-brown, but its streak will consistently be reddish-brown, distinguishing it from other dark minerals. Similarly, pyrite, or “fool’s gold,” has a metallic yellow body color but leaves a greenish-black streak, immediately differentiating it from true gold, which leaves a yellow streak.
The Role of Mineral Hardness
The ability of a mineral to leave a streak is governed by its hardness relative to the porcelain plate. Mineral hardness is measured using the Mohs Hardness Scale, a relative scale from 1 (talc) to 10 (diamond). The unglazed porcelain used for a standard streak plate possesses a fixed hardness, typically falling between 6.5 and 7 on the Mohs scale.
A streak is only produced when the mineral being tested is softer than the plate. When a softer mineral is rubbed against the plate, the harder porcelain surface acts as an abrasive, pulverizing the mineral’s structure into a fine powder. This powder is the observed streak. If the mineral’s hardness is less than 6.5, the test works reliably, yielding the diagnostic powder color.
If a mineral is harder than the streak plate, the physical mechanism reverses. The mineral scratches the plate instead of being scratched. The mineral sample leaves no powdered residue of itself; instead, it gouges a faint line into the porcelain itself. This action may leave a mark that appears white or colorless, but this mark is actually the powdered dust of the porcelain plate, not the mineral. The plate’s hardness establishes a physical threshold for the streak test’s effectiveness.
Identifying Minerals That Leave No Streak
Minerals that are harder than the porcelain plate (Mohs hardness of 7 or greater) will fail to produce a true streak. A common example is quartz (hardness 7). Other hard minerals that scratch the plate include topaz (hardness 8), corundum (hardness 9), and diamond (hardness 10).
When a mineral does not leave a powder, the result is interpreted as having “no streak” or a “colorless streak.” This outcome is a valuable diagnostic property, indicating that the unknown mineral’s hardness is 7 or greater. For these harder samples, the streak test’s limitation necessitates the use of other identification methods. The primary technique used is the hardness test, where the mineral is tested against reference materials to pinpoint its exact value on the Mohs scale. The absence of a streak directs the identification process toward measuring greater physical resistance.