Copper is a metal recognized for its exceptional electrical conductivity and malleability. It is a native element, meaning it can be found in nature in its uncombined metallic form, making it a mineral. Identifying copper and other minerals relies on evaluating distinct physical properties. One of the most reliable of these properties is the mineral’s “streak,” which reveals the substance’s true color beneath any surface changes.
Understanding the Mineral Streak
The mineral streak is defined as the color of a mineral when it is reduced to a fine powder. This property is determined by conducting a simple test. The methodology involves scraping a mineral sample firmly across a piece of unglazed porcelain, commonly referred to as a streak plate.
This action creates a powdered line on the surface of the plate, and the color of that powder is the mineral’s characteristic streak. The streak test is often a more trustworthy diagnostic tool than observing the mineral’s surface color. The external appearance of a mineral can be misleading due to trace impurities, weathering, or surface tarnish.
Surface tarnish, caused by exposure to the elements, can drastically alter the outward color of a specimen. However, when the mineral is ground into a powder, the effect of these surface impurities is diminished. This consistency makes the streak color a fundamental and reliable characteristic for mineral identification, even when the specimen is highly altered.
The True Powder Color of Copper
The streak of native copper is described as a metallic copper-red or a bright reddish-brown. Some descriptions specify a pink, rose, or light copper color, reflecting the distinct hue of the pure metal. This specific color is a direct result of copper’s elemental nature, as the powder retains the appearance of the freshly exposed, untarnished metal.
This true powder color often contrasts sharply with the appearance of a naturally occurring copper specimen. Exposed native copper frequently develops tarnish (dull brown, black, or vibrant green/blue) due to oxidation and the formation of secondary copper minerals like malachite and azurite. The streak test effectively bypasses this surface alteration, revealing the underlying elemental color.
Native copper is relatively soft, possessing a Mohs hardness ranging from 2.5 to 3.0. This low hardness allows the streak test to be performed easily, as the mineral is softer than the unglazed porcelain streak plate (hardness 6.5 to 7). The softness ensures the copper is ground into a powder rather than scratching the streak plate, which is a common issue with much harder minerals.
Identifying Copper Using the Streak Test
The streak test serves as a powerful diagnostic tool for distinguishing native copper from minerals that share a similar appearance. For instance, the brassy yellow mineral chalcopyrite, a common copper sulfide ore, can be mistaken for native copper, but its streak color is blackish-green. The clear difference in streak color immediately separates the two.
The consistency of the copper-red streak is valuable because it remains the same regardless of how weathered or oxidized the sample is. A piece of native copper covered in green tarnish will still produce the same reddish streak as a freshly polished sample. This reliability makes the streak an important factor in confirming the identity of a specimen.
For geologists and prospectors, the streak test is a quick method for ruling out visually similar but chemically distinct substances. It helps differentiate native copper from other metallic-looking minerals and oxidized copper ores. The characteristic metallic copper-red powder is a definitive indicator of the presence of the native element.