Which two minerals can be scratched with your fingernail?

The two minerals that can be scratched with your fingernail are talc and gypsum. These are among the softest minerals found in nature.

Understanding Mineral Hardness

Mineral hardness is a measure of its resistance to scratching, primarily determined by the strength of the bonds between its atoms. The most common way to measure this is using the Mohs Hardness Scale, which ranks minerals from 1 (softest) to 10 (hardest) based on their ability to scratch one another.

On this scale, common objects have an approximate hardness:

• Your fingernail: Approximately 2.5
• Copper penny: Approximately 3.5
• Steel knife: Approximately 5.5
• Glass: Approximately 5.5

A mineral can be scratched by any object that has an equal or higher Mohs hardness value. Since your fingernail has a hardness of about 2.5, it can easily scratch minerals with a hardness of 2 or lower.

The Softest Minerals: Talc and Gypsum

Talc and gypsum are notably soft because their atomic structures allow for relatively weak bonds, making them susceptible to scratching by everyday objects like a fingernail.

Talc

• Mohs Hardness: 1
• Characteristics: Talc is the softest known mineral. It often has a soapy or greasy feel and can be white, gray, or pale green. It forms from the alteration of magnesium-rich minerals like serpentine or pyroxene.
• Common Uses: Due to its softness, smoothness, and ability to absorb moisture, talc is widely used in products like talcum powder, cosmetics, ceramics, paints, and as a filler in paper and rubber.

Gypsum

• Mohs Hardness: 2
• Characteristics: Gypsum is also a very soft mineral, typically white or colorless, and can form in various crystalline structures. It often occurs in sedimentary rock formations, especially in evaporite deposits where ancient seas have dried up.
• Common Uses: Gypsum is essential in the construction industry. It's the primary component of plaster of Paris, drywall (also known as plasterboard or Sheetrock), and agricultural soil conditioners.

Why Your Fingernail Can Scratch Them

As your fingernail has a hardness of roughly 2.5, it is harder than talc (1) and gypsum (2). This difference in hardness means that when you drag your fingernail across the surface of these minerals, the stronger bonds in your fingernail are able to break the weaker bonds in the mineral, resulting in a visible scratch.

Here's a quick comparison:

This demonstrates why your fingernail is an effective tool for identifying these two particularly soft minerals.