Question

Salt is a mineral, but the plastic making up an inexpensive pen is not. Why not?

Short answer

Plastic in inexpensive pens is not a mineral because it is man-made, organic, has a variable composition, and lacks a definite crystalline structure. In contrast, salt is a mineral as it is naturally occurring, inorganic, has a specific chemical composition, and a definite crystalline structure.

Step-by-step solution

Define a Mineral

In order to differentiate between a mineral (salt) and non-mineral (plastic), we first need to understand the characteristics and definition of a mineral. A mineral is a naturally occurring inorganic substance that has a specific chemical composition and a definite crystalline structure.

Characteristics of a Mineral

Minerals have some characteristics that can help us distinguish them. These characteristics include: 1. Naturally occurring - Minerals are formed through natural geological processes. 2. Inorganic - Minerals are not made of organic compounds, meaning they do not come from living organisms. 3. Chemical composition - Each mineral has a specific chemical composition, which means it is made of certain chemical elements or compounds. 4. Crystalline structure - Minerals have a definite structure in which their atoms or ions are arranged in an organized and repeating pattern.

Characteristics of Plastic

Now, let's take a look at the characteristics of plastic to understand why it is not a mineral. 1. Man-made - Plastics are synthetic materials, which means that they are made by humans through chemical processes, not naturally occurring geological processes. 2. Organic - Plastics are made from organic materials, most commonly being derived from petroleum products. This means that the source of plastics is from living organisms (plants and animals). 3. Variable composition - Plastics can have different compositions, depending on the type and purpose of the plastic. 4. Non-crystalline structure - Most plastics do not have a definite crystalline structure as minerals do; instead, they often have an amorphous or disordered molecular arrangement.

Comparison and Conclusion

Based on the characteristics of minerals and plastic, we can conclude that plastic found in inexpensive pens is not a mineral because it is man-made, organic, has a variable composition, and does not have a definite crystalline structure. In contrast, salt is a mineral because it meets all the criteria: it is naturally occurring, inorganic, has a specific chemical composition, and a definite crystalline structure.

Key concepts

Mineral characteristics

Minerals are fascinating natural substances that exhibit unique and defining characteristics. To be classified as a mineral, a substance must meet several strict criteria:

• **Naturally Occurring**: Minerals form through natural geological processes. They cannot be synthetic or artificial.
• **Inorganic**: This means that minerals are not made from organic, or living, matter. They are purely chemical compounds.
• **Definite Chemical Composition**: Each mineral is made up of specific elements. These elements combine in certain proportions unique to each mineral.
• **Crystalline Structure**: Minerals have an ordered atomic arrangement. This crystalline construction dictates how the mineral will grow and form.

These characteristics distinguish minerals from other materials, such as glass or plastic, which do not meet these criteria.

Inorganic substances

Inorganic substances are those that are not derived from living organisms. They consist of mineral compounds that form independently of any biological process.

• **Origin**: Inorganic substances are typically formed by geological activities such as volcanic eruptions, sedimentation, and chemical precipitation.
• **Composition**: Although they include a wide range of compounds, they do not contain carbon-hydrogen bonds typical of organic matter.
• **Mineral Examples**: Common inorganic minerals include quartz, feldspar, and salts like halite (table salt).
• **Applications**: Many inorganic substances are vital for industry, like talc and graphite in manufacturing.

In contrast, organic substances, like those used for making plastics, are carbon-based and often derived from living matter.

Crystalline structure

The crystalline structure of a substance refers to the regular, repeating arrangement of atoms in a mineral. This organized pattern impacts various physical properties:

• **Determines Shape**: Many minerals naturally form crystals that reflect their internal symmetry, leading to distinct shapes, such as cubes or hexagons.
• **Affects Hardness**: The strength of the bonds between the atoms can influence how hard or soft a mineral is.
• **Influences Cleavage and Fracture**: Crystals may break along smooth planes (cleavage) or irregular surfaces (fracture) depending on their structural arrangement.
• **Optical Properties**: The internal arrangement of atoms affects how light passes through the crystal, impacting its color and luster.

These qualities contribute to the diagnostic process in mineral identification, setting mineral crystals apart from amorphous substances.

Synthetic materials

Synthetic materials, such as plastic, are artificially made and differ vastly from naturally occurring minerals. Unlike minerals, they do not originate from geological processes but are of human creation.

• **Manufacturing**: Synthetics like plastics are produced through chemical reactions. These often involve organic compounds derived from fossil fuels.
• **Variability**: Synthetic materials can be engineered to meet a wide array of needs, with tailored properties like flexibility, durability, and color.
• **Lack of Order**: Many synthetic substances, including plastics, lack a structured, crystalline form. They often have an amorphous, irregular molecular structure.
• **Eco-Friendly Innovations**: While traditionally made from petroleum, there is a growing movement towards bio-based synthetics that are more environmentally friendly.

Understanding the differences between synthetic and mineral materials helps highlight the diversity of materials available for various applications.