what does the acid test tell you about a mineral

Some minerals react with acid, but what does the acid test tell you about a mineral? Minerals can be found in many different things.

They can be found in food and water, but in this article, we will be mainly looking at minerals in rocks and stones. And how tests, like the acid test, can help with determining the type of mineral and rock.

Geologists use the acid test often to determine the type of mineral they’re working with. The acid test can be a great help in determining the type of mineral. Rocks are built up of two or more minerals, so the acid test can help in determining the type of rock.

What are Minerals?

A mineral is a chemical element or a combination of multiple chemical elements that form and exist in nature.

Minerals are solids, but they can be dissolved in water. In drinking water, there are small concentrations of minerals present.

Some metals or rocks are built up of one sole mineral. For example, gold and silver are minerals.

rock vs minerals

A pure piece of silver is built up solely of silver minerals (Ag). These types of minerals are called native elements.

Structure of Minerals

When the atoms, molecules and/or ions that form a mineral rank themselves in a neat and orderly way, they are called crystal lattices.

Based on how the mineral ranks itself, the structure as we see it will change. For example, salt is a mineral formed by cubically ordered atoms: Natrium (Na) and Chloride (Cl) form a structured new molecule: NaCl.

The composition of a mineral decides the properties of the mineral, for example, the colour and the hardness of the mineral.

The latter is expressed on Moh’s scale. From 1 to 10 where 10 are the materials the most resistant against scratches.

Diamond is a mineral build-up of very well-structured Carbon (C) atoms, it scores a 10 on the scale, and with that is generally seen as one of the strongest materials.

Minerals are built up of one or more chemical elements. Every mineral has different properties, partially based on its structure.

Minerals in Rocks

minerals rocks

Rocks are built up of more than one mineral. If a “rock” is built up of one mineral, like quartz, for example, it is a pure mineral, technically not a rock.

Minerals are most often completely bound with other minerals into the rock. But sometimes they can form distinct layers in the rock, like with schist for example.

The minerals in rocks are generally mixed up by large geological processes over a long time span.

These processes include but are not limited to volcanism, plate tectonics and earthquakes.

Rock Types

There are three main categories of rocks:

  • Magmatic Rocks
  • Sedimentary Rocks
  • Metamorphic Rocks
types of rocks

1. Magmatic Rocks

The magmatic rocks are pretty self-explanatory. They form when magma solidifies. If that happens deep inside the ground, rocks like granite and porphyry form.

When magma reaches the surface, it’s called lava. If this lava solidifies, it forms rocks like basalt, obsidian and pumice stone.

2. Sedimentary Rocks

The sedimentary rocks are split up into two categories. There are loose rocks like sand, clay and gravel.

These rocks form layers when deposited by rivers into large water basins, like oceans and seas.

When there is pressure on such a layer, the rock goes through a phase of diagnosis. The resulting rocks are claystone, conglomerate and sandstone.

There are also sedimentary rocks formed by biological processes, like coal and limestone.

3. Metamorphic Rocks

Finally, there are the metamorphic rocks. As the name suggests, these rocks are formed when going through a metamorphic phase.

When the solid types of sedimentary rocks or granite are under high pressure, the bindings inside the rock change. The resulting rocks are marble (from limestone), slate (from claystone) and gneiss (from granite).

There are a lot of different rock types. The categories are obtained by the origin and phase of the rocks.

What is the Acid Test?

The acid test is a test that geologists use to not only help determine the type of minerals present in a rock but also help in deriving the type of rock itself based on the types of minerals.

The acid test isn’t a conclusive test. It only indicates if a certain type of mineral is present in the rock.

The acid that is mostly used for the acid test is hydrochloric acid (HCl), with a concentration between 5% and 10%.

When a small amount of this substance is dropped onto the rock, it can either do nothing or start bubbling.

What Does the Acid Test Tell You About a Mineral?

When there is a bubbling effect when the acid is dropped onto the rock, it indicates the presence of carbonate minerals (minerals that contain the carbonate ion: CO3).

The main carbonate minerals are:

  • Aragonite (CaCO3)
  • Calcite (CaCO3)
  • Dolomite (CaMg(CO3)2)
  • Azurite (Cu3(CO3)2(OH)2)

This isn’t the full list of carbonate minerals, but these are the most important. On top of that, these minerals have the strongest reactions with the acid.

For other carbonate minerals, the reaction is sometimes too weak to even observe with the naked eye.

A magnifying glass may be needed to see the bubbling in those cases. To strengthen the reaction, you could warm the acid, the reaction is strengthened when using higher temperatures.

The acid test consists of pouring acid over a mineral or rock. If it starts bubbling, carbonate minerals are present.

The Chemics Behind the Acid Test

When the mineral comes into contact with the acid, they initiate a chemical reaction.

The mineral breaks apart, and forms, together with the HCl four new compounds. Aside from the Cl ions and Metal ions, water (H2O) and carbon dioxide (CO2) are formed.

Carbon dioxide is a gas, in combination with the water that is formed too, the bubbles arise.

Some minerals react more heavily, resulting in more bubbles forming. Hereunder is an example of the reaction between Magnesite (MgCO3) and HCl.

acid test of magnesite

The Acid Test and Rocks

Rocks contain two or more minerals. You can perform the acid test on a rock to discover if there are carbonate minerals present in the rock.

This solely won’t tell you which type of rock it is, but in combination with other determination methods, it can help.

The acid test can be a very important tool to determine if a mineral is a carbonate mineral. It can also be an important step in the determination of rocks because there are many rock types built up of different minerals with different properties.

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Final Thoughts

To get to the answer to the question: what does the acid test tell you about a mineral, we had to cross different disciplines.

The acid test is a perfect example of chemics used in other disciplines. Geologists are dependent on the chemics behind the reaction between the mineral and acid to help determine minerals and rocks.

It’s important to be said that the acid test isn’t a definitive test, you can only use it to determine if there are carbonate minerals in the rock, it can’t be used to determine the rock type.

But it’s perfect in a sequence of determination experiments and is used that way most of the time.