Why calcite fizzes

Limestone is a very common sedimentary rock composed mostly of the mineral calcite. Most limestones have a marine origin, being formed by the accumulation of shells and shell fragments of once-living marine organisms such as molluscs, bryozoans and corals.

Other limestones, such as cold-seep carbonates, have been derived from non-biogenic mineral formation as a result of direct crystallisation of calcium carbonate from water. Find out more in the articles Limestone origins and New Zealand limestone origins. If most limestones have a biogenic origin, it may be of value to discover how marine organisms like bryozoans and molluscs extract calcium carbonate from seawater and lay it down in crystalline form to form a hard protective outer casing or shell.

By understanding the biological and chemical principles that are at work, these natural processes may be able to be reproduced in the lab to develop a new generation of materials for everyday use. Find out more in the article Calcium carbonate biomineralisation. Calcium carbonate has a wide range of uses, and a study of its physical and chemical properties will help to explain why it has so many applications. Find out more in the article Carbonate chemistry. Pure lime is calcium oxide, which is produced industrially by the thermal decomposition of limestone.

Find out more in the article Lime — a time-tested chemical. Limestone is quarried for aggregate and building stone for use in road and building construction, as well as being used for making agricultural fertiliser, cement, steel and glass.

High-purity limestone is used as a mineral filler in paper, paint, plastics, rubber and carpet backings as well as in water treatment and as a dusting agent in coal mining to prevent fires. Find out more in the article Limestone uses.

When testing material that has been exposed at Earth's surface, it is very important to test unweathered material. A fresh surface can usually be obtained by breaking the rock. Some rocks are porous and contain a reservoir of air. Small amounts of air escaping into a drop of acid from below can give the appearance of a gentle acid reaction. Don't be fooled. If you place a drop of acid on some sandstones, a few bubbles will emerge out of pore spaces.

It's not a carbonate cement. To avoid this problem scratch the rock across a streak plate and test the powder or the grains that are produced. The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology. However, calcite is a ubiquitous mineral and it is often present as an intimate part of other mineral specimens and rocks.

These can produce a false acid reaction. To avoid being misled, students should always be cautioned to confirm a specimen's identity with multiple properties. If a specimen fizzes with acid but has a Mohs hardness of seven and breaks with a conchoidal fracture, then it certainly isn't calcite! Depending upon the experience of the students, specimens that are very true to their properties can be presented to the class, or specimens with some challenges can be used. Lots of minerals found in the field will not be absolutely true to properties.

It's better to learn that lesson in the lab and go into the field with wisdom. This can place small amounts of calcite potentially on every unknown specimen in the lab! Don't assume that a single acid reaction is correct.

Test the specimen in a second location if you suspect that contamination has occurred. In a mineral identification lab, barite is commonly confused with calcite because of contamination. The barite might naturally contain small amounts of calcite, or the hardness testing of a previous student might have left small amounts of calcite on a barite specimen. Students are often drawn to an identification as "calcite" simply because of the acid test. If that mineral exhibits a bit of cleavage and is not very hard, then many students will arrive at an incorrect identification.

Most students are intrigued with the acid test and want to try it. To limit frivolous acid use, students should be instructed to use a single drop of acid for the test and to only test specimens when carbonate minerals are suspected.

If that is not done, some students will use the acid frivolously. This behavior is encouraged if the classroom is equipped with large acid bottles that are filled to the top. However, if the acid bottles are small and nearly empty at the beginning of class, students usually ration their use of the acid to appropriate amounts. Small, nearly empty bottles makes less acid available to spill. Acid dispensing bottles: Small acid dispensing bottles work well for the acid test.

They dispense the acid one-drop-at-a-time and will not spill if they are knocked over. If you are a teacher supervising the acid test in a classroom, give students small bottles that are nearly empty. That will reduce the amount of frivolous acid use that might otherwise occur.

Label the bottles clearly and instruct students in acid use before making them available. The type of bottle selected for dispensing the acid is important. Laboratory supply stores sell bottles that are designed for dispensing acid one-drop-at-a-time. The lid is always on these bottles except when they are being cleaned or refilled , and they do not produce a spill when they are knocked over.

Bottles with a removable lid that has a squeeze bulb dispenser will be occasionally knocked over when the lid is off if they are being used by normal humans. Acid dispensing bottles should be made of rigid plastic with a small opening which allows acid to be easily dispensed one-drop-at-a-time. Soft dispensing bottles or bottles with a larger opening can dispense a large amount of acid with an accidental squeeze. The best place to purchase commercially prepared solutions is from a laboratory supply company.

Purchasing it ready-for-use is the recommended way to obtain it. Don't try to prepare your own solution if you don't know exactly what you are doing and have an equipped laboratory. Your chemistry department might be able to assist you with ordering acid. Mineral specimens that are used properly in the science classroom or laboratory will need to be replaced frequently.

Students will be investigating them with hardness tests, streak tests, acid tests and other experiments. All of these tests damage the specimen and make it less fit for the next group of students. To keep the acid test from fouling your entire collection, ask students to rinse specimens after testing with acid and limit testing to only when it is needed. Test results can vary because of weathering, previous testing, contamination, and specimen purity.

It can be used instead of hydrochloric acid for introducing students to the acid test. Vinegar is easy to obtain, inexpensive, and safer to use than hydrochloric acid. The effervescence using vinegar usually requires a hand lens for clear observation and is only observable with carbonate minerals that have a strong reaction with hydrochloric acid.

Vinegar is often used when the acid test is part of a precollege course. Protective gloves, glasses, paper towels, and immediate access to an eyewash station are recommended. The only problem with weak acids like lemon juice and vinegar is that sometimes it is harder to see the reaction the bubbles. The best way to fix this problem is to create a fresh surface for the kids to test.

When you scratch the surface you are removing some of the older weathered exterior or areas that were already tested and giving the children a fresh surface to test. Once they scratch off a small area to test, they can use a dropper bottle or a straw to place a few drops of lemon juice or vinegar on the surface. If it bubbles, you know that there is the mineral calcite in your rock. This is a great test for limestones and marbles which are made completely of calcite.

If you need help teaching kids how to identify rocks and minerals check out this book which will give you all of the details you need. Filed under: Education , Rocks on September 27th,