The "Acid Test" for carbonate Minerals and also Carbonate Rocks

Article by: Hobart M. King, PhD, RPG

Hydrochloric mountain on limestone: This video demonstrates what happens as soon as one fall of dilute (10%) hydrochloric mountain is inserted on a item of limestone. Security glasses and also gloves space recommended once doing the acid test.

You are watching: When in contact with hydrochloric acid, which mineral gives off bubbles of carbon dioxide gas?


What is the acid Test?

To many geologists, the term "acid test" way placing a drop of dilute (5% to 10%) hydrochloric acid on a rock or mineral and also watching because that bubbles the carbon dioxide gas to it is in released. The bubbles signal the existence of carbonate minerals such as calcite, dolomite, or one of the minerals listed in Table 1.


The bubbling relax of carbon dioxide gas can be therefore weak the you require a hand lens to observe solitary bubbles slowly farming in the autumn of hydrochloric mountain - or for this reason vigorous the a flash of effervescence is produced. This variations in effervescence vigor room a result of the type of lead carbonate minerals present, the amount of carbonate present, the bit size the the carbonate, and also the temperature that the acid.


Hydrochloric acid on limestone: This video clip demonstrates what happens once one drop of dilute (10%) hydrochloric acid is placed on a piece of limestone. Safety glasses and also gloves are recommended once doing the mountain test.


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Magnesite: The mineral magnesite, which has a chemical composition the MgCO3, will effervesce weakly with warmth hydrochloric acid and really weakly through cold acid. Specimen is about 6.4 cm across.


What reasons the Fizz?

Carbonate minerals space unstable in contact with hydrochloric acid. Once acid begins to effervesce (fizz) top top a specimen, a reaction similar to the one shown listed below is taking place.

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On the left next of this reaction, the mineral calcite (CaCO3) is in call with hydrochloric acid (HCl). These react to type carbon dioxide gas (CO2), water (H2O), liquified calcium (Ca++), and dissolved chlorine (Cl--). The carbon dioxide bubbles the you observe are proof that the reaction is taking place. When that occurs, calcite or another carbonate mineral is present.

Many various other carbonate minerals react v hydrochloric acid. Each of these minerals is composed of one or more metal ions combined with a carbonate ion (CO3--). The chemistry of this reactions is similar to the calcite reaction above. The mineral reacts through hydrochloric mountain to develop carbon dioxide gas, water, a dissolved metal ion, and dissolved chlorine. The reactions because that magnesite (MgCO3) and also siderite (FeCO3) are presented below.

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Calcite: This transparent specimen that calcite reflects cleavage the is characteristics of the mineral. Calcite, v a ingredient of CaCO3, will certainly react strongly through either cold or heat hydrochloric acid. Specimen measures around 10 centimeter across.

Acid reaction of carbonate Minerals
MineralChemical CompositionCold acid ReactionWarm mountain Reaction
AragoniteCaCO3strongstrong
AzuriteCu3(CO3)2(OH)2yesstrong
CalciteCaCO3strongstrong
DolomiteCaMg(CO3)2weakyes
MagnesiteMgCO3very weakweak
MalachiteCu2CO3(OH)2yesyes
RhodochrositeMnCO3weakyes
SideriteFeCO3very weakweak
SmithsoniteZnCO3weakyes
StrontianiteSrCO3yesyes
WitheriteBaCO3weakweak
Table 1: A list of commonly and also occasionally encountered lead carbonate minerals with their chemistry formula and also reactions to cold and also warm hydrochloric acid. Check results can vary since of weathering, vault testing, contamination, and specimen purity.

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The Vigor of carbonate Reactions

Careful observation is important because some carbonate minerals reaction vigorously and also others barely react with cold acid.

The lead carbonate mineral that is most frequently encountered by geologists is calcite (CaCO3). Calcite is a "ubiquitous" mineral. Ubiquitous means "found everywhere." Calcite wake up in igneous, metamorphic, and sedimentary rocks and also is the most generally encountered lead carbonate mineral. If you ar one fall of cold hydrochloric mountain on calcite, the whole drop of acid will certainly erupt through bubbles and also a vigorous fizz will last for a couple of seconds.

Dolomite CaMg(CO3)2 is an additional commonly encountered carbonate mineral. If you ar one autumn of cold hydrochloric mountain on a item of dolomite, the reaction is weak or no observed. Instead of seeing an noticeable fizz, friend will see a fall of acid on the surface of the mineral that might have a couple of bubbles of carbon dioxide gas slowly cultivation on the dolomite surface.

However, if heat acid is placed on dolomite, an evident fizz will occur. This occurs due to the fact that the acid and also rock react an ext vigorously at higher temperatures.

If you place a fall of hydrochloric acid on powdered dolomite, a clearly shows reaction will certainly occur. This is since the surface ar area has actually been increased, making more dolomite available to the acid. (You can conveniently make dolomite powder by scratching a specimen that dolomite across a streak plate. Then test the powder by placing a fall of hydrochloric acid on the powder. One more easy method to produce a small amount the mineral flour is to scratch the specimen v a nail.)

Different carbonate minerals have different responses to hydrochloric acid. A list of common and also occasionally encountered carbonate minerals is offered in Table 1 v their chemical composition and their loved one reaction v cold and also warm hydrochloric acid.

When a mineral has actually a weak solution to acid, you should be observant and patient to check out it. Because that example, magnesite has actually a an extremely weakreaction through cold HCl. If you powder a little amount the magnesite on a streak plate and place a drop of mountain on it, you might not check out any action for several seconds. Then, as tiny bubbles start to type on corpuscle of magnesite, the drop of mountain will appear to prosper larger in size. That occurs as carbon dioxide is liberated indigenous the mineral and also displaces the water. Observing the formation of bubbles v a hand lens deserve to be helpful.


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Dolostone: Dolostone is a sedimentary absent composed mostly of the mineral dolomite, which has a chemical composition that CaMg(CO3)2. Dolomite will certainly effervesce weakly through cold hydrochloric acid, producing a few bubbles. The reaction is more noticeable once the acid is warm and/or the rock is powdered. The specimen in the photo is about 10 centimeter across.


The acid Test ~ above Rocks

LIMESTONE, DOLOSTONE, and MARBLE

Some rocks contain lead carbonate minerals, and also the mountain test can be supplied to aid identify them. Limestone is composed virtually entirely that calcite and will produce a vigorous fizz through a fall of hydrochloric acid. Dolostone is a rock created of practically entirely of dolomite. The will create a an extremely weak fizz when a drop of cold hydrochloric acid is placed upon it, a much more obvious fizz once powdered dolostone is tested, and also a stronger fizz when hot hydrochloric mountain is used.

Limestone and also dolostone can be a little more complex. Castle are periodically composed of a mixture of calcite and dolomite and also have acid reactions that room deceptive. A dolostone deserve to contain enough calcite come fool you right into calling the a limestone. For these rocks the mountain test could not be sufficient for a confident identification - however at least you will recognize that the rock has actually a significant carbonate mineral content.

Marble is a limestone or a dolostone that has actually been metamorphosed. The will have an acid reaction that is comparable to the limestone or dolostone native which it to be formed.


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Other Applications the the "Acid Test": Geologists deserve to use dilute hydrochloric acid to help identify the cementing agent of sandstones. They ar a drop of dilute HCl top top the sandstone and closely observe. If calcite is the cementing agent, an effervescence will certainly occur and some the the sand grains might be liberated. A hand lens or small microscope is supplied to do the observations. The photo above is a amplified view of a piece of Oriskany Sandstone, an Ordovician-age absent unit from the Appalachian basin that serves as a organic gas reservoir and a organic gas warehouse unit. Oriskany sandstone is frequently cemented through calcite.


OTHER ROCKS that FIZZ

Always remember that "calcite is ubiquitous." (Ubiquitous method that it is found practically everywhere.)

Many rocks contain little amounts of calcite or various other carbonate minerals. All of these can create a fizz also though the lead carbonate is just a minor part of a rock"s composition. These rocks can contain tiny veins or crystals of lead carbonate minerals that produce a fizz in contact with acid. These veins and crystals have the right to be for this reason tiny that they space not clearly shows to the unaided eye. This small amount of carbonate can fizz the an initial time a autumn of mountain is applied but it is in depleted and not fizz if acid is used a second time to the same location on the rock.

Some sedimentary rocks space bound in addition to calcite or dolomite cement. Sandstone, siltstone, and also conglomerate sometimes have calcite cement that will produce a vigorous fizz v cold hydrochloric acid. Some conglomerates and also breccias contain clasts of lead carbonate rocks or minerals the react through acid.

Many shales to be deposited in maritime environments and contain sufficient calcium carbonate to produce a vigorous mountain fizz. This shales were created when mud was deposited in an environment comparable to or nearby to where limestone was formed. They room composed of sedimentary clay mineral intermixed with a small amount the calcite. They are recognized as "calcareous shales."

Don"t enable an acid fizz to guide the identification process. In many instances it will certainly instead add detail come your observation such as: "calcareous shale" or "sandstone with lead carbonate cement." This is beneficial information.


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Vinegar can be supplied for the acid test: Vinegar can be a safe, economical and also easy-to-obtain "acid" because that identifying calcite and also dolomite. Vinegar is dilute acetic acid that to produce a very weak reaction through calcite and also dolomite - ideal observed through a hand lens.

The "Vinegar Test"

Vinegar is a dilute acetic acid systems (about 5% to 10%) the produces a weak effervescent reaction through calcite and dolomite. It deserve to be used instead of hydrochloric mountain for introducing students come the mountain test. Vinegar is straightforward to obtain, inexpensive, and safer to usage than hydrochloric acid.The effervescence utilizing vinegar usually calls for a hand lens because that clear observation and also is just observable v carbonate mineralsthat have a strong reaction through hydrochloric acid. Vinegar is often used as soon as the mountain test is part of a precollege course. Protective gloves, glasses, paper towels, and immediate accessibility to one eyewash station are recommended.

EXTREME acid REACTIONS

A few rocks have the right to produce an extreme reaction v hydrochloric acid. These space usually rocks composed of calcite or aragonite withabundant pore room or extremely high surface areas. Part specimens the chalk, coquina, oolite, and also tufa room examples. As soon as a autumn of dilute hydrochloric mountain is placed on this specimens, an eruption of mountain foam can rise up turn off of the rock and spread come an unforeseen diameter. The reaction is really brief (and might not it is in repeatable), yet it is so sudden and also vigorous the it deserve to surprise one inexperienced person. This description is for one autumn of acid. If much more is offered an even an ext vigorous reaction will occur. (These too much reactions will certainly not happen with every specimen of this rocks. Be aware when experimentation them or presenting them to students for testing.)

The incredibly vigorous reaction of cold hydrochloric acid v these specimens occurs since the rocks room so porous or since they have actually a an extremely high surface area under a single drop of acid.

TEST UNWEATHERED MATERIAL

Calcite and also other lead carbonate minerals have actually a short resistance to weathering and also can be attacked by acids in organic waters and soils. When testing material that has been exposed at Earth"s surface, the is an extremely important to check unweathered material. A new surface have the right to usually be obtained by break the rock.

DECEIVED by POROSITY!

Some rocks space porous and also contain a reservoir that air. Small amounts of wait escaping right into a fall of acid from listed below can provide the figure of a gentle acid reaction. Don"t it is in fooled. If you place a autumn of acid on some sandstones, a few bubbles will emerge out of sharp spaces. It"s not a lead carbonate cement. To avoid this difficulty scratch the rock across a streak plate and test the flour or the seed that space produced.


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Contamination in Mineral identification Labs

When student are given minerals come identify, two instances can reason problems through their work.

1) In mineral identification labs, some students are ready to call any mineral that produces an acid reaction "calcite" or an additional carbonate. However, calcite is a ubiquitous mineral and also it is often existing as one intimate component of various other mineral specimens and rocks. These can produce a false acid reaction. To prevent being misled, students should constantly be cautioned to check a specimen"s identity with many properties. If a specimen fizzes v acid yet has a Mohs hardness that seven and also breaks with a conchoidal fracture, climate it certainly isn"t calcite!

Depending upon the endure of the students, specimens the are really true to your properties can be gift to the class, or specimens with some challenges can be used. Several minerals uncovered in the field will not be certain true to properties. It"s much better to discover that class in the lab and go right into the field with wisdom.

2) due to the fact that calcite is among the table of contents minerals that the Mohs Hardness Scale, it is often used to test the hardness of mineral specimens. This can place little amounts that calcite perhaps on every unknown specimen in the lab! Don"t assume the a solitary acid reaction is correct. Check the specimen in a second location if you suspect that contamination has actually occurred.

In a mineral to know lab, barite is frequently confused v calcite due to the fact that of contamination. The barite might naturally contain little amounts that calcite, or the hardness testing of a ahead student might have left little amounts that calcite on a barite specimen.Students space often attracted to one identification as "calcite" simply since of the acid test. If the mineral exhibits a little ofcleavage and also is not an extremely hard, then many students will certainly arrive at an not correct identification.


Acid check Safety

Hydrochloric acid, appropriately diluted come a 10% concentration, can reason irritation if it contact the skin or eyes. It can additionally fade clothing. Hydrochloric acid need to kept in clearly-labeled dispensing bottles and also used v quick and easy access to file towels, water, and an eyewash station. Security glasses and protective gloves room recommended. If skin call occurs, the area need to be purged with plenty of water. If eye contact occurs, the eye must be flushed for 15 minutes with plenty that water. If a call lens is worn, the eye should be flushed, contact lens removed, and also flushing continued. Seek prompt medical attention because that eye contact.Specimens that space tested through acid should be rinsed after testing to eliminate or dilute unreacted acid.

Limiting Frivolous Acid usage in Labs

Most students space intrigued v the mountain test and want to try it. To border frivolous acid use, students should be instructed to use a single drop of acid for the test and to only test specimens once carbonate minerals space suspected. If the is no done, part students will usage the acid frivolously. This habits is motivated if the classroom is fitted with large acid bottles that are filled come the top. However, if the mountain bottles are tiny and virtually empty in ~ the start of class, students commonly ration their usage of the acid to suitable amounts. Small, nearly empty party makes less acid obtainable to spill.


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Acid dispensing bottles: little acid dispensing bottles work-related well because that the mountain test. Lock dispense the mountain one-drop-at-a-time and also will no spill if they room knocked over. If you space a teacher supervising the mountain test in a classroom, provide students little bottles that are almost empty. The will mitigate the quantity of frivolous acid use that can otherwise occur. Brand the bottles plainly and instruct students in mountain use prior to making them available.


Acid party Selection

The form of bottle selected for dispensing the mountain is important. Laboratory supply stores offer bottles that space designed for dispensing acidone-drop-at-a-time. The lid is always on these bottles (except once they room being cleaned or refilled), and also they execute not produce a pour out whenthey are knocked over. Bottles with a removable lid that has actually a squeeze pear dispenser will certainly be sometimes knocked over once the lid is offif they space being provided by normal humans.

Acid dispensing bottles must be make of rigid plastic v a little opening which permits acid to be conveniently dispensed one-drop-at-a-time. Soft dispensing bottles or bottles with a larger opening can dispense a big amount that acid with an accidental squeeze.

Sources that Hydrochloric Acid

Hydrochloric acid diluted come a 10% equipment cannot it is in purchased in most communities. The finest place to acquisition commercially ready solutions is from a activities supply company. Purchasing that ready-for-use is the recommended means to obtain it. Don"t shot to prepare your very own solution if friend don"t know precisely what you are doing and have an fitted laboratory. Her chemistry department might be able to assist you through ordering acid. Part generous chemists will certainly prepare a 10% solution for you.

Mineral Specimens together "Consumables"

Mineral specimens that are provided properly in the scientific research classroom or activities will should be changed frequently.Students will be investigating them with hardness tests, streak tests, mountain tests and also other experiments. All of thesetests damages the specimen and make it less fit because that the next team of students. To save the acid test native fouling your whole collection, asking students come rinse specimens after testing with acid and also limit testing to only as soon as it is needed.


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