The "Acid Test" for carbonate Minerals and Carbonate Rocks

Article by: Hobart M. King, PhD, RPG

Hydrochloric mountain on limestone: This video clip demonstrates what happens as soon as one drop of dilute (10%) hydrochloric mountain is put on a item of limestone. Protective glasses and also gloves are recommended as soon as doing the acid test.

You are watching: When carbonate minerals come into contact with hydrochloric acid they


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 absent or mineral and watching for bubbles of 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 detailed in Table 1.


The bubbling release of carbon dioxide gas deserve to be for this reason weak the you need a hand lens come observe solitary bubbles slowly farming in the fall of hydrochloric acid - or so vigorous that a flash of effervescence is produced. This variations in effervescence vigor are a result of the form of lead carbonate minerals present, the quantity of lead carbonate present, the bit size the the carbonate, and also the temperature the the acid.


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


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Magnesite: The mineral magnesite, which has actually a chemistry composition that MgCO3, will effervesce weakly with warmth hydrochloric mountain and really weakly v cold acid. Specimen is around 6.4 cm across.


What causes the Fizz?

Carbonate minerals are unstable in contact with hydrochloric acid. As soon as acid starts 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 side of this reaction, the mineral calcite (CaCO3) is in contact with hydrochloric acid (HCl). These reaction to type carbon dioxide gas (CO2), water (H2O), dissolved calcium (Ca++), and also dissolved chlorine (Cl--). The carbon dioxide bubbles the you watch are proof that the reaction is acquisition place. Once that occurs, calcite or an additional carbonate mineral is present.

Many various other carbonate mineral react through hydrochloric acid. Each of these minerals is composed of one or much more metal ions linked with a lead carbonate ion (CO3--). The chemistry of these reactions is similar to the calcite reaction above. The mineral reacts v hydrochloric acid to produce carbon dioxide gas, water, a liquified metal ion, and dissolved chlorine. The reactions for magnesite (MgCO3) and also siderite (FeCO3) are displayed below.

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Calcite: This transparent specimen that calcite mirrors cleavage that is properties of the mineral. Calcite, through a composition of CaCO3, will certainly react strongly through either cold or warmth hydrochloric acid. Specimen measures around 10 centimeter across.

Acid reaction of lead carbonate Minerals
MineralChemical CompositionCold acid ReactionWarm acid 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 occasionally encountered lead carbonate minerals through their chemistry formula and also reactions to cold and warm hydrochloric acid. Test results deserve to vary because of weathering, vault testing, contamination, and specimen purity.

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The Vigor of carbonate Reactions

Careful monitoring is important because some lead carbonate minerals reaction vigorously and also others proper react with cold acid.

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

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

However, if warm acid is put on dolomite, an noticeable 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 mountain on powdered dolomite, a clearly shows reaction will occur. This is because the surface ar area has been increased, making more dolomite easily accessible to the acid. (You can easily make dolomite flour by scratching a specimen of dolomite throughout a streak plate. Climate test the powder by place a drop of hydrochloric acid on the powder. An additional easy method to create a tiny amount that mineral powder is to scratch the specimen with a nail.)

Different lead carbonate minerals have various responses come hydrochloric acid. A list of common and occasionally encountered carbonate minerals is given in Table 1 v their chemistry composition and also their relative reaction v cold and warm hydrochloric acid.

When a mineral has a weak response to acid, you should be observant and also patient to watch it. For example, magnesite has actually a very weakreaction v cold HCl. If you flour a little amount of magnesite top top a streak plate and place a fall of mountain on it, you could not check out any activity for number of seconds. Then, together tiny bubbles begin to kind on corpuscle of magnesite, the drop of acid will show up to prosper larger in size. That occurs as carbon dioxide is liberated from the mineral and displaces the water. Observing the development of bubbles through a hand lens have the right to be helpful.


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Dolostone: Dolostone is a sedimentary absent composed mainly of the mineral dolomite, which has a chemistry composition of CaMg(CO3)2. Dolomite will effervesce weakly v cold hydrochloric acid, developing a few bubbles. The reaction is much more noticeable when the mountain is warmth and/or the stone is powdered. The specimen in the picture is about 10 centimeter across.


The mountain Test ~ above Rocks

LIMESTONE, DOLOSTONE, and also MARBLE

Some rocks contain lead carbonate minerals, and the acid test can be offered to assist identify them. Limestone is composed nearly entirely of calcite and will create a vigorous fizz through a autumn of hydrochloric acid. Dolostone is a rock written of almost entirely of dolomite. It will develop a really weak fizz once a drop of cold hydrochloric mountain is inserted upon it, a much more obvious fizz as soon as powdered dolostone is tested, and a stronger fizz when warm hydrochloric mountain is used.

Limestone and also dolostone deserve to be a little more complex. Castle are sometimes composed that a mixture the calcite and also dolomite and have acid reactions that are deceptive. A dolostone can contain sufficient calcite come fool you right into calling it a limestone. Because that these rocks the mountain test could not be enough for a i was sure identification - but at least you will understand that the rock has actually a significant carbonate mineral content.

Marble is a limestone or a dolostone that has been metamorphosed. That will have an mountain reaction that is similar to the limestone or dolostone native which it to be formed.


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Other Applications of the "Acid Test": Geologists deserve to use dilute hydrochloric mountain to assist identify the cementing agent of sandstones. They ar a drop of dilute HCl on the sandstone and closely observe. If calcite is the cementing agent, an effervescence will occur and some that the sand grains might be liberated. A hand lens or small microscope is supplied to make the observations. The photo over is a intensified view that a item of Oriskany Sandstone, one Ordovician-age rock unit from the Appalachian basin that serves as a natural gas reservoir and also a natural gas warehouse unit. Oriskany sandstone is regularly cemented by calcite.


OTHER ROCKS that FIZZ

Always remember the "calcite is ubiquitous." (Ubiquitous means that that is found practically everywhere.)

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

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

Many shales were deposited in marine environments and also contain sufficient calcium carbonate to produce a vigorous acid fizz. This shales were developed when mud was deposit in an environment similar to or nearby to where limestone to be formed. They space composed of sedimentary clay mineral intermixed through a little amount of calcite. Lock are recognized as "calcareous shales."

Don"t allow an acid fizz to overview the identification process. In many situations it will instead add detail come your observation such as: "calcareous shale" or "sandstone with carbonate cement." This is an useful information.


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

The "Vinegar Test"

Vinegar is a dilute acetic acid equipment (about 5% to 10%) the produces a weak effervescent reaction with calcite and also dolomite. It can be used rather of hydrochloric mountain for presenting students to the mountain test. Vinegar is basic to obtain, inexpensive, and safer to usage than hydrochloric acid.The effervescence utilizing vinegar usually requires a hand lens for clear observation and also is just observable with carbonate mineralsthat have actually a strong reaction with hydrochloric acid. Vinegar is frequently used once the acid test is part of a precollege course. Protective gloves, glasses, paper towels, and immediate accessibility to an eyewash station room recommended.

EXTREME mountain REACTIONS

A couple of rocks can produce an extreme reaction with hydrochloric acid. These room usually rocks created of calcite or aragonite withabundant pore an are or incredibly high surface areas. Some specimens that chalk, coquina, oolite, and tufa space examples. Once a autumn of dilute hydrochloric mountain is inserted on this specimens, one eruption of mountain foam deserve to rise up turn off of the rock and also spread come an unexpected diameter. The reaction is very brief (and may not be repeatable), yet it is so sudden and vigorous the it can surprise an inexperienced person. This description is for one drop of acid. If much more is used an even an ext vigorous reaction will certainly occur. (These too much reactions will certainly not take place with every specimen of this rocks. Be conscious when experimentation them or presenting them to students for testing.)

The incredibly vigorous reaction that cold hydrochloric acid with these specimens occurs because the rocks room so porous or due to the fact that they have actually a very high surface area under a solitary drop that acid.

TEST UNWEATHERED MATERIAL

Calcite and other carbonate minerals have a short resistance to weathering and can be attacked by acids in natural waters and also soils. When experimentation material that has actually been exposed in ~ Earth"s surface, that is really important to test unweathered material. A new surface deserve to usually be derived by breaking the rock.

DECEIVED through POROSITY!

Some rocks space porous and contain a reservoir of air. Little amounts of wait escaping right into a drop of mountain from listed below can give the appearance of a gentle mountain reaction. Don"t be fooled. If you place a autumn of mountain on part sandstones, a couple of bubbles will emerge out of sharp spaces. It"s not a carbonate cement. To prevent this difficulty scratch the rock across a streak plate and also test the powder or the seed that room produced.


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The best means to learn about minerals is to examine with a repertoire of small specimens that you deserve to handle, examine, and also observe their properties. Cheap mineral collection are obtainable in the beer-selection.com Store.


Contamination in Mineral to know Labs

When students are given minerals come identify, two instances can cause problems with their work.

1) In mineral identification labs, part students are ready to call any mineral the produces an acid reaction "calcite" or another carbonate. However, calcite is a common mineral and it is often current as an intimate component of various other mineral specimens and rocks. This can create a false mountain reaction. To avoid being misled, student should constantly be cautioned to check a specimen"s identity with many properties. If a specimen fizzes v acid however has a Mohs hardness that seven and also breaks with a conchoidal fracture, climate it absolutely isn"t calcite!

Depending top top the suffer of the students, specimens the are an extremely true to their properties can be gift to the class, or specimens v some difficulties can it is in used. Numerous minerals discovered in the field will not be absolutely true to properties. It"s much better to learn that lesson in the lab and go into the ar with wisdom.

2) since calcite is one of the table of contents minerals of the Mohs Hardness Scale, the is frequently used to test the hardness the mineral specimens. This have the right to place tiny amounts of calcite potentially on every unknown specimen in the lab! Don"t assume the a solitary acid reaction is correct. Check the specimen in a 2nd location if you doubt that contamination has actually occurred.

In a mineral identification lab, barite is frequently confused v calcite since of contamination. The barite could naturally contain little amounts of calcite, or the hardness experimentation of a ahead student can have left tiny amounts that calcite ~ above a barite specimen.Students space often attracted to one identification as "calcite" simply due to the fact that of the mountain test. If the mineral exhibits a little bit ofcleavage and also is not very hard, then numerous students will arrive in ~ an incorrect identification.


Acid test Safety

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

Limiting Frivolous Acid use in Labs

Most students space intrigued with the acid test and also want to shot it. To limit frivolous acid use, students must be instructed to use a single drop of mountain for the test and also to just test specimens as soon as carbonate minerals room suspected. If that is no done, some students will usage the acid frivolously. This behavior is motivated if the classroom is equipped with huge acid party that room filled to the top. However, if the acid bottles are small and virtually empty in ~ the start of class, students usually ration their use of the mountain to proper amounts. Small, almost empty party makes much less acid accessible to spill.


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Acid dispensing bottles: tiny acid dispensing bottles occupational well for the acid test. Lock dispense the acid one-drop-at-a-time and also will not spill if they are knocked over. If you space a teacher supervising the mountain test in a classroom, give students little bottles the are virtually empty. That will alleviate the quantity of frivolous acid usage that might otherwise occur. Brand the bottles clearly and instruct college student in mountain use before making castle available.


Acid party Selection

The type of bottle selected for dispensing the mountain is important. Laboratory supply stores sell bottles that space designed because that dispensing acidone-drop-at-a-time. The lid is constantly on these bottles (except when they space being cleaned or refilled), and they carry out not produce a spill whenthey space knocked over. Bottles v a removable lid that has actually a squeeze pear dispenser will be periodically knocked over as soon as the lid is offif they are being provided by regular humans.

Acid dispensing bottles should be do of strict plastic through a little opening which permits acid to be easily dispensed one-drop-at-a-time. Soft dispensing party or bottles with a larger opening deserve to dispense a huge amount the acid through an inadvertently squeeze.

Sources that Hydrochloric Acid

Hydrochloric acid diluted come a 10% equipment cannot it is in purchased in many communities. The ideal place to acquisition commercially all set solutions is native a laboratory supply company. Purchasing the ready-for-use is the recommended means to achieve it. Don"t shot to prepare your very own solution if girlfriend don"t know exactly what you room doing and have an fitted laboratory. Her chemistry room might be able to assist you through ordering acid. Part generous chemists will prepare a 10% equipment for you.

Mineral Specimens together "Consumables"

Mineral specimens that are used properly in the scientific research classroom or laboratory will need to be changed frequently.Students will be investigating them through hardness tests, streak tests, acid tests and also other experiments. All of thesetests damage the specimen and make it much less fit for the next group of students. To save the acid test indigenous fouling your whole collection, asking students come rinse specimens after trial and error with acid and limit experimentation to only as soon as it is needed.


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