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 acid is placed on a piece of limestone. Safety glasses and also gloves are recommended once doing the mountain test.

You are watching: Which common mineral fizzes when dilute hydrochloric acid is placed on it


What is the mountain Test?

To most geologists, the ax "acid test" method placing a autumn of dilute (5% come 10%) hydrochloric acid on a rock or mineral and watching because that bubbles that carbon dioxide gas to it is in released. The bubbles signal the presence of carbonate minerals such as calcite, dolomite, or among the minerals noted in Table 1.


The bubbling release of carbon dioxide gas deserve to be therefore weak that you need a hand lens to observe single bubbles slowly growing in the drop of hydrochloric mountain - or for this reason vigorous the a flash of effervescence is produced. These variations in effervescence vigor are a result of the type of carbonate minerals present, the quantity of lead carbonate present, the particle size the the carbonate, and also the temperature that the acid.


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


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Magnesite: The mineral magnesite, which has a chemical composition the MgCO3, will effervesce weakly with heat hydrochloric acid and an extremely weakly with cold acid. Specimen is around 6.4 cm across.


What reasons the Fizz?

Carbonate minerals are unstable in call with hydrochloric acid. When acid starts to effervesce (fizz) ~ above a specimen, a reaction similar to the one shown listed below is acquisition 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 kind carbon dioxide gas (CO2), water (H2O), liquified calcium (Ca++), and also dissolved chlorine (Cl--). The carbon dioxide bubbles that you watch are evidence that the reaction is taking place. Once that occurs, calcite or another carbonate mineral is present.

Many other carbonate mineral react v hydrochloric acid. Each of this minerals consists of one or more metal ions linked with a carbonate ion (CO3--). The chemistry of this reactions is comparable to the calcite reaction above. The mineral reacts through hydrochloric acid to create carbon dioxide gas, water, a liquified metal ion, and also dissolved chlorine. The reactions because that magnesite (MgCO3) and also siderite (FeCO3) are shown below.

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Calcite: This transparent specimen that calcite reflects cleavage that is characteristics of the mineral. Calcite, with a composition of CaCO3, will certainly react strongly with either cold or heat hydrochloric acid. Specimen measures about 10 cm across.

Acid reaction of carbonate Minerals
MineralChemical CompositionCold mountain 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 carbonate minerals with their chemistry formula and also reactions to cold and also warm hydrochloric acid. Test results can vary due to the fact that of weathering, ahead testing, contamination, and also specimen purity.

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The Vigor of lead carbonate Reactions

Careful monitoring is important due to the fact that some lead carbonate minerals reaction vigorously and others barely react with cold acid.

The carbonate mineral the is most commonly 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 frequently encountered carbonate mineral. If you place one drop of cold hydrochloric mountain on calcite, the entire drop the acid will erupt with bubbles and also a vigorous fizz will last because that a couple of seconds.

Dolomite CaMg(CO3)2 is an additional commonly encountered lead carbonate mineral. If you ar one autumn of cold hydrochloric mountain on a piece of dolomite, the reaction is weak or no observed. Rather of seeing an evident fizz, girlfriend will watch a fall of acid on the surface ar of the mineral that can have a few bubbles of carbon dioxide gas slowly farming on the dolomite surface.

However, if warm acid is put on dolomite, an noticeable fizz will certainly occur. This occurs due to the fact that the acid and rock react an ext vigorously at greater temperatures.

If you place a autumn of hydrochloric mountain on powdered dolomite, a clearly shows reaction will occur. This is since the surface area has been increased, making more dolomite accessible to the acid. (You can quickly make dolomite flour by scratching a specimen of dolomite throughout a streak plate. Then test the flour by place a autumn of hydrochloric mountain on the powder. One more easy way to develop a small amount of mineral flour is to scratch the specimen with a nail.)

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

When a mineral has actually a weak response to acid, you should be observant and patient to watch it. Because that example, magnesite has a really weakreaction v cold HCl. If you flour a small amount the magnesite top top a streak plate and also place a autumn of mountain on it, you could not view any activity for number of seconds. Then, as tiny bubbles begin to kind on corpuscle of magnesite, the autumn of acid will show up to prosper larger in size. The occurs as carbon dioxide is liberated from the mineral and displaces the water. Observing the formation of bubbles with a hand lens deserve to be helpful.


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Dolostone: Dolostone is a sedimentary absent composed primarily of the mineral dolomite, which has actually a chemistry composition of CaMg(CO3)2. Dolomite will certainly effervesce weakly through cold hydrochloric acid, producing a couple of bubbles. The reaction is an ext noticeable once the acid is heat and/or the rock is powdered. The specimen in the photograph is around 10 cm across.


The acid Test ~ above Rocks

LIMESTONE, DOLOSTONE, and MARBLE

Some rocks contain carbonate minerals, and also the acid test deserve to be offered to assist identify them. Limestone is composed nearly entirely of calcite and also will develop a vigorous fizz with a autumn of hydrochloric acid. Dolostone is a rock composed of nearly entirely that dolomite. That will develop a really weak fizz as soon as a autumn of cold hydrochloric acid is placed upon it, a an ext obvious fizz when powdered dolostone is tested, and a stronger fizz when warm hydrochloric mountain is used.

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

Marble is a limestone or a dolostone that has been metamorphosed. That will have an mountain reaction the is comparable 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 help identify the cementing certified dealer of sandstones. They ar a autumn of dilute HCl ~ above the sandstone and also closely observe. If calcite is the cementing agent, one effervescence will certainly occur and some of the sand grains could be liberated. A hand lens or little microscope is offered to make the observations. The photo above is a amplified view of a piece of Oriskany Sandstone, one Ordovician-age absent unit native the Appalachian container that serves together a herbal gas reservoir and also a herbal gas storage unit. Oriskany sandstone is regularly cemented by calcite.


OTHER ROCKS that FIZZ

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

Many rocks contain tiny amounts the calcite or other carbonate minerals. All of these can create a fizz also though the lead carbonate is just a minor component of a rock"s composition. This rocks might contain tiny veins or crystals of carbonate minerals that create a fizz in contact with acid. These veins and crystals deserve to be therefore tiny that they are not clearly shows to the unaided eye. This small amount the carbonate could fizz the an initial time a fall of acid is used but it is in depleted and not fizz if acid is applied a 2nd time to the same ar on the rock.

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

Many shales were deposited in maritime environments and contain enough calcium carbonate to produce a vigorous acid fizz. This shales were formed when dirt was deposited in one environment comparable to or nearby to wherein limestone was formed. They space composed of sedimentary clay minerals intermixed through a tiny amount the calcite. Lock are known as "calcareous shales."

Don"t enable an mountain fizz to overview the to know process. In many instances it will certainly instead include detail to your observation such as: "calcareous shale" or "sandstone with lead carbonate cement." This is beneficial information.


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Vinegar can be used for the mountain test: Vinegar can be a safe, economical and easy-to-obtain "acid" because that identifying calcite and dolomite. Vinegar is dilute acetic mountain that produces a really weak reaction through calcite and dolomite - best observed with a hand lens.

The "Vinegar Test"

Vinegar is a dilute acetic acid systems (about 5% come 10%) the produces a weak effervescent reaction through calcite and dolomite. It deserve to be used instead of hydrochloric mountain for introducing students to the mountain test. Vinegar is simple to obtain, inexpensive, and safer to usage than hydrochloric acid.The effervescence utilizing vinegar usually needs a hand lens for clear observation and also is just observable through carbonate mineralsthat have a solid reaction v hydrochloric acid. Vinegar is frequently used when the mountain test is part of a precollege course. Security gloves, glasses, file towels, and immediate access to one eyewash station space recommended.

EXTREME acid REACTIONS

A few rocks can produce severe reaction with hydrochloric acid. These room usually rocks written of calcite or aragonite withabundant pore room or exceptionally high surface areas. Some specimens that chalk, coquina, oolite, and tufa space examples. As soon as a autumn of dilute hydrochloric acid is inserted on these specimens, an eruption of mountain foam have the right to rise up turn off of the rock and spread come an unforeseen diameter. The reaction is an extremely brief (and may not be repeatable), but it is so sudden and vigorous that it have the right to surprise one inexperienced person. This description is for one drop of acid. If much more is used an even more vigorous reaction will certainly occur. (These too much reactions will not take place with every specimen of these rocks. Be mindful when trial and error them or presenting them to students for testing.)

The exceptionally vigorous reaction that cold hydrochloric acid v these specimens occurs due to the fact that the rocks space so porous or due to the fact that they have actually a an extremely high surface area under a solitary drop of acid.

TEST UNWEATHERED MATERIAL

Calcite and other lead carbonate minerals have actually a short resistance to weathering and can be assaulted by acids in organic waters and soils. When testing material that has actually been exposed in ~ Earth"s surface, that is very important to test unweathered material. A fresh surface have the right to usually be acquired by breaking the rock.

DECEIVED by POROSITY!

Some rocks room porous and contain a reservoir of air. Tiny amounts of air escaping into a fall of mountain from listed below can give the figure of a gentle acid reaction. Don"t it is in fooled. If you location a fall of mountain on part sandstones, a couple of bubbles will arise out of spicy spaces. It"s no a carbonate cement. To prevent this difficulty scratch the rock across a streak plate and test the flour or the seed that room produced.


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The best way to learn around minerals is to examine with a arsenal of little specimens the you can handle, examine, and also observe their properties. Inexpensive mineral collection are available in the lifwynnfoundation.org Store.


Contamination in Mineral to know Labs

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

1) In mineral to know labs, some students are prepared to call any mineral the produces an mountain reaction "calcite" or one more carbonate. However, calcite is a common mineral and also it is often existing as one intimate part of other mineral specimens and rocks. These can develop a false acid reaction. To protect against being misled, student should always be cautioned to check a specimen"s identity with many properties. If a specimen fizzes with acid however has a Mohs hardness of seven and breaks v a conchoidal fracture, then it certainly isn"t calcite!

Depending upon the suffer of the students, specimens the are really true to their properties deserve to be presented to the class, or specimens v some obstacles can it is in used. Many minerals discovered in the ar will no be certain true come properties. It"s much better to discover that class in the lab and go into the field with wisdom.

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

In a mineral identification lab, barite is typically confused v calcite because of contamination. The barite might naturally contain tiny amounts the calcite, or the hardness experimentation of a ahead student can have left small amounts of calcite on a barite specimen.Students room often attracted to an identification as "calcite" simply since of the acid test. If that mineral exhibits a little bit ofcleavage and also is not an extremely hard, then plenty of students will certainly arrive at an not correct identification.


Acid check Safety

Hydrochloric acid, effectively diluted to a 10% concentration, can cause irritation if it contacts the skin or eyes. The can additionally fade clothing. Hydrochloric acid need to kept in clearly-labeled dispensing bottles and used through quick and also easy access to paper towels, water, and also an eyewash station. Safety glasses and protective gloves are recommended. If skin call occurs, the area have to be flushed with many of water. If eye contact occurs, the eye should be flushed for 15 minutes through plenty of water. If a call lens is worn, the eye must be flushed, call lens removed, and flushing continued. Look for prompt medical attention because that eye contact.Specimens that room tested v acid have to be rinsed after trial and error to remove or dilute unreacted acid.

Limiting Frivolous Acid use in Labs

Most students space intrigued through the acid test and also want to shot it. To limit frivolous acid use, students should be instructed to usage a single drop of acid for the test and also to only test specimens once carbonate minerals space suspected. If that is not done, some students will usage the acid frivolously. This actions is urged if the class is equipped with big acid party that are filled to the top. However, if the acid bottles are little and nearly empty at the start of class, students typically ration their usage of the acid to proper amounts. Small, practically empty bottles makes much less acid easily accessible to spill.


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


Acid party Selection

The kind of party selected for dispensing the mountain is important. Activities supply stores offer bottles that room designed for dispensing acidone-drop-at-a-time. The lid is constantly on this bottles (except when they room being cleaned or refilled), and they execute not develop a spill whenthey space knocked over. Bottles with a removable lid that has a squeeze pear dispenser will be occasionally knocked over as soon as the lid is offif they space being supplied by normal humans.

Acid dispensing bottles should be make of rigid plastic through a little opening which enables acid to be quickly dispensed one-drop-at-a-time. Soft dispensing party or bottles with a bigger opening can dispense a large amount of acid v an inadvertently squeeze.

Sources of Hydrochloric Acid

Hydrochloric acid diluted to a 10% systems cannot be purchased in most communities. The ideal place to acquisition commercially ready solutions is native a laboratory supply company. Purchasing it ready-for-use is the recommended method to attain it. Don"t shot to prepare your very own solution if friend don"t know specifically what you space doing and also have an equipped laboratory. Her chemistry department might have the ability to assist you v ordering acid. Part generous chemists will prepare a 10% solution for you.

Mineral Specimens together "Consumables"

Mineral specimens the are supplied properly in the science classroom or activities will have to be changed frequently.Students will be investigate them with hardness tests, streak tests, acid tests and other experiments. Every one of thesetests damages the specimen and also make it less fit for the next group of students. To keep the acid test from fouling your entire collection, asking students come rinse specimens after testing with acid and also limit experimentation to only as soon as it is needed.


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