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

Post by: Hobart M. King, PhD, RPG

Hydrochloric acid on limestone: This video demonstrates what happens when one drop of dilute (10%) hydrochloric acid is put on a piece of limerock. Protective glasses and gloves are recommfinished as soon as doing the acid test.

You are watching: What does the acid test tell you about a mineral


What is the Acid Test?

To most geologists, the term "acid test" indicates placing a drop of dilute (5% to 10%) hydrochloric acid on a rock or mineral and also watching for bubbles of carbon dioxide gas to be released. The bubbles signal the presence of carbonate minerals such as calmention, dolomite, or one of the minerals detailed in Table 1.


The bubbling release of carbon dioxide gas deserve to be so weak that you need a hand lens to observe single bubbles slowly prospering in the drop of hydrochloric acid - or so vigorous that a flash of effervescence is developed. These variations in effervescence vigor are an outcome of the form of carbonate minerals current, the amount of carbonate existing, the ppost dimension of the carbonate, and also the temperature of the acid.


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


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Magnesite: The mineral magnesite, which has actually a chemical composition of MgCO3, will effervesce weakly via warm hydrochloric acid and very weakly through cold acid. Speciguys is about 6.4 cm across.


What Caprovides the Fizz?

Carbonate minerals are unsecure in call with hydrochloric acid. When acid begins to effervesce (fizz) on a speciguys, a reactivity comparable to the one shown listed below is taking place.

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On the left side of this reaction, the mineral calpoint out (CaCO3) is in contact through hydrochloric acid (HCl). These react to develop carbon dioxide gas (CO2), water (H2O), dissolved calcium (Ca++), and liquified chlorine (Cl--). The carbon dioxide bubbles that you observe are proof that the reaction is occurring. When that occurs, calpoint out or one more carbonate mineral is existing.

Many other carbonate minerals react through hydrochloric acid. Each of these minerals is composed of one or more steel ions unified through a carbonate ion (CO3--). The chemistry of these reactions is similar to the calpoint out reaction above. The mineral reacts through hydrochloric acid to create carbon dioxide gas, water, a liquified steel ion, and also liquified chlorine. The reactions for magnewebsite (MgCO3) and also siderite (FeCO3) are displayed listed below.

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Calcite: This transparent specimen of calmention shows cleavage that is characteristic of the mineral. Calpoint out, through a composition of CaCO3, will certainly react strongly with either cold or warm hydrochloric acid. Specimales steps around 10 cm across.

Acid Reactions of Carbonate Minerals
MineralChemical CompositionCold Acid ReactionWarm Acid Reaction
AragoniteCaCO3strongstrong
AzuriteCu3(CO3)2(OH)2yesstrong
CalciteCaCO3strongstrong
DolomiteCaMg(CO3)2weakyes
MagnesiteMgCO3exceptionally weakweak
MalachiteCu2CO3(OH)2yesyes
RhodochrositeMnCO3weakyes
SideriteFeCO3incredibly weakweak
SmithsoniteZnCO3weakyes
StrontianiteSrCO3yesyes
WitheriteBaCO3weakweak
Table 1: A list of generally and also periodically encountered carbonate minerals via their chemical formula and also reactions to cold and heat hydrochloric acid. Test results deserve to vary because of weathering, previous testing, contamination, and also speciguys purity.

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The Vigor of Carbonate Reactions

Careful observation is crucial bereason some carbonate minerals react strongly and others bacount react via cold acid.

The carbonate mineral that is the majority of generally encountered by geologists is calpoint out (CaCO3). Calcite is a "ubiquitous" mineral. Ubiquitous suggests "discovered everywhere." Calcite occurs in igneous, metamorphic, and also sedimentary rocks and is the most frequently encountered carbonate mineral. If you place one drop of cold hydrochloric acid on calpoint out, the entire drop of acid will certainly erupt through bubbles and a vigorous fizz will last for a few secs.

Dolomite CaMg(CO3)2 is one more frequently encountered carbonate mineral. If you place one drop of cold hydrochloric acid on a piece of dolomite, the reaction is weak or not observed. Instead of seeing an noticeable fizz, you will view a drop of acid on the surface of the mineral that might have actually a few bubbles of carbon dioxide gas progressively flourishing on the dolomite surchallenge.

However before, if heat acid is put on dolomite, an noticeable fizz will happen. This occurs bereason the acid and also rock react even more intensely at better temperatures.

If you place a drop of hydrochloric acid on powdered dolomite, a visible reaction will certainly happen. This is bereason the surface location has been raised, making more dolomite obtainable to the acid. (You deserve to easily make dolomite powder by scratching a specimen of dolomite across a streak plate. Then test the powder by placing a drop of hydrochloric acid on the powder. Anvarious other simple way to produce a little amount of mineral powder is to scrape the specimen with a nail.)

Different carbonate minerals have various responses to hydrochloric acid. A list of widespread and periodically encountered carbonate minerals is provided in Table 1 with their chemical complace and their family member reactivity via cold and also warm hydrochloric acid.

When a mineral has a weak response to acid, you need to be observant and also patient to watch it. For example, magnesite has a really weakreactivity via cold HCl. If you powder a little amount of magnewebsite on a streak plate and place a drop of acid on it, you might not check out any type of action for several secs. Then, as tiny bubbles begin to develop on pshort articles of magnesite, the drop of acid will show up to flourish larger in dimension. That occurs as carbon dioxide is liberated from the mineral and displaces the water. Observing the formation of bubbles through a hand lens have the right to be useful.


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Dolostone: Dolorock is a sedimentary rock composed primarily of the mineral dolomite, which has a chemical complace of CaMg(CO3)2. Dolomite will certainly effervesce weakly through cold hydrochloric acid, developing a couple of bubbles. The reaction is even more noticeable once the acid is warm and/or the rock is powdered. The speciguys in the photo is around 10 cm throughout.


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The Acid Test on Rocks

LIMESTONE, DOLOSTONE, AND MARBLE

Some rocks contain carbonate minerals, and also the acid test have the right to be used to assist identify them. Limerock is composed virtually entirely of calcite and also will certainly develop a vigorous fizz via a drop of hydrochloric acid. Dolorock is a rock written of virtually completely of dolomite. It will certainly develop an extremely weak fizz once a drop of cold hydrochloric acid is inserted upon it, an extra apparent fizz as soon as powdered dolostone is tested, and a more powerful fizz once warm hydrochloric acid is provided.

Limestone and also dolostone can be a little even more complicated. They are occasionally created of a mixture of calcite and also dolomite and have actually acid reactions that are deceptive. A dolostone deserve to contain sufficient calmention to fool you into calling it a limestone. For these rocks the acid test might not be enough for a confident identification - but at leastern you will certainly understand that the rock has a far-reaching carbonate mineral content.

Marble is a limestone or a dolostone that has been metamorphosed. It will have actually an acid reaction that is similar to the limestone or dolorock from which it was developed.


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Other Applications of the "Acid Test": Geologists can use dilute hydrochloric acid to aid identify the cementing agent of sandstones. They place a drop of dilute HCl on the sandrock and also closely observe. If calcite is the cementing agent, an effervescence will occur and some of the sand also grains might be liberated. A hand also lens or small microscopic lense is used to make the monitorings. The photo above is a amplified watch of a piece of Oriskany Sandstone, an Ordovician-age rock unit from the Appalachian Container that serves as a natural gas reservoir and also a herbal gas storage unit. Oriskany type of sandrock is often cemented by calcite.


OTHER ROCKS THAT FIZZ

Almethods remember that "calpoint out is common." (Ubiquitous means that it is discovered virtually almost everywhere.)

Many rocks contain tiny amounts of calcite or other carbonate minerals. All of these deserve to develop a fizz also though the carbonate is only a minor component of a rock"s complace. These rocks can contain little veins or crystals of carbonate minerals that create a fizz in call with acid. These veins and also crystals deserve to be so tiny that they are not visible to the unaided eye. This tiny amount of carbonate might fizz the first time a drop of acid is used but be depleted and not fizz if acid is applied a second time to the exact same area on the rock.

Some sedimentary rocks are bound along with calmention or dolomite cement. Sandstone, siltrock, and also conglomerate periodically have calcite cement that will certainly create a vigorous fizz with cold hydrochloric acid. Some conglomerates and also breccias contain clasts of carbonate rocks or minerals that react through acid.

Many shales were deposited in marine settings and contain enough calcium carbonate to create a vigorous acid fizz. These shales were formed once mud was deposited in an setting equivalent to or adjacent to where limerock was formed. They are created of sedimentary clay minerals interblended through a tiny amount of calmention. They are well-known as "calcareous shales."

Don"t permit an acid fizz to guide the identification process. In many instances it will rather add detail to your monitoring such as: "calcareous shale" or "sandrock via carbonate cement." This is helpful information.


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Vinegar can be provided for the acid test: Vinegar have the right to be a safe, economical and easy-to-attain "acid" for identifying calcite and dolomite. Vinegar is dilute acetic acid that produces an extremely weak reactivity via calpoint out and also dolomite - best oboffered via a hand also lens.

The "Vinegar Test"

Vinegar is a dilute acetic acid solution (about 5% to 10%) that produces a weak effervescent reaction through calpoint out and also dolomite. It can be provided instead of hydrochloric acid for presenting students to the acid test. Vinegar is straightforward to achieve, inexpensive, and also safer to use than hydrochloric acid.The effervescence using vinegar commonly needs a hand lens for clear monitoring and also is just observable via carbonate mineralsthat have a solid reactivity with hydrochloric acid. Vinegar is frequently offered as soon as the acid test is component of a precollege course. Protective gloves, glasses, paper towels, and immediate accessibility to an eyewash terminal are recommended.

EXTREME ACID REACTIONS

A few rocks can develop an extreme reactivity with hydrochloric acid. These are usually rocks composed of calpoint out or aragonite withnumerous pore room or very high surface locations. Some specimens of chalk, coquina, oolite, and tufa are examples. When a drop of dilute hydrochloric acid is inserted on these specimens, an eruption of acid foam deserve to climb up off of the rock and also spcheck out to an unsupposed diameter. The reactivity is incredibly brief (and might not be repeatable), however it is so sudden and vigorous that it deserve to surpincrease an inproficient perkid. This description is for one drop of acid. If more is supplied an even more vigorous reactivity will certainly take place. (These extreme reactions will certainly not happen through every specimen of these rocks. Be conscious once trial and error them or presenting them to students for experimentation.)

The very vigorous reaction of cold hydrochloric acid via these specimens occurs because the rocks are so porous or bereason they have actually a really high surconfront area under a solitary drop of acid.

TEST UNWEATHERED MATERIAL

Calcite and also various other carbonate minerals have actually a low resistance to weathering and also can be attacked by acids in herbal waters and soils. When experimentation material that has actually been exposed at Earth"s surchallenge, it is extremely vital to test unweathered material. A fresh surface can normally be derived by breaking the rock.

DECEIVED BY POROSITY!

Some rocks are porous and also contain a reservoir of air. Small quantities of air escaping into a drop of acid from below can give the appearance of a gentle acid reactivity. Don"t be fooled. If you area a drop of acid on some sandstones, a couple of bubbles will certainly emerge out of pore spaces. It"s not a carbonate cement. To prevent this difficulty scratch the rock throughout a streak plate and also test the powder or the grains that are produced.


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The finest way to learn about minerals is to research through a collection of tiny specimens that you deserve to handle, study, and also observe their properties. Inexpensive mineral collections are accessible in the slrfc.org Store.


Contamination in Mineral Identification Labs

When students are provided minerals to recognize, two situations have the right to cause troubles through their work.

1) In mineral identification labs, some students are prepared to speak to any mineral that produces an acid reactivity "calcite" or an additional carbonate. However, calmention is a common mineral and also it is regularly current as an intimate component of various other mineral specimens and also rocks. These deserve to produce a false acid reaction. To avoid being misled, students have to always be cautioned to confirm a specimen"s identification via multiple properties. If a speciguys fizzes via acid but has a Mohs hardness of seven and also breaks through a conchoidal fracture, then it certainly isn"t calcite!

Depending upon the endure of the students, specimens that are very true to their properties have the right to be presented to the class, or specimens with some obstacles can be offered. Lots of minerals uncovered in the area will not be absolutely true to properties. It"s much better to learn that lesson in the lab and go into the area through wisdom.

2) Since calpoint out is among the index minerals of the Mohs Hardness Scale, it is often used to test the hardness of mineral specimens. This can area little amounts of calpoint out possibly on eincredibly unrecognized specimales in the lab! Don"t assume that a solitary acid reaction is correct. Test the specimales in a second place if you suspect that contamination has developed.

In a mineral identification lab, barite is typically perplexed with calpoint out because of contamicountry. The barite could normally contain tiny quantities of calpoint out, or the hardness trial and error of a previous student could have actually left tiny amounts of calpoint out on a barite speciguys.Students are frequently attracted to an identification as "calcite" ssuggest bereason of the acid test. If that mineral exhibits a little bit ofcleavage and also is not exceptionally difficult, then many type of students will arrive at an incorrect identification.


Acid Test Safety

Hydrochloric acid, correctly diluted to a 10% concentration, deserve to reason irritation if it contacts the skin or eyes. It can also fade clothes. Hydrochloric acid need to maintained in clearly-labeled dispensing bottles and supplied via quick and also simple accessibility to paper towels, water, and an eyewash terminal. Safety glasses and also protective gloves are recommfinished. If skin call occurs the area have to be fluburned through plenty of water. If eye contact occurs the eye must be fluburned for 15 minutes via plenty of water. If a call lens is worn, the eye must be fluburned, call lens rerelocated, and flushing ongoing. Seek prompt medical attention for eye call.Specimens that are tested via acid should be rinsed after testing to remove or dilute unreacted acid.

Limiting Frivolous Acid Use in Labs

Most students are intrigued through the acid test and want to attempt it. To limit frivolous acid use, students should be instructed to use a single drop of acid for the test and also to only test specimens once carbonate minerals are suspected. If that is not done, some students will certainly usage the acid frivolously. This behavior is encouraged if the classroom is equipped with huge acid bottles that are filbrought about the height. However before, if the acid bottles are small and also almost empty at the beginning of class, students usually ration their use of the acid to appropriate quantities. Small, nearly empty bottles provides much less acid available to spill.


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Acid dispensing bottles: Small acid dispensing bottles job-related well for the acid test. They dispense the acid one-drop-at-a-time and will certainly not spill if they are knocked over. If you are a teacher supervising the acid test in a classroom, provide students tiny bottles that are nearly empty. That will minimize the amount of frivolous acid use that might otherwise occur. Label the bottles clearly and also instruct students in acid usage prior to making them easily accessible.


Acid Bottle Selection

The form of bottle selected for dispensing the acid is important. Laboratory supply stores market bottles that are designed for dispensing acidone-drop-at-a-time. The lid is always on these bottles (other than as soon as they are being cleaned or refilled), and they carry out not create a spill whenthey are knocked over. Bottles via a removable lid that has a squeeze bulb dispenser will certainly be sometimes knocked over when the lid is offif they are being supplied by normal humans.

Acid dispensing bottles have to be made of rigid plastic with a little opening which permits acid to be quickly dispensed one-drop-at-a-time. Soft dispensing bottles or bottles via a larger opening deserve to dispense a big amount of acid with an accidental squeeze.

Sources of Hydrochloric Acid

Hydrochloric acid diluted to a 10% solution cannot be purchased in a lot of communities. The best place to purchase commercially prepared solutions is from a laboratory supply company. Purchasing it ready-for-usage is the recommended method to obtain it. Don"t attempt to prepare your own solution if you don"t recognize specifically what you are doing and have an equipped laboratory. Your chemistry department might have the ability to assist you via ordering acid. Some generous chemists will prepare a 10% solution for you.

Mineral Specimens as "Consumables"

Mineral specimens that are offered appropriately in the scientific research classroom or laboratory will must be replaced generally.Students will certainly be investigating them with hardness tests, streak tests, acid tests and other experiments. All of thesetests damage the speciguys and make it less fit for the next group of students. To keep the acid test from fouling your whole collection, ask students to rinse specimens after testing with acid and limit trial and error to only when it is necessary.


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