When students are given minerals to identify, two situations can cause problems with their work.

1) In mineral identification labs, some students are ready to call any mineral that produces an acid reaction “calcite” or another carbonate. 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.

2) Since calcite is one of the index minerals of the Mohs Hardness Scale, it is often used to test the hardness of mineral specimens. 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.