Which mineral scratches dolomite and is scratched by olivine?
It’s a question that pops up in field‑work notebooks and rock‑hounding forums alike. The answer isn’t as obvious as it looks, so let’s dig into the hardness scale, the logic behind scratch tests, and the real‑world candidates that fit the bill.
What Is the Scratch Test and Why It Matters
When geologists field‑sample, one of the first things they do is a quick scratch test. It’s that simple: take a known‑hard mineral (usually a piece of a hard rock or a piece of a tool) and try to scratch the sample. Also, if the scratch appears, the sample is softer; if not, it’s harder. The test is a fast, low‑cost way to get a rough idea of a rock’s composition before you bring it into a lab.
The scratch test relies on Mohs hardness, a scale that ranks minerals from 1 (talc) to 10 (diamond). Each mineral can scratch anything softer than itself but can’t be scratched by anything harder. So if Mineral A scratches Mineral B, A is at least one step harder. If Mineral B is scratched by Mineral C, C is at least one step harder than B Surprisingly effective..
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Now, the puzzle: Which mineral scratches dolomite (hardness ~4) and is itself scratched by olivine (hardness ~6.5–7)? Put another way, we’re looking for a mineral with a hardness between 4 and 6.5.
The Hardness Ranges of the Candidates
| Mineral | Mohs Hardness | Typical Use in Scratch Tests |
|---|---|---|
| Dolomite | 3.0 | Common in igneous rocks; a good “hard” reference |
| Apatite | 5.0 | Common in sedimentary rocks; often used as a “middle‑ground” reference |
| Olivine | 6.0 | A common feldspar in granites |
| Calcite | 3.5–4.5–7.0 | Often found in sedimentary and metamorphic rocks |
| Orthoclase | 6.0 | Softer than dolomite; scratches by dolomite |
| Quartz | 7. |
You can already spot the two obvious suspects: apatite and orthoclase. Both sit comfortably between dolomite and olivine on the scale.
Why Knowing This is Useful
- Field Identification – If you have a hand sample that looks like dolomite but you’re unsure, spotting a mineral that scratches it tells you it’s harder than dolomite. Then, seeing that same mineral get scratched by a piece of olivine confirms its hardness is below olivine’s range.
- Mineral Sorting – In a mixed‑rock quarry, you can quickly separate dolomite‑rich layers from olivine‑rich ones by simple scratch tests.
- Educational Value – Teaching students about the Mohs scale becomes interactive when they can actually see the scratches in real time.
How to Perform the Scratch Test Safely
1. Gather Your Tools
- A clean, flat piece of olivine (or a polished quartz surface if olivine isn’t available).
- A clean piece of dolomite (or a piece of calcite as a softer control).
- A clean, flat mineral that you suspect might be the middle ground (e.g., a small chunk of apatite or orthoclase).
- A magnifying glass or hand lens (optional but handy).
2. Clean the Surfaces
Dust or oil can hide scratches. A quick wipe with a lint‑free cloth and a drop of rubbing alcohol does the trick.
3. Test the Hardness of Dolomite
- Use the olivine piece to scratch the dolomite. If you see a clear line, olivine is harder (as expected).
- Try the suspected middle mineral on dolomite. If it scratches, you’re on the right track.
4. Test the Hardness of Olivine
- Now scratch the suspected mineral with the olivine. If the line disappears or is faint, olivine is harder, confirming the mineral’s position.
5. Document
Take a photo of each scratch. Note the depth, clarity, and any color changes Simple, but easy to overlook..
Common Mistakes in the Field
| Mistake | Why It Happens | How to Fix It |
|---|---|---|
| Using a dirty surface | Dust can mask scratches. | |
| Ignoring temperature | Cold minerals can be harder temporarily. | |
| Using too much force | Can fracture the sample, not just scratch. | |
| Misreading the scale | Some people think a higher number means “softer. | |
| Assuming “scratch” means “damage” | A scratch is a permanent mark, not a flaw. On top of that, | Treat the sample as a reference, not a specimen. In practice, ” |
Practical Tips That Actually Work
- Use a Reference Plate – Keep a small slab of quartz (hardness 7) handy. If your suspect mineral can scratch quartz, it’s definitely harder than olivine, so ditch the candidate.
- Look for Color Change – Some minerals, like feldspar, may show a faint color shift when scratched. It’s a subtle cue that you’re looking at a feldspar.
- Check for Pitting – Apatite often leaves a shallow pit rather than a deep line. If you see a shallow mark, you’re likely dealing with apatite.
- Cross‑Reference with Other Tests – A weak magnetic response can rule out magnetite or hematite, narrowing the field.
- Keep a Scratch Log – Record each test in a notebook. Over time, patterns emerge that help you identify new samples faster.
Which Mineral Is It? Apatite or Orthoclase?
Both apatite and orthoclase fit the criteria. Still, in most field situations, the most common answer people look for is apatite. Here’s why:
- Frequency – Apatite is a common accessory mineral in sedimentary and metamorphic rocks, especially in dolomitic formations.
- Distinctive Appearance – It often shows a dull, greenish‑gray color that makes it stand out.
- Scratch Characteristics – It scratches dolomite with a clean, shallow line and gets scratched cleanly by olivine.
Orthoclase (a feldspar) is also a valid answer, but it’s less common in dolomite‑rich contexts and can be confused with other feldspars Worth keeping that in mind. Turns out it matters..
FAQ
Q1: Can I use a glass slide instead of olivine?
A1: Glass has a hardness of about 5.5, so it can scratch dolomite but may not scratch orthoclase or apatite cleanly. It’s best to stick with a hard mineral like olivine or quartz.
Q2: What if the mineral I suspect is a mixture of apatite and orthoclase?
A2: Mixed samples will show a combination of scratch patterns. Look for a shallow pit (apatite) and a slightly deeper line (orthoclase) on the same surface Most people skip this — try not to..
Q3: Does temperature affect the scratch test?
A3: Minor temperature changes (e.g., from a cold morning to a warm afternoon) can slightly alter hardness readings, but the differences are usually negligible for field work.
Q4: Is there a way to confirm the mineral without a scratch test?
A4: Yes—use a hand lens to look for diagnostic features like crystal habit, cleavage, or color. For a definitive ID, a thin section and X‑ray diffraction are the gold standard Not complicated — just consistent..
Q5: Can weathering change the hardness of a mineral?
A5: Weathering can soften a surface, making scratches appear less pronounced. That’s why it’s important to test fresh, unweathered areas of the sample Most people skip this — try not to..
Wrap‑Up
The scratch test remains a rock‑hounding classic because it’s quick, inexpensive, and surprisingly informative. If you’re trying to figure out which mineral sits between dolomite and olivine on the Mohs scale, think of apatite first—especially if your sample is from a dolomitic setting. Now, keep your tools clean, your tests consistent, and your observations sharp, and you’ll be identifying minerals in the field faster than ever. Still, orthoclase is a close second, but it’s less likely to show up in those specific rocks. Happy scratching!
