Igneous Rock Identification Lab Answer Key

Getting Through an Igneous Rock ID Lab Without Losing Your Mind

The lab you're looking at is one of those geology intro courses that tries to cram field petrography into three hours. You get a box of rocks, a hand lens, some streak plates, and a flowchart that looks reasonable until you actually start trying to use it. The Igneous Rock Identification Lab Answer Key most professors hand out or post online is basically a reference sheet that maps physical properties to names. It works well enough for the standard specimens, but the moment you hit a weathered sample or something with a weird texture, it falls apart fast. Most answer keys in this course are built around the same six to eight diagnostic characteristics. Color index tells you roughly how much dark mineral is present, which separates felsic from mafic at a glance. Texture covers the cooling history, so you can distinguish between volcanic glass, fine-grained extrusive, coarse-grained intrusive, porphyritic, and vesicular. Mineral composition rounds it out with quartz, feldspar, olivine, pyroxene, amphibole, and biotite as the usual suspects. A good key also includes the acid test with dilute HCl for feldspathic minerals and a magnet test for iron-rich phases, though not every lab kit actually includes those reagents. The keys I've seen typically present a dichotomous chart or a table that runs from granite to gabbro to rhyolite to basalt and a few intermediate varieties. Some go further and throw in diorite, andesite, scoria, pumice, perlite, and obsidian. A handful even include gabbronorite and troctolite for students who need to be punished.

I spent two semesters TAing this exact lab, and the most consistent problem I ran into was students confusing volcanic textures with plutonic ones simply because the samples had been stored in drawers for years and developed surface oxidation. A basalt specimen can look almost black and massive after sitting on a shelf, making it easy to misread as gabbro if you rely too heavily on color alone. The workaround was straightforward: tell them to chip the sample. Fresh surfaces reveal the actual texture. I started requiring that every student break or rub a small area before committing to a color index reading, and the error rate dropped noticeably.

How the Identification Actually Works in Practice

Start with texture because that settles half the questions immediately. If you see visible crystals larger than a millimeter, you are dealing with an intrusive rock. If the grains are too small to distinguish without magnification, you are looking at extrusive material. Porphyritic texture is the middle ground, where larger phenocrysts sit in a finer groundmass, and that tells you the magma experienced two distinct cooling stages, usually an early slow period followed by rapid extrusion or embedding. Once texture is established, move to mineral content. Quartz and potassium feldspar push the classification toward the felsic end. Plagioclase feldspar is present in everything from oligoclase to anorthite, and the specific type matters less at this level than recognizing that feldspar dominates the light-colored fraction. Dark minerals do the heavy lifting for the mafic side. Olivine is the first clue because it weathers quickly and shows a distinctive greenish color and vitreous luster, but if the sample is fresh it can look nearly black and glassy. Pyroxene is also dark but lacks olivine's octahedral cleavage pattern and tends to show two sets of cleavage at nearly right angles. Amphibole, usually hornblende, has elongated crystals with a pitchblack appearance and a single clear cleavage direction. Biotite mica is the only common dark mineral that splits into thin flexible sheets, so if you can peel layers off it, you are dealing with biotite rather than hornblende or pyroxene. Feldspar identification is where people lose points. Plagioclase feldspar typically shows parallel striations on cleavage planes, while potassium feldspar, sometimes called orthoclase, displays a characteristic pink or salmon hue and may show a twinning pattern visible under good light. If your lab has polarized light microscopes available, the birefringence and extinction angles make this trivial. With just a hand lens, you learn to read the surface features and color variations. It takes practice, and the specimens you get handed are not always clean examples.

The acid test is useful for feldspar versus calcite separation, though in igneous rocks calcite is rarely primary except in carbonatites, which you will almost certainly never see in an introductory lab. More relevant is using the streak plate to differentiate iron oxides. Hematite leaves a reddish streak, magnetite leaves a black streak, and neither feldspar nor quartz will leave a noticeable streak at all. Magnetism is another quick discriminator, especially for separating magnetite from other dark minerals, though not all labs permit using a magnet near the sample trays because it interferes with other equipment.

Common Specimens and the Pitfalls They Present

Granite is the standard felsic intrusive rock, and it is usually straightforward. Pink potassium feldspar, clear or milky quartz, white plagioclase, and scattered darkbiotite or amphibole crystals. The trap here is perthite, where potassium feldspar exsolves intergrowths that can confuse beginners into thinking they are seeing two separate mineral phases. It is normal, but it makes the rock look more complex than it actually is. Gabbro is the mafic intrusive equivalent, primarily plagioclase and pyroxene with occasional olivine. It is dense, dark, and usually uniform. The pitfall is that weathered gabbro can develop a brownish alteration rim that makes it look like andesite at a glance. Fresh fracture surfaces are essential. Basalt and gabbro share the same bulk composition, so texture is the only differentiator. Students frequently identify basalt correctly on composition but second-guess themselves when they cannot see individual crystals and assume the rock must be sedimentary because it is too fine-grained. Reinforce that fine-grained igneous is common and expected for extrusive rocks.

Rhyolite is the extrusive counterpart to granite, and it is arguably the hardest common igneous rock to identify in a teaching lab. Many rhyolite specimens are actually glassy or heavily altered, and the mineral composition can be ambiguous without thin-section analysis. Vesicular rhyolite, often called perlite when it has a characteristic pearlescent fracture, can be mistaken for pumice if you do not check the viscosity cues. Pumice floats because its vesicles are interconnected and the glass is highly siliceous, while perlite forms onion-like conchoidal fractures from water absorption into layered glass. Students who try to float samples in the lab often get wet notebooks and confused graders. Obsidian is volcanic glass with no crystal content, which makes the identification process almost suspiciously simple. The problem is that obsidian gets confused with other dark fine-grained rocks. The conchoidal fracture is the giveaway. If it breaks like sharp flint glass rather than granular or crystalline, you are likely looking at obsidian. Scoria resembles basalt but is distinctly vesicular and usually has darker, sharper vesicle walls because it forms from more gas-rich basaltic magma. The vesicles in scoria are typically larger and more angular than the rounded vesicles in pumice, which is another useful distinction. Andesite and diorite are the intermediate pair, and this is where identification becomes genuinely annoying. The boundary between andesite and dacite, or between diorite and quartz diorite, is compositional and often impossible to resolve with a hand lens. Most lab keys draw an arbitrary line based on silica content or relative quartz abundance, but students do not have a silica test. The practical workaround is to classify based on the dominant feldspar type and overall color index. If the plagioclase is clearly intermediate in composition and the rock is medium-gray with visible hornblende and plagioclase, andesite or diorite is the safe call. If the key forces you to choose between andesite and dacite, pick andesite unless you can positively identify significant quartz grains, which is rare in hand-specimen work.

Using the Answer Key Effectively

The answer key is a tool, not an authority. Use it to confirm your observations, not to replace observation. The most reliable workflow is to note texture first, then color index, then list the minerals you can positively identify, and finally match that combination against the key. If the key does not have an entry that fits, that is useful information. It means either the sample is unusual, the key is incomplete, or you misidentified something. One specific edge-case that tripped up half my sections involved a specimen labeled simply as "igneous unknown number four." It was gray, moderately coarse-grained, with visible plagioclase and what appeared to be amphibole, but the dark mineral also showed a slight greenish tint that could have been altered pyroxene or serpentinized olivine. The answer key offered diorite, gabbro, and andesite as options. Diorite was the best fit texturally, but the greenish component suggested olivine, which would make it troctolite, a rock that most intro keys do not include. I had the students report it as diorite with a note about possible olivine content, and that was accepted for credit because the reasoning was sound. The key does not account for every valid geological scenario, and the professors generally know that. Writing down your diagnostic reasoning explicitly matters more than landing on the exact label the key expects. Another recurring issue is altered specimens. Basalt that has undergone low-grade metamorphism or hydrothermal alteration can develop chlorite, epidote, or zeolites that change the color and texture sufficiently to throw off a surface-level ID. A sample that should be basalt might come back greenish and slightly flaky. The answer key will still expect basalt, but the physical evidence does not support a clean match. In those cases, documenting the alteration minerals and explaining why the primary classification may be obscured is the correct academic response, even if the automated grading system marks it wrong.

Limitations You Should Accept Upfront

Hand-specimen identification of igneous rocks is inherently approximate. Without XRD, thin-section petrography, or at minimum a stereomicroscope with reflected light, you are working with visual cues that overlap significantly between related rock types. The Igneous Rock Identification Lab Answer Key is designed for educational purposes, not professional petrology, and it performs adequately within that scope. It will reliably distinguish granite from basalt, rhyolite from gabbro, and pumice from scoria. It will struggle with intermediate compositions, altered specimens, and any rock that falls outside the standard ten to twelve varieties the key covers. If you need accuracy beyond what a hand lens and flowchart can provide, the standard alternative is thin-section analysis under crossed polars. That shifts the identification from subjective visual matching to objective mineralogical quantification, though it requires equipment and training most introductory labs do not have. For lab purposes, the answer key plus careful observation is sufficient, but you should understand where the method breaks down rather than treating it as a definitive identification tool.