Aplite
Aplite is a fine-grained, sugary rock of quartz and feldspar that fills fractures and dikes in granite. Because it has almost no dark minerals it is usually pale pink or cream, and its even, saccharoidal texture explains the name, from the Greek for 'simple.'
Aplite is a fine-grained, sugary rock of quartz and feldspar that fills fractures and dikes in granite. Because it has almost no dark minerals it is usually pale pink or cream, and its even, saccharoidal texture explains the name, from the Greek for "simple."
🪨 Geology Guide Specimen: Open Aplite in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 7 subgroups of Igneous, and test your eye in LithoClade.
Sugary Texture: Why Is Aplite Fine-Grained?
Aplite dikes commonly fill cracks that open in a granite body that is already largely solid. When a fracture opens, pressure on the remaining water-rich melt drops suddenly and water escapes, which raises the temperature at which the melt crystallizes. Many tiny crystals nucleate at once, giving a uniform grain usually under a millimeter across.
Companion of Pegmatite: How Are the Two Related?
Aplite and pegmatite frequently occur together in composite dikes, with an aplite margin and a coarse pegmatite core, or in layered "line rock." Both crystallize from the silica-rich residual melt of a granite, but under different conditions of water content and cooling, which is why one is minute-grained and the other gigantic.
Field Recognition: How Do You Spot It?
Look for thin, pale, sharp-sided dikes cutting through darker granite. The rock looks like coarse sugar, weathers to resistant ledges, and may contain scattered small garnets. It has little biotite or hornblende, which separates it from fine-grained mafic dikes.
Aplite at a Glance
| Property | Value |
|---|---|
| Type / variety | Saccharoidal Quartz–Feldspar Aplite |
| Composition | Equigranular Quartz + Alkali Feldspar (devoid of dark mafic minerals) |
| Geology Guide group | Igneous Rocks (Plutonic & Volcanic) |
| Rock Classification | Felsic Plutonic (Granitoids) |
| Mohs hardness | Mohs 6.5–7 |
| Texture / habit | Sugary (saccharoidal) fine equigranular (<1 mm) |
| Density | 2.60–2.65 g/cm³ |
| Lustre and cleavage | Uniform pale cream/pink sugary appearance |
| Geological setting | Late-stage cross-cutting dikes and veins inside batholiths |
| Diagnostic feature (Geology Guide) | Pale, sugary-textured intrusive dike rock composed almost entirely of uniform sub-millimeter quartz and alkali feldspar grains with virtually zero dark ferromagnesian minerals. |
| Texture name | Saccharoidal (sugary) |
| Typical form | Dikes and veins in granite |