Bostonite Identification Guide

What Bostonite Looks Like

Bostonite is a fine-grained, light-colored alkali-feldspar dike rock — essentially the fine-grained, near-aphyric equivalent of a syenite or a sodic trachyte that occurs as dikes and sills. It is typically gray, pinkish-gray, buff, or pale brown, with a dull to slightly sugary appearance. The rock is dominated by alkali feldspar laths with very little quartz and few dark (mafic) minerals, giving it a pale, even tone. A characteristic trachytic texture (sub-parallel alignment of tiny feldspar laths) may be visible with a lens or felt as a faint sheen.

Step-by-Step Field ID Checklist

1. Confirm a pale, fine-grained igneous rock with few or no dark minerals.
2. Use a hand lens to see tiny aligned feldspar laths (trachytic texture).
3. Note the near-absence of quartz and scarcity of mica or amphibole.
4. Check the field setting: bostonite occurs as dikes and sills, often near alkaline complexes.
5. Test hardness and acid reaction to rule out carbonate rocks.

Key Diagnostic Tests

• Mohs hardness: feldspar-dominated, ~6; scratches glass and resists a knife.
• Streak: white to pale gray.
• Cleavage/fracture: feldspar grains show two cleavages at ~90°; the rock fractures irregularly.
• Acid: no reaction with dilute HCl.
• Density: moderate (SG ~2.6), typical of feldspathic rock.
• Texture: the aligned-lath trachytic fabric plus low quartz content is the best field clue.

Common Look-Alikes and How to Tell Them Apart

• Aplite: a fine-grained granitic dike rock but quartz-rich; bostonite is quartz-poor, so look for the absence of glassy quartz grains.
• Felsite/rhyolite: felsic volcanic with quartz and often phenocrysts; bostonite is hypabyssal, quartz-poor, and dominated by alkali feldspar.
• Trachyte: chemically similar but a true lava flow; bostonite is the intrusive dike equivalent and is often more sugary-textured.
• Syenite: the coarse-grained plutonic equivalent; bostonite is its fine-grained cousin.

Where Bostonite Is Found

Bostonite is named from Marblehead Neck near Boston, Massachusetts. It occurs as dikes and sills associated with alkaline igneous provinces, often together with camptonite and other lamprophyres, in regions such as New England, the British Isles, and other rifted or alkaline terranes.