Bostonite
Fine-grained alkali feldspar trachytoid dike rock
A fine-grained, feldspar-rich dike rock with a trachytic texture, essentially a hypabyssal equivalent of trachyte or syenite.
- Mohs hardness
- 6
- Color
- Pale gray, pinkish to buff
- Type
- igneous
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Overview
Bostonite is a fine-grained, light-colored dike (hypabyssal) rock composed almost entirely of alkali feldspar with little or no dark minerals. It has a characteristic trachytic texture in which lath-shaped feldspar microlites show subparallel flow alignment, giving it a feel intermediate between trachyte (volcanic) and syenite (plutonic).
The rock takes its name from the Boston area of Massachusetts, USA, where it was described as a distinctive dike rock. Its pale, almost mafic-mineral-free composition makes it notably leucocratic (light-toned).
Bostonite is essentially a textural and field-naming term for these feldspathic dike rocks and is mainly of academic and collector interest.
Formation & geology
Bostonite forms from alkali-rich, feldspar-dominated magmas that intrude as dikes and sheets at shallow to moderate depths (hypabyssal level). The relatively rapid cooling in these thin bodies produces a fine grain size and the flow-aligned, trachytic arrangement of feldspar laths.
It commonly occurs in association with alkaline intrusive complexes and syenite or trachyte provinces, where late-stage feldspathic melts are squeezed into fractures. Besides the Boston-area type occurrences in Massachusetts, comparable bostonitic dikes are reported from many alkaline igneous provinces worldwide.
How to identify it
Bostonite is a pale gray, pinkish, or buff fine-grained rock that is conspicuously poor in dark minerals, giving it a light, almost uniform appearance. A hand lens may reveal tiny aligned feldspar laths producing a faint streaky (trachytic) texture.
Hardness is about 6 (feldspar). Distinguish it from felsite or rhyolite by its near absence of quartz and its alkali-feldspar dominance, and from trachyte by its dike-rock (hypabyssal) setting. It resembles a fine-grained syenite but is finer and occurs as narrow intrusions rather than large plutons.
Uses & significance
Bostonite has no significant commercial use as an ornamental or industrial stone. Its importance is descriptive and scientific: it is a recognized field term for pale, feldspar-rich trachytoid dike rocks and helps geologists map alkaline intrusive activity.
For collectors, bostonite is a minor curiosity representing the hypabyssal end of the trachyte-syenite family. It has no established metaphysical tradition and is of interest mainly to petrologists, students, and those documenting dike-rock suites in alkaline provinces.
Frequently asked questions
What is bostonite?
It is a fine-grained, feldspar-rich dike rock with a trachytic texture, essentially a shallow-intrusive equivalent of trachyte or fine syenite.
Where does the name come from?
It is named after the Boston area of Massachusetts, USA, where the distinctive dike rock was first described.
Does bostonite contain dark minerals?
Very little. It is a leucocratic (light-colored) rock dominated by alkali feldspar with few or no mafic minerals.
How is bostonite different from syenite?
Both are alkali-feldspar-rich, but bostonite is much finer grained and forms as thin dikes, whereas syenite is a coarse-grained plutonic rock.
Bostonite guides
In-depth guides for identifying, valuing, and understanding Bostonite.
