gabbro
Enable an AI agent to recognise gabbro, assess the condition and representativeness of a particular occurrence or specimen, and select justified investigation, handling or material-use actions.
Research draft, second pass
A second pass drafted this model: the structure a model of this thing needs, and what is known about it in the world. The line under this one says how the second half was obtained - researched against sources, or recalled without web access, in which case nothing here was read anywhere and every claim is a lead to verify. Unreviewed either way.
Researched by: Codex + Grok
Purpose and description
Enable an AI agent to recognise gabbro, assess the condition and representativeness of a particular occurrence or specimen, and select justified investigation, handling or material-use actions.
Gabbro is a coarse-grained, plutonic mafic igneous rock composed mainly of plagioclase (typically labradorite-bytownite) and clinopyroxene, with or without olivine, orthopyroxene, and opaque oxides, corresponding to the plutonic equivalent of basalt in the IUGS QAPF and gabbroic-rock classifications.
It can be Assign or revise a provisional gabbro identification from documented petrographic evidence; Choose fresh, altered and compositionally contrasting sampling locations; Request thin-section examination, modal analysis or plagioclase chemistry to resolve a specific classification ambiguity; Identify fractures, weathered zones and veins that constrain block selection or further testing; Compare measured properties with an explicitly supplied material-use requirement; Preserve an oriented reference specimen and link destructive-test results to its source material.
Distinguishing features
Check crystalline grain size and texture alongside mineral identity when distinguishing gabbro from basalt or microgabbro; record the classification convention rather than treating colour as decisive.
Distinguish gabbro from diorite using measured plagioclase composition: BGS specifies average anorthite content above 50% for gabbro within the applicable classification field. [BGS gabbro definition](https://webapps.bgs.ac.uk/bgsrcs/rcs_details.cfm?code=GB)
Distinguish gabbro from anorthosite using modal mafic-mineral abundance: BGS assigns gabbro when this exceeds 10%, subject to the other classification criteria. [BGS gabbro definition](https://webapps.bgs.ac.uk/bgsrcs/rcs_details.cfm?code=GB)
Measure clinopyroxene, orthopyroxene and olivine separately before assigning a narrow subtype; BGS distinguishes gabbro sensu stricto using their relative proportions. [BGS gabbro sensu stricto definition](https://webapps.bgs.ac.uk/bgsrcs/rcs_details.cfm?code=GBS)
Inspect preserved igneous texture, replacement minerals and deformation before accepting an altered or metamorphosed specimen as presently identifiable gabbro rather than a rock with an inferred gabbro protolith.
Scope
+ Evidence supporting or challenging identification as gabbro
+ Mineral proportions, plagioclase composition and crystalline texture
+ Variation across specimens, blocks and exposed gabbro
+ Weathering, mineral replacement, veins and fractures
+ Measured material properties and their relevance to a stated use
- Independent definitions of constituent minerals such as plagioclase and pyroxene
- Complete histories and geometries of intrusions, crustal sections or tectonic settings
- Quarry operations, extraction rights and production planning
- Design and certification of aggregate products or finished stone components
- Soils and sediments produced after the rock has disaggregated
Characteristics
- Petrographic identification
- provisional gabbro | supported gabbro | ambiguous | alternative identification; named classification convention Controls whether gabbro-specific interpretation is justified.
- Primary mineral mode
- volume % plagioclase, clinopyroxene, orthopyroxene, olivine, quartz, alkali feldspar and other identified phases; method and uncertainty Supports classification and exposes mixtures or subtypes hidden by a field label.
- Plagioclase composition
- anorthite mol %; measured range, zoning and stated averaging method Helps resolve the gabbro-diorite boundary without relying on appearance.
- Grain-size distribution
- mm by mineral phase, with observed range and measurement method Supports textural classification and selection of a representative analytical sample.
- Igneous fabric
- equigranular | inequigranular | ophitic | subophitic | layered | other described | unresolved; multiple values allowed Records recognition evidence and directions along which sampling or performance may differ.
- Secondary mineral replacement
- estimated volume % replaced, identified secondary phases and affected primary minerals Separates current material composition from reconstructed original composition.
- Weathering condition
- fresh | surface-weathered | penetratively weathered | disaggregating | unresolved; grading method recorded Prevents a weathered surface or fresh core from standing for the entire material.
- Discontinuity geometry
- spacing and aperture in mm or m, orientation in degrees, persistence in m; observation scale Supports assessment of block integrity, sampling access and differences between intact rock and rock mass.
- Specimen-to-occurrence relationship
- sampled from | loose near | supplied as | unknown origin; linked locality, interval or material lot Limits how far specimen observations can be generalised.
- Measured material performance
- density in kg/m³, water absorption in mass %, strength in MPa or use-specific test result; method, moisture state and orientation Supports proposed uses through actual measurements rather than a rock-name assumption.
Also called
Where this came from
wikidata · CC0 1.0
Drafted structure
Bundle to layer to finding to question, as the second pass will find it: 5 bundles · 9 layers · 16 findings · 26 questions.
Gabbro identity Evidence that the material belongs under the registered gabbro concept.
A dark crystalline appearance or supplier label cannot establish the mineralogical boundaries of gabbro.
Mineralogical boundaries
Composition observations needed to distinguish neighbouring rock identities.
Diagnostic mineral evidence
Record observed mineral modes and plagioclase chemistry separately from inferred or unavailable values.
- What measured mineral proportions and plagioclase compositions support the gabbro identification, and with what uncertainty? measurement
- Under the chosen classification, do these observations exclude diorite, anorthosite and other neighbouring rock classes? boundary
Texture and name resolution
Textural support and the intended breadth of the gabbro name.
Classification resolution
Record grain-scale evidence, the naming convention and whether a broad or narrow identification is justified.
- Does gabbro here mean the broad class, gabbro sensu stricto or an unverified field or commercial name? definition
- What grain-size and texture observations distinguish this material from basalt or microgabbro under that convention? boundary
- Which additional petrographic observation would resolve the remaining naming ambiguity? action
Gabbro variability and sampling The relationship between measured material and the gabbro occurrence it represents.
Mineral segregation, layering and local inclusions can make one specimen misleading for a larger block or exposure.
Mineral and textural variation
Spatial changes that affect identification and sampling coverage.
Gabbro domains
Distinguish observed domains by mineral abundance, grain size, fabric, veins and inclusions without presuming a formation mechanism.
- How do plagioclase and mafic-mineral proportions and grain sizes vary across the observed material? measurement
- Which layers, veins or inclusions require separate observations rather than inclusion in a single averaged gabbro description? boundary
Sample representativeness
Traceable sampling context and limits on extrapolation.
Sampled domain coverage
Link each specimen to its location, orientation, sampled domain and fresh or weathered condition.
- Was this specimen collected in place, recovered as a loose fragment or supplied from a documented lot, and what source relationship is verified? provenance
- Which mineralogical or textural domains remain unsampled before the observations can represent the proposed block, interval or lot? action
Gabbro alteration and integrity Changes to primary minerals and the continuity of the rock.
Recognition of an original gabbro composition does not establish the identity or integrity of the material now present.
Primary mineral preservation
Preserved igneous evidence versus secondary replacement and weathering.
Replacement and identity
Record which feldspar and mafic grains survive, what replaces them and whether the original composition remains recoverable.
- What fractions of plagioclase, pyroxene and any olivine remain identifiable, and what secondary phases replace them? measurement
- Is gabbro supported as the current rock identity, only as a protolith interpretation, or neither? boundary
Fractures and weathering depth
Discontinuities and deterioration relative to fresh interiors.
Intact gabbro extent
Record connected fractures, vein interfaces and weathering penetration at the scale relevant to the intended action.
- What fracture spacing, aperture, infill and weathering depth separate intact interiors from compromised zones? measurement
- Where should a fresh comparison sample or an integrity test be taken before selecting this material for cutting or mechanical testing? action
Gabbro investigation and use Evidence needed to choose tests and evaluate proposed material actions.
The gabbro name alone does not establish strength, durability, finish quality or suitability for a particular use.
Property evidence
Measured performance tied to composition, fabric and condition.
Use-specific performance
Associate property results with the tested gabbro domain, specimen preparation, moisture condition and orientation.
- Which measured properties address the proposed aggregate, dimension-stone or research use, and which relevant properties remain untested? measurement
- Do the results apply to intact fresh specimens, altered material, finished surfaces or the fractured rock mass? boundary
- Does the tested material meet the supplied use criteria, or is a further test needed before a suitability judgement? action
Targeted material actions
Investigation and preparation choices that preserve interpretable evidence.
Evidence-preserving test selection
Choose sampling and preparation around unresolved mineralogy, layering, replacement or fractures, retaining a reference where material will be consumed.
- Should the next action target mineral identification, plagioclase chemistry, replacement phases or mechanical behaviour to resolve the stated decision? action
- What specimen orientation, fresh-to-weathered comparison and retained reference are needed before sectioning, crushing or destructive testing? action
Evidence and external alignment What the world already says about this thing, gathered so the model can be checked against it.
A model that cannot be lined up against existing standards, identifiers and practice cannot be adopted by anyone who already uses them.
Reported evidence
Findings from the breadth pass, kept separate from the structural claims.
Kinds and varieties
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- olivine gabbro
- norite (orthopyroxene-dominant gabbroic rock)
- gabbronorite
- troctolite
- hornblende gabbro
- quartz gabbro
- essexite / alkali gabbro
- layered (cumulate) gabbro
- Which of these kinds and varieties hold for the sense of gabbro this model covers, and on what evidence? provenance
Identifiers and schemes
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Wikidata - Q191478 - gabbro
- Which of these identifiers and schemes hold for the sense of gabbro this model covers, and on what evidence? provenance
Standards and regulation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- IUGS Subcommission on the Systematics of Igneous Rocks - QAPF and gabbroic-rock classification
- Which of these standards and regulation hold for the sense of gabbro this model covers, and on what evidence? provenance
Real-world use
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Crushed stone and dimension stone; oceanic crust Layer 3; Ni-Cu-PGE host in layered mafic intrusions
- Which of these real-world use hold for the sense of gabbro this model covers, and on what evidence? provenance
Typical measurements
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- SiO2 - 45-52 - wt%
- Which of these typical measurements hold for the sense of gabbro this model covers, and on what evidence? provenance
Failure modes and hazards
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Polishing and low skid resistance as road aggregate; possible asbestiform amphibole in altered gabbros
- Which of these failure modes and hazards hold for the sense of gabbro this model covers, and on what evidence? provenance
Regional variation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- British usage historically treated norite separately; oceanic vs continental gabbro suites
- Which of these regional variation hold for the sense of gabbro this model covers, and on what evidence? provenance
Neighbouring kinds and how to tell them apart
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- diorite - plagioclase more calcic (An50+) and mafic minerals more abundant in gabbro
- basalt - gabbro is phaneritic/plutonic; basalt is the fine-grained volcanic equivalent
- Which of these neighbouring kinds and how to tell them apart hold for the sense of gabbro this model covers, and on what evidence? provenance
Sources
- Igneous Rocks: A Classification and Glossary of Terms (IUGS recommendations) - Official IUGS mineralogical definition and named gabbroic varieties
What the second pass must settle
- Does vr.tr.gabbro denote the broad gabbro class or gabbro sensu stricto, and how does the registry place norite, troctolite and other related names?
- Which classification convention and grain-size boundary should govern separation from microgabbro or dolerite?
- What evidence threshold should move strongly altered or recrystallised material to a neighbouring model while retaining a gabbro-protolith relationship?
- Which sampling coverage is sufficient for layered or heterogeneous gabbro at specimen, block and material-lot scales?
- Which proposed uses and acceptance criteria must this model support, and what measured evidence is required for each?