Read rock composition evidence

A sample contains three copper pieces and no other material. Does having three pieces make it a mixture?

A mixture contains different substances, not simply separate pieces

A whole piece and what makes it up

A specimen is the selected piece being studied. A constituent is a material that makes up part or all of that specimen

A rock is a natural solid geological material. Many rocks contain several mineral types, but a rock can contain just one mineral type

Minerals such as quartz and calcite are natural solids with characteristic compositions and ordered internal structures. Ordered means their tiny building units follow a repeating arrangement

Here the mineral identities are supplied in reports. A colour, grain shape or mineral name on a display label is not by itself a complete composition analysis

Read the roles in a report

A labelled limestone specimen is the whole piece being studied. A supplied report identifies calcite as a mineral within that specimen

Explain which term names the specimen and which names an identified constituent

The selected whole piece is the limestone specimen

Calcite is identified within that piece, so it is a constituent; the report does not identify every constituent

One specimen, two constituents

A grain is not a mineral type

A grain is a small part distinguished in an observation of the material. Several grains may all be the same mineral type

A grain can also be a rock fragment containing several minerals. It is not necessarily one crystal or one mineral type, and it is not one chemical particle

To count mineral types, count distinct identified mineral names. To count grains, count the observed parts under the stated counting rule

Count identities rather than grains

Inspect one counted fragment

One small rock fragment is counted as one grain in a sediment observation. A separate identification report shows quartz and calcite within that fragment

Explain how this one counted grain can contain more than one mineral type

The observation counts one fragment, while the report identifies quartz and calcite inside it

The fragment contains at least two mineral types, showing that one counted grain need not represent one mineral type

Compare two complete records

This model specimen stays fixed. Each counted grain contains only the named mineral, and the whole-model record includes every grain with no other material present. Compare the north patch with the whole specimen. The table reports identified grains, not masses, particles or a picture of their positions

Which conclusion follows from comparing both records?

Ask what the record covers

Scope means the part examined: a small patch, one fragment or the whole specimen. A result for a patch does not automatically describe the rest

Completeness means whether all constituents are established. Identifying quartz does not exclude other material if the report leaves some of it unidentified

Some questions stipulate a complete idealised composition. Use that stated information for the model, but do not assume that a real measurement detects every trace impurity

Only a complete one-substance record justifies calling the whole model sample pure. A known second substance establishes a mixture even if its amount is small

Use a complete idealised record

Leave unidentified material unresolved

Keep a local result local

Check the kind of evidence

Colour and texture describe appearance. An identification report establishes named constituents. A label may name the main mineral without listing every inclusion, a piece of other material enclosed within the specimen

Grain number is not mass or volume. Several tiny grains need not contain more material than one larger grain

A later investigation can identify something that was present all along. More complete knowledge does not necessarily mean the specimen itself changed

Do not turn colour into an identification

Include the inclusion

Avoid an unsupported mass claim

Recognise a changed report

Review a composition claim

Identify the specimen and the part the report covers, then list the constituent identities it actually establishes

Distinguish the number of grains from the number of mineral types. A grain is not a chemical particle, and grain count does not determine mass

Check whether the record is complete. A local test, appearance or display label does not establish the complete composition of a whole specimen

A rock can contain one or several mineral types. Some rocks also contain natural glass, which lacks the ordered structure of a mineral, or organic material, as in coal