Gordaite

Key takeaways

Gordaite belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Gordaite to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Gordaite from memory before moving on to harder problems.

Reference excerpt

Gordaite is a sulfate mineral composed primarily of hydrous zinc sodium sulfate chloride hydroxide with formula: NaZn4(SO4)(OH)6Cl·6H2O. It was named for the discovery location in the Sierra Gorda district of Chile. Gordaite forms as tabular trigonal crystals. Gordaite first appeared after a research dive in September 1984 from the Juan de Fuca Ridge of the northeastern side of the Pacific Ocean. Gordaite was also described from weathered slag deposits as a result of copper smelting in Hettstedt, Germany. The mineral exhibits a hexagonal shape with clear or white (green if cuprian – Cu2+) crystals ranging from planar to broad habit and has a point group of 3. Gordaite commonly occurs near minerals such as sphalerite, boleite and gypsum. The most recent finding occurred in the San Francisco mine in Chile where copper-zinc sulfide deposits were found.

Composition The 1998 Nasdala article examines the ALV 1457-5R sample found on the sea floor of the Juan de Fuca Ridge. The sample itself was taken from a sulfide chimney with mass of about 2.5 kg. The chimney was composed of about 98% permeable sulfides and 2% sulfates. The sulfides were primarily composed of iron sulfides such as pyrite and sphalerite and were present in the core of the chimney. The sulfate portions were found in concentric layers on the outside of the sample, alternating between [Zn4(OH)6(SO4)Cl]- and [Na(H2O)6]- interlayers. Table 1: Chemical Composition of the Gordaite Mineral

(1) San Francisco mine, Chile; by electron microprobe, H2O by CHN analyzer; low analytical total due to loss of H2O during grinding, Na too high due to peak overlaps with Zn; after adjusting Na2O to 5.5% from AA, and partitioning H between H2O and (OH)- according to crystal-structure analysis, corresponds to Na1.54Zn3.39(S04)(OH)6Cl0.95·6H20. (2) NaZn4(SO4)(OH)6Cl·6H20. Table 1 is taken from a study done based on the findings in the San Francisco mine. Section 1 displays the results based from an electron microprobe test which resulted in loss of water. Section 2 displays the normalized results taken from crystal structure analysis. Recognition of gordaite crystals is problematic due to its association with tabular baryte. Based on the findings of the dive at Juan de Fuca Ridge, gordaite was formed due to the reaction of discharging hydrothermal fluids with sea water. The overwhelming presence of baryte crystals confirms the idea that gordaite must be more than a byproduct of weathering.

Structure The gordaite mineral from Juan de Fuca has a trigonal crystal structure and its formula is Zn4Na(OH)6(SO4)Cl·6H2O. Utilizing electron microprobe analysis to define gordaite’s chemical composition is difficult because of its intricate chemical layering. According to the study done using the Philips XL 30 SEM (scanning electron microscope), the analysis of sodium could not be completed because the lines of zinc (Zn-Lα) were so intense that they overlapped the Na-K lines. The sample from Germany is a trigonal structure with lattice perimeters: a = 8.364 Å and c = 13.046 Å respectively. Gordaite contains octahedra with edges that share CDI2 – like sheets and have Zn2+ ions in their centers, where five hydroxide ions coordinate the zinc ions. Throughout the sulfate groups are connected to octahedral sheets. Sodium ions lie between the layers in order to make up for the negative charges and are coordinated by six water molecules.

Physical properties Gordaite belongs to the point group P3. This indicates that the mineral has a primitive lattice and belongs to the hexagonal/trigonal point group. Gordaite crystals are typically white or opaque and can sometimes exhibit a greenish color if trace copper is present. The crystals are broad and flat and have a vitreous luster. The mineral shows a cleavage plain parallel to {0001}, is flexible and has a Mohs hardness of about 2.5. The measured specific gravity is about 2.627 whereas the calculated value is about 2.640. Its crystallographic axes are a = 8.3556 and c = 13.025 angstroms, similar to that is the sample from Germany.

Geologic occurrences Gordaite has appeared in oxidized deposits of Cu-Zn sulfide in the mines of San Francisco, Chile. Deposits have also been found on the eroded mine dumps in Hettstedt and Helbra Germany. The most notable incidence occurred on the sea floor of the Juan de Fuca Ridge. This occurrence is important because it shows the reaction of ocean water with hydrothermal fluids in the exterior oxidized portions of the chimney. Gordaite was named after the Sierra Gorda District of the Tocopilla Province of the Antofagasta region of Chile where it has been reported from the San Francisco mines and other locations. It was approved by the International Mineralogical Association in 1996. Gordaite has since been reported from mine dumps in Germany and the Juan de Fuca Ridge.

References

Illustrations

Worked examples

Example 1 — a first encounter with Gordaite

Start with the simplest possible case. Write down what Gordaite claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Gordaite before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Gordaite ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Gordaite

In research: Gordaite appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Gordaite in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Gordaite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Minerals described in 1997, Minerals in space group 147, Sodium minerals, so understanding it makes those chapters shorter.
In everyday life: Look for Gordaite outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Gordaite in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Gordaite means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Gordaite out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Gordaite in simple terms?

Gordaite is a sulfate mineral composed primarily of hydrous zinc sodium sulfate chloride hydroxide with formula: NaZn4(SO4)(OH)6Cl·6H2O. It was named for the discovery location in the Sierra Gorda district of Chile.

Why does Gordaite matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Gordaite?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Gordaite.