The development of well-formed crystal faces in minerals, especially in rock-forming minerals, can indeed be a challenge when differentiating between sulcate (such as sulfates) and non-sulcate minerals. Several factors contribute to the difficulty in identifying these minerals based on crystal morphology:
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Crystal Habit: Many rock-forming minerals tend to form in a massive or granular habit rather than as individual crystals with well-defined faces. This is particularly true for minerals that crystallize in a high-temperature, high-pressure environment where space is limited, leading to the formation of grains rather than well-formed crystals.
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Conditions of Formation: Rock-forming minerals often form in environments such as igneous, metamorphic, and sedimentary processes. These conditions can be very different from those that lead to the growth of minerals with well-defined crystal faces, such as those found in evaporative or hydrothermal deposits where conditions allow for more orderly growth.
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Time and Space Constraints: The availability of space and time for crystal growth can be restricted in natural settings. Rapid cooling of magma, for example, does not allow for the development of large crystals, leading to the formation of aphanitic textures where crystals are too small to be identified.
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Intergrowths: Rock-forming minerals often grow in intergrowths with other minerals, complicating the ability to isolate and observe individual crystal faces. This can hinder identification based solely on crystal morphology.
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Metamorphism and Weathering: Rock-forming minerals can undergo metamorphic processes or weathering, which can alter their original crystal forms and create new textures that may obscure the original crystal faces.
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Mineral Variability: Many minerals exhibit variability in their crystal forms. For example, polymorphism can occur in some minerals, leading to different crystal structures under different conditions, which can confuse identification efforts based on crystal faces alone.
In summary, while crystal faces are often used in mineral identification, the context in which rock-forming minerals develop—combined with environmental factors and physical constraints—often results in a lack of well-defined crystal morphology that is necessary for clear categorization into sulcate and non-sulcate groups. For accurate identification, other methods such as chemical analysis or the use of optical properties may be necessary.